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Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products
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Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products
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  • Mitchell W. Alton
    Mitchell W. Alton
    Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States
    Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States
    More by Mitchell W. Alton
    Orcidhttps://orcid.org/0000-0002-7119-3706
  • Eleanor C. Browne*
    Eleanor C. Browne
    Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States
    Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States
    *Email: [email protected]. Tel: 303-735-7685.
    More by Eleanor C. Browne
    Orcidhttps://orcid.org/0000-0002-8076-9455
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ACS Environmental Au

Cite this: ACS Environ. Au 2022, 2, 3, 263–274
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https://doi.org/10.1021/acsenvironau.1c00043
Published February 17, 2022

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Abstract

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Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals that have come under scrutiny due to their widespread use and environmental persistence. Significant data on environmental concentrations and persistence of these chemicals exists, but their oxidation mechanism is poorly understood, preventing a comprehensive understanding of the environmental fate and impact of cVMS. We performed experiments in an environmental chamber to characterize the first-generation oxidation products of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under different peroxy radical fates (unimolecular reaction or bimolecular reaction with either NO or HO2) that approximate a range of atmospheric compositions. While the identity of the oxidation products from D3 changed as a function of the peroxy radical fate, the identity and yield of D4 and D5 oxidation products remained largely constant. We compare our results against the output from a kinetic model of cVMS oxidation chemistry. The reaction mechanism used in the model is developed using a combination of previously proposed cVMS oxidation reactions and standard atmospheric oxidation radical chemistry. We find that the model is unable to reproduce our measurements, particularly in the case of D4 and D5. The products that are poorly represented in the model help to identify possible branching points in the mechanism, which require further investigation. Additionally, we estimated the physical properties of the cVMS oxidation products using structure–activity relationships and found that they should not be significantly partitioned to organic or aqueous aerosol. The results suggest that cVMS first-generation oxidation products are also long-lived in the atmosphere and that environmental monitoring of these compounds is necessary to understand the environmental chemistry and loading of cVMS.

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1. Introduction

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Cyclic volatile methyl siloxanes (cVMS) are high-production volume chemicals (1,2) that are common components of consumer and industrial products such as deodorants, lotions, sealants, and lubricants. cVMS have high vapor pressures and low Henry’s solubilities, leading to preferential partitioning into the atmosphere (3) where they primarily degrade through reactions with OH. (4) cVMS lifetimes with respect to oxidation by OH are between 4 and 10 days (assuming a 24 h average OH concentration of 1.2 × 106 molecules cm–3), (4−12) and consequently, cVMS are globally distributed and have been identified in remote environments such as the arctic. (13−15) These abundant chemicals are also bioaccumulative and toxic. (13,16−18) Consequently, the European Union placed restrictions on the use of cVMS in certain cosmetic products in 2016, with recommendations in 2021 to restrict the use in certain industrial processes. (19−21) Owing to their widespread use, toxicity, and environmental persistence, there has been significant interest in understanding the environmental fate of cVMS. (5,12,13,16,17,22−38)
Previous research has largely focused on quantifying ambient concentrations of parent cVMS (13,23,26,39) and understanding their oxidation kinetics; (5,10,11) significantly less is understood about the identity of cVMS oxidation products and their environmental chemistry. (36,37) In the atmosphere, cVMS oxidation is initiated by reactions with OH or Cl to produce an alkyl radical (R3SiCH2•), which quickly reacts with O2 to produce a peroxy radical (R3SiCH2O2•, or more generally, RO2). The subsequent reactions depend on atmospheric composition: reactions with NO will dominate under high NO mixing ratios, such as in urban areas; reactions with HO2 will dominate where high HO2 mixing ratios are found, such as in areas dominated by biogenic emissions; and unimolecular reactions can dominate in areas with low mixing ratios of NO or HO2, such as remote regions or even in urban areas for reactions with sufficiently fast unimolecular reactions. (40)
Laboratory investigations of cVMS oxidation have observed numerous oxidation products but typically identify siloxanol (−CH3 replaced with −OH) as the main product. (4,7,8) Two experimental studies have also reported the formate ester (−CH3 replaced with −CH(O)H) product. (4,5) Additionally, oxidation products such as hydroperoxides, alcohols (R3SiCH2OH), and products with an increased number of silicon atoms have been detected. (8,41) Siloxanol formation has often been ascribed to the hydrolysis of formate ester, (4,7) a process that likely occurs on surfaces. Attempts to develop an oxidation mechanism based on these observations and known atmospheric peroxy radical chemistry have largely been unsuccessful, in part, because the RO2 fate has been unclear in many of the past experiments. It has generally been concluded that Si in cVMS allows for unique chemistry to occur and several unusual reactions have been proposed based on the laboratory results. (4,8,18) Recent theoretical investigations of organosilicon chemistry provide evidence that organosilicon compounds undergo unique rearrangements inaccessible to carbon-based compounds. (36,37) The net effect of the mechanism proposed by the theoretical investigations oxidation under high NOx (=NO + NO2) conditions includes the production of HO2 and the oxidation of two NO radicals to NO2. (36,37) This net effect is inconsistent with laboratory measurements showing that at most one NO is oxidized to NO2 during cVMS oxidation and that cVMS reduces O3 production and OH radical concentrations. (18) However, as cVMS oxidation products were not measured, there is still an opportunity for better online characterization of the chemistry.
Without an understanding of the cVMS oxidation mechanism and products, it is difficult to predict the fate of the oxidation products. For instance, one possibility is that the oxidation products contribute to aerosol mass. Based on observations of Si in urban nanoparticles, it has been suggested that oxidation products of cVMS from personal care products are present in aerosol, (29,39,42) while other studies have suggested that silicon in aerosol from organosilicon compounds is minimal. (43,44) There are substantial variations in the reported aerosol yields of cVMS, which range from approximately 0–50%. (45) The range in the reported yields is likely due to experimental conditions creating different oxidation products. Without knowledge of the oxidation mechanism and measurements of the oxidation products, however, this hypothesis is difficult to test.
In this study, we conducted a series of atmospheric chamber experiments designed to investigate the reaction mechanisms of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under conditions of different RO2 lifetimes and reaction partners. We also oxidized fully deuterated hexamethyldisiloxane (D18L2) with Cl atoms to provide some constraint on the identity of the oxidation products. Using a combination of our measurements, kinetic modeling, and constraints from the literature, we propose a simplified oxidation mechanism for cVMS that can be used in modeling atmospheric cVMS chemistry. We identify points in the cVMS oxidation mechanism that require further investigation. The environmental fate of the observed products is investigated through estimations of vapor pressure and water solubility, with the results suggesting that the first generation of cVMS oxidation does not create oxidation products that will participate in aerosol growth.

2. Materials and Methods

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Experiments were performed in a ∼1 m3 FEP Teflon chamber at ambient lab temperature (295 ± 3 K) and pressure (∼860 mbar) at low relative humidity (<5% RH). A low RH was used to minimize the wall loss of the oxidation products. The chamber, previously described in Alton and Browne, (5) was run in either batch mode, in which the chamber was slowly collapsed as air was sampled from it, or semibatch mode, in which the sampled air was continuously replaced. To correct for dilution in semibatch mode, the dilution rate was quantified by sampling for 30 min after completion of the experiment and determining the first-order loss constant for each product. This rate constant represents both loss to the walls and dilution and was used to correct the measured values for individual species. Experiments for D3 were conducted in both batch and semibatch modes and product distributions were determined to be consistent between the two methods of chamber operation on the timescale we are interested in (∼3 h). Experimental conditions are summarized in Table 1.
Table 1. Radical Precursor Concentrations, Estimated Oxidant Concentrations, and Estimated Ratios of the Reaction between RO2 and NO to HO2 and HO2 to other RO2
cVMSaoxidant precursor and concentration (ppbv)b[OH] or [Cl] (molecules cm–3)[NO], [NO2] (ppb)NO/HO2cτRO2 (s)d
D3*eCl2, 151.0 × 106 0.4:127
D3*H2O2, 10008.0 × 107 0.04:14
D3*HONO, 4005.0 × 107200,150200:10.02
D3Cl2, 151.0 × 106 0.4:127
D4Cl2, 156.0 × 105 0.9:140
D4H2O2, 10008.0 × 1070.05, 80.04:14
D4HONO, 4005.0 × 107230,190100:10.02
D4Cl2, 100 CH2O, 10007.0 × 105 0.02:11.6
D5Cl2, 154.0 × 1050.05, 0.051:146
D5H2O2, 10008.0 × 1070.05, 40.04:14
D5HONO, 4005.0 × 107250,180100:10.02
a

Approximately 80 ppbv of cVMS was added to the chamber.

b

The initial mixing ratios of NO and NO2 before the lights were turned on in HONO experiments were ∼50 and ∼100 ppbv, respectively, except for D3, which had ∼100 ppbv of NO. Differences in mixing ratios are due to the inconsistency of HONO generation using HNO3 and NaNO2. NO and NO2 were only measured in experiments for which mixing ratios are reported. For other experiments, background mixing ratios of 50 pptv were assumed.

c

Estimated radical concentrations were determined at the point that 10% of the cVMS has reacted, as calculated from the KinSim model.

d

Rate constants used in calculating the RO2 lifetimes were obtained from Ziemann and Atkinson: (46)kRO2+HO2 = 1.5 × 10–11 molecules cm–3 s–1, kRO2+NO = 9 × 10–12 molecules cm–3 s–1, kRO2+RO2 = 1 × 10–14 molecules cm–3 s–1, and kRO2+OH/Cl = 2 × 10–10 molecules cm–3 s–1.

e

Experiments marked with (*) were performed in semibatch mode, in which sampled air is continually replaced with clean air to maintain a constant chamber volume for the duration of the experiment.

D3 (98%, Acros Organics), D4 (98%, Acros Organics), D5 (97%, Sigma-Aldrich) were diluted in acetonitrile (99.8%, Sigma-Aldrich) to 5% (w/w) and injected into the chamber using a gently heated borosilicate glass tube with a stream of zero-air (AADCO Instruments, 737 series) transporting the evaporated cVMS to the chamber. We saw no evidence for thermal degradation of the cVMS from heating of the tube (maximum temperature estimated at less than 60 °C). The oxidation chemistry of each precursor was investigated under three different RO2 fates. Peroxy radical lifetimes were calculated using measured concentrations of NO or estimated concentrations of reactive species from a kinetic model, which will be discussed later in Section 3.3. Experiments using Cl2 as an oxidant precursor were designed to favor unimolecular RO2 reactions. For these experiments, we calculate a RO2 lifetime with respect to bimolecular reactions of about 0.4 min with ∼55% of the RO2 reacting with HO2 and 20% undergoing unimolecular rearrangements (assuming a rate constant of 8 × 10–3 s–1 for isomerization), (36) and the rest reacting with background NO. Experiments using H2O2 as an oxidant precursor probed conditions where >90% of RO2 react with HO2, while experiments using HONO resulted in ∼99% of RO2 reacting with NO.
Cl2 was added by a flow of N2 over a gravimetrically calibrated permeation device (VICI Metronics). Hydrogen peroxide was added via a solution added into the heated borosilicate glass tube with a zero-air flow. HONO was generated by the addition of 40 μL of a 1.5 M sodium nitrite solution (>99%, Sigma-Aldrich) to a 2 M nitric acid solution (70%, Fischer Scientific) while passing zero-air over the headspace of the solution, transporting any evolved gases into the chamber. Cl2 and HONO were photolyzed into Cl atoms and OH + NO, respectively, with 370 nm fluorescent lights (General Electric, F40BL) positioned below the chamber. H2O2 was photolyzed with 254 nm lights (General Electric, G36T8) to generate OH. Control experiments were performed to ensure the parent cVMS were stable in the presence of UV light. To test for photostability of hydroperoxides generated from D4 oxidation, we performed a control experiment where we photolyzed Cl2 with 370 nm lights with ∼1 ppmv of added formaldehyde to generate HO2. Partway through the experiment, 254 nm lights were also turned on with 370 nm lights. The production of hydroperoxides increased at the same rate as siloxanol, suggesting that more Cl2 was photolyzed with the additional lights increasing total oxidation, but there was no additional loss of the hydroperoxide from photolysis.
As in our previous work, (5) a high-resolution long time-of-flight chemical ionization mass spectrometer (CIMS; Aerodyne Research, Inc. and Tofwerk AG; resolving power ∼8000 m/z/Δm/z) using protonated toluene as the reagent ion measured the cVMS parent compounds and oxidation products during the experiments. Compounds are detected as [M + H]+ products. The ionization is relatively soft; we do not detect the methane fragment (R3Si+) from the parent cVMS that is generally seen in other proton-transfer ionization schemes. We did observe R3Si+ as a fragment of the siloxanol product, though less than 3% of the siloxanol signal was detected as this fragment. The CIMS sampled a total of 1.7 slpm, with 700 sccm of that flow consisting of humidified zero-air and the rest sampled from the chamber. The addition of water vapor enhances the instrument response (counts per second per ppbv normalized to the toluene reagent ion signal; ncps) for detection of cVMS and the oxidation products. Data was postprocessed in Tofware v3.2.3 (Tofwerk AG) in the IGOR Pro environment (Wavemetrics, v8.0.4.2) using fully constrained peak fitting and allocation of isotope signals. An example of the peak fitting is shown in Section S1 of the Supporting Information (Figure S1). NO and NO2 concentrations were measured using a chemiluminescence NO and NO2 analyzer with a blue light converter for true NO2 measurements (Teledyne, T200UP) with 1 min resolution and 50 pptv limit of detection.
We used KinSim v4.14 (47) in Igor Pro to simulate gas-phase chemistry for each experiment. KinSim is an open-source solver for kinetics modeling. Rate constants for non-cVMS species were obtained from Atkinson et al. (48) and Atkinson. (49) The mechanism used in the KinSim simulations is presented in Section S2 of the Supporting Information. The lifetime of cVMS species with respect to oxidation by OH (∼days) requires the use of elevated radical concentrations to ensure sufficient oxidation on the timescale of our experiment. However, the experiments were designed to avoid potentially unrepresentative reactions in the atmosphere (such as RO2 + RO2 or RO2 + OH). We assumed that the chamber was well mixed. Wall-partitioning calculations for all cVMS products were included using KinSim’s built-in wall-partitioning functions. (50) The reversible vapor–wall interactions were calculated upon mechanism compilation using the first-order vapor condensation rate coefficient (1 × 10–3 s–1), (51) the mass vapor saturation concentration of the partitioning molecule, and enthalpy of vaporization, with the latter two values estimated by the melting point, boiling point, and vapor pressure module (MPBPWIN v1.44) in the Environmental Protection Agency’s (EPA) Estimations Programs Interface for Windows (EPIWIN v4.11). Rate constants for peroxy radical reactions with NO and HO2 were taken from Atkinson and Ziemann (46) and were kept constant. As the oxidation rate constants for the different parent cVMS are known, (5) the decay of the cVMS was used to constrain the concentration of oxidants, which was then used to determine the photolysis rates of the oxidant precursors. An average photolysis rate was used for all experiments using the same oxidant precursor (7 × 10–5, 2 × 10–4, and 4 × 10–4 s–1 for H2O2, Cl2, and HONO, respectively).
For estimating partitioning between condensed phases in the atmosphere, we used EPIWIN to calculate the water solubility with the Water Solubility and Octanol-Water Partition Coefficient (WSKOW v1.43) and the Henry’s law constant with the HenryWin v3.21 module. In addition to EPIWIN, the EPA’s Toxicity Estimation Software Tool (TEST) was used to estimate the vapor pressures, boiling points, and water solubilities of the cVMS and their oxidation products. TEST does not estimate Henry’s Law constants; however, a proxy of the ratio between the vapor pressure and the water solubility of the compound can be used to compare between the EPIWIN and TEST. (52) TEST reports a consensus value where it estimates the values using multiple methods then averages those results together. These estimations are further discussed in Section 3.4.

3. Results and Discussion

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3.1. cVMS Oxidation Products

Figure 1 shows an example time series from a D3 oxidation experiment with H2O2 as the oxidant precursor. The only product to exhibit significant loss after the lights are turned off is the difunctional siloxanol + formate ester product. As oxidants are continuously generated during these experiments, the first-generation oxidation products will undergo further oxidation. To minimize the impact of this oxidation, we analyze the product signals at the point where only 10% of the parent cVMS has been oxidized. This metric was chosen as only 4% of the oxidation products are expected to have reacted at this point due to the continued generation of oxidants. Additionally, although the oxidation products are expected to have lifetimes with respect to wall loss greater than 1 h, measuring the products after only 10% of the cVMS reacted (∼30 min) minimizes the impacts of wall loss on the measured concentrations. Comparing the results at the point where 10% of the cVMS has reacted allows for an easier intercomparison between the different experiments, as we are achieving the same oxidant exposures.

Figure 1

Figure 1. Example time series of D3 oxidation with H2O2 as oxidant precursors. The shading signifies when the lights are on, and the dashed vertical line signifies when 10% of D3 had been oxidized. Signals averaged to 1 min time resolution are shown.

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Possible isomers of select D3 oxidation products are shown in Figure 2. These assignments are proposed based on previous works (4,7,8) and further informed through isotopically labeled linear VMS experiments (described in Section S3 of the Supporting Information). We did not detect the formation of any oxidation products that contained either fewer or greater silicon atoms than the parent compound. For instance, we did not observe the formation of either D4 or a product with 10 silicon atoms during the oxidation of D5.

Figure 2

Figure 2. Example cVMS oxidation product structures.

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The CIMS was calibrated for the parent siloxanes. The sensitivity to the parent compounds in these experiments was lower than reported in previous works (5) as the ion optics were tuned for optimal resolving power, which sacrificed sensitivity. Authentic standards for cVMS oxidation products are not commercially available, and thus, we are unable to quantify their concentrations. However, after normalization to the reagent ion and when working in the linear response region, the instrument response to a specific compound will be linear with respect to changes in concentration. As the reagent ion exhibited no significant depletion and signals were significantly above the limit of detection, these experiments are within the linear response region. Thus, although the oxidation products cannot be quantified, differences across RO2 fate conditions in the signal of a given product can be interpreted as changes in the relative amount of a product being formed. Moreover, as shown in Section S4 of the Supporting Information, when 10% of the cVMS had reacted, the total signal of the oxidation products detected normalized to the mixing ratio of cVMS reacted, varied by <20% across the experimental conditions (Figures S3 and S4) even when there are significant changes in the compositions of the ions formed (particularly for D3). This finding suggests that variations in instrument response to different products were relatively small and that we were capturing most of the oxidation products.
Given the absence of authentic calibration standards, we elect to report the product abundance in terms of the normalized signal yield. This quantity is calculated as the signal of an individual product divided by the sum of the total product signals and would be approximately equal to the molar yield if we had equal sensitivity to every oxidation product. Figure 3 shows the normalized signal yields of each of the products for the different cVMS studied.

Figure 3

Figure 3. Signal yield of different cVMS oxidation products when RO2 was most likely to react with (a, d, g) NO, (b, e, h) HO2, or (c, f, i) when unimolecular reactions are favored, divided by the signal of the siloxane lost at that point. Panels (a–c) are for D3, (d–f) for D4, and (g–i) for D5 oxidation. The signals for siloxanediol, ether hydroperoxide, and ether nitrate are multiplied by 10 and striped for visual clarity.

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The oxidation mechanism of D3 has been studied with theoretical calculations and will be discussed first. The most intense product signal formed in the D3 experiments under conditions that favored unimolecular reactions, as seen in Figure 3c, was (C5H14O6Si3)H+. We assign this formula as a difunctional product: siloxanol and formate ester. In the RO2 + NO and RO2 + HO2 conditions, the product with the most intense signal was (C6H16O5Si3)H+, which corresponds to the formate ester product. The next most intense signal in the experiments was (C5H16O4Si3)H+, which we identified as siloxanol. We attribute (C6H18O6Si3)H+ to an ether hydroperoxide and (C6H18O5Si3)H+ to the hydroperoxide. The hydroperoxide was identified by Sommerlade et al., (8) though the ether hydroperoxide was not. No nitrogen-containing peaks were identified when D3 was oxidized in high NOx (∼100 ppbv NOx) conditions. This observation may suggest that the formation of organic nitrates was unfavorable. Alternatively, organic nitrates may be detected with low efficiency because of either fragmentation or low ionization efficiency. The presence of and instrument response to organic nitrates will be discussed further in the context of D4 and D5 oxidation.
The assignment of (C6H16O5Si3)H+ as a formate ester rather than a carboxylic acid was informed by previous experimental and theoretical work. (4,37) Additionally, we oxidized fully deuterated hexamethyldisiloxane (D18L2) with Cl atoms in a similar experiment to Atkinson et al. (4) and observed (C6D16O3Si2)H+ as the major ion. No detectable signal existed for ions with exchangeable hydrogens. The 1H in the formula originated from the proton-transfer reaction leading to ionization in the CIMS. If the product were instead a carboxylic acid, we would expect to observe (C6D15HO3Si)H+. Further details regarding this experiment are in Section S3 of the Supporting Information.
The product distribution in the D4 and D5 oxidation experiments differed substantially from the D3 experiments. Monofunctional products, siloxanol and to a lesser extent formate ester, dominated the product signals. Additionally, the signal yields of siloxanol and formate ester showed little sensitivity to the RO2 lifetime and reaction partner. In the D4 and D5 experiments, signals for (C8H23O7Si4N)H+ and (C10H29O8Si5N)H+, attributed to the organic nitrates, and (C8H23O8Si4N)H+ and (C10H29O9Si5N)H+, attributed to the ether organic nitrates, were detected. However, these compounds appeared to be formed with significantly smaller yields than the siloxanol and formate ester products. Only the ether nitrate yields are shown in Figure 3. Proton-transfer ionization can lead to fragmentation of organic nitrates through nitric acid loss. As a result, instrument response for the protonated organic nitrate can be low with high limits of detection. We detected the nitric acid loss fragment, (C10H28O6Si5)H+, for the ether nitrate product at ∼25% of the ether nitrate signal for D5, which suggests that some fragmentation did occur. We did not observe ions consistent with either water- or NO2-loss fragmentation pathways or charge-transfer products. Although the reagent ion may be ineffective at detecting organic nitrates, we expect that the yield of organic nitrates in these experiments was small given that both the organic nitrate signal and the total product signals were similar under the different RO2 fates investigated, which ranged from 99% of RO2 reacting with NO in the RO2 + NO conditions to ∼5% reacting with NO in the RO2 + HO2 experiments. More discussion on the instrument response is in Section S4 of the Supporting Information. Additionally, previous work suggests that organic nitrate yields are low. Organic nitrates have not been detected before this work, even when cVMS was oxidized under high NO conditions. (4,18) Carter et al. (53) performed environmental chamber experiments investigating how siloxanes alter ozone formation in high NOx conditions. Through a model-measurement comparison, they determined that their results were inconsistent with organic nitrate formation; however, siloxane oxidation products were not measured.
In works that identified products with an increased number of silicon atoms, RO2 + RO2 reactions or reactions within the condensed phase likely occurred. In our experiments, RO2 + RO2 reactions were minimal under all conditions (<1%) and no aerosol was formed; thus, we did not expect to observe products with an increased number of silicon atoms.

3.2. Discussion of the Reaction Mechanism

In most previous studies, the major oxidation products have typically been attributed to siloxanol and formate ester, though formate ester has only been detected experimentally twice. (4,5) The siloxanol has been suggested to be a hydrolysis product of formate ester. (4) Other works have suggested unusual reactions that could potentially explain the formation of the siloxanol and the formate ester products; (8,36,37) however, a lack of controlled and varied RO2 fates in these experiments prevents a comprehensive assessment of the mechanism. In this work, we varied the lifetime and reaction partner of the RO2 radical to gain insight into these reactions. Additionally, our real-time measurement technique has the advantage that conversion of formate ester to siloxanol on surfaces will be minimized. In this section, we discuss the potential cVMS oxidation mechanism as compiled from previous experimental and theoretical studies. (4,8,18,36,37) The schemes presented in this section are based on these previously proposed reactions as well as typical oxidation pathways in the atmosphere. In the following section, we implement the reaction mechanism in a zero-dimensional kinetic box model and compare it to our measurements.

3.2.1. RO2 + NO

As shown in Scheme 1, cVMS oxidation is initiated by OH or Cl abstracting a hydrogen from one of the methyl groups (R1). The resulting alkyl radical quickly reacts with O2 forming RO2 (R2). When the NO/HO2 ratio is greater than 2, such as urban locations, the primary reaction of RO2 is with NO. (46) This reaction has two channels, one that forms an alkoxy radical (RO) and NO2 (R4), and one that forms an organic nitrate (-ONO2; not shown). Based on our measurements and as discussed in Section 3.1, we suggest that the organic nitrate channel is minor and that reaction with NO mainly proceeds through reaction R4. Possible fates for the R3SiCH2O• radical include reaction with O2, decomposition, and isomerization. Isomerization reactions were expected to dominate as reactions with O2 to form R3SiCHO (not shown) and decomposition have been calculated to be at least 10 orders of magnitude slower than isomerization. (36) Possible isomerization pathways include a hydrogen shift from an adjacent methyl group or a unique rearrangement (R6) to form a carbon-based radical. R6 is inaccessible to carbon-based volatile organic compounds (VOCs). The calculated lifetime (from theoretically determined rate constants) for R3SiCH2O• with respect to R6 was ∼6 × 10–12 s for D3, and this isomerization is expected to be the major RO fate. (36) The most intense hydroperoxide and nitrate signals detected during oxidation in this work are consistent with the formation of ether hydroperoxide and ether nitrate, consistent with R6 occurring rapidly. Note that the peroxy radicals formed from R5 followed by R2′ and R6 followed by R2″ are isomers. However, based on theoretical calculations, (36) R5 is expected to be slow relative to R6, and thus, further reactions and products from R2′ were not considered. The carbon-centered radical formed by R6, R3SiOCH2•, will subsequently add O2 (R2″) to form a new ether peroxy radical. (36,37) This radical can react with NO/HO2 again (R4′) to form an ether alkoxy radical. The formate ester forms when the ether alkoxy radical reacts with O2 (R7). The ether peroxy radical can also react with HO2 to form ether hydroperoxide (R3′) or with NO to form ether nitrate (not shown).

Scheme 1

Scheme 1. Potential Reactions of cVMS in High NOx/HO2 Conditions
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3.2.2. RO2 + HO2

The main channel of the RO2 reaction with HO2 typically results in the formation of a hydroperoxide (R3). (54,55) In our experiments, the observed intensity of this expected product was low. While this may be due to a low instrument response to hydroperoxides when using proton-transfer ionization, the fact that similar products in similar yields are formed under both RO2 + NO (<1% of RO2 reacting with HO2) and RO2 + HO2 (>90% of RO2 reacting with HO2) conditions leads us to suggest that RO2 radicals can react with HO2 to form the RO radical, OH, and O2 (R4). The formation of the RO radical from the reaction of RO2 with HO2 has been observed previously in oxygenated organic molecules, leading to a radical recycling mechanism. (55,56) Once R6 occurs, the reactions are the same in the NO case with the addition of R3′ to form ether hydroperoxide.

3.2.3. Conditions Favoring Unimolecular Reactions

Under conditions of low HO2 or NO concentrations, the lifetime of the RO2 radical with respect to bimolecular reactions is long and unimolecular reactions may become important. Possible unimolecular reactions of cVMS-derived RO2 identified through quantum chemical calculations include 1,3-, 1,5-, and 1,7-hydrogen shifts to form QOOH with a propagated radical. However, the more favorable (lower energy barrier) isomerization was predicted to be the unusual pathway shown in R8 (Scheme 2). (36)

Scheme 2

Scheme 2. Potential Reactions of cVMS in Low NOx/HO2 Condition
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After undergoing R8, the resulting radical is expected to quickly proceed through R9, generating a siloxy radical and formaldehyde. We suggest that the siloxy radical can undergo reaction R11 to make the siloxanol product after reaction with H2O or HO2. The reaction of RO with H2O (R11) has been proposed to occur on silica surfaces, (57) while Carter et al. (53) previously proposed the reaction of RO with HO2 to make siloxanol. Fu et al. (36) suggested RO could undergo a reaction and abstract hydrogen from a methyl group on a nearby silicon atom (R10, 1,5 H-shift) or on the same silicon atom (1,3 H-shift), which is less favorable than the 1,5 H-shift. Both reactions propagate the radical to create another functional group on the cVMS molecule, and our measurement technique cannot distinguish which methyl groups have been modified; therefore, we did not consider this difference. Another unusual isomerization of the RO2 radical (R8a) that was proposed initially by Atkinson et al. (4) and used to explain observation of the formate ester product was previously investigated with quantum chemical calculations and determined to be energetically unfavorable. (36,37)

3.3. Kinetic Model Using the Proposed Mechanism

We implement the reactions in Schemes 1 and 2, with rate constants informed from previously published quantum chemical calculations, (36) in a kinetic model (KinSim in Igor Pro) to investigate if the proposed mechanism can reproduce the changing intensity of different products under different RO2 fates. The full mechanisms with rate constants for the three VMS studies are presented in Section S2 of the Supporting Information. Reactions to form organic nitrate with NO were excluded from the model. RO2 + RO2 reactions were minimal in all experiments (<0.5% of the RO2 reactions were with another RO2), and therefore were not considered in the mechanism. In the model, we held bimolecular reaction rates (RO2 + HO2 and RO2 + NO) constant. As reaction R8a was previously determined to not be energetically favorable, (36) this reaction was not used. We included the oxidation of first-generation products with rate constants equal to the parent cVMS. As there is uncertainty in molar yields due to a lack of calibration standards, we focus on comparing the relative changes in signal yields as a function of RO2 fate. Figure 4 shows the signal yields of the siloxanol and formate ester products from our experiments and the kinetic model at the point 10% of the cVMS was oxidized, normalized to the signal yield in the RO2 + NO conditions. By observing relative changes between the conditions, the absolute calibration of the oxidation products should not affect the interpretation of the data.

Figure 4

Figure 4. Signal yields of formate ester (a–c) and siloxanol (d–f) for all three cVMS from the experimental results and kinetic modeling using the mechanism presented, normalized to the results from the RO2 + NO conditions. Panels (a) and (d) are for D3 oxidation, (b) and (e) are for D4 oxidation, and (c) and (f) for D5. Note the log axis on siloxanol results. Only the rate constants for reactions with OH/Cl and wall loss constants were changed between cVMS. In the model results that had a source of siloxanol from the RO2 + NO reaction, 5% of the reaction product made siloxanol directly, and the other 95% made RO.

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In our experiments with D3, we detect that the signal corresponding to the formate ester decreases with an increasing RO2 lifetime, which is consistent with formate ester being formed after RO2 reacts with NO/HO2. Consequently, the model captures this decrease albeit the measured decrease in formate ester yield with increasing RO2 lifetime is greater than the decrease predicted by the model (Figure 4). For D5, we observed an increase in formate ester with increasing RO2 lifetime, which contrasts with the model prediction of a decreasing trend. The model also fails to capture the trends in the relative siloxanol yield for all three cVMS species; the model predicts an increase in siloxanol yield with increasing RO2 lifetime, while the measurements suggest that the siloxanol yield is essentially independent of RO2 lifetime and fate. In fact, the model predicts negligible siloxanol yield under RO2 + NO conditions (Figure S5), which causes the modeled siloxanol yield normalized to RO2 + NO conditions to change by orders of magnitude as the RO2 lifetime increases. The model also predicts significantly less siloxanol (∼100 × less) formation than in our experiments, if we assume that we detect all of the products with equal sensitivity. Siloxanol formation has previously been attributed to hydrolysis of formate ester prior investigations of siloxane chemistry. (4,7,18) Our high time-resolution measurements of formate ester and siloxanol are inconsistent with this hypothesis since we observed the simultaneous formation of both formate ester and siloxanol.
The model-measurement gap could potentially be explained by an unknown formation mechanism of siloxanol in high NO/HO2 conditions. As a thought experiment, if a branching ratio is added to produce 5% of siloxanol and 95% of the RO after the reaction of RO2 with NO, then the amount of siloxanol formed between experiments is slightly closer to the experimental results, though it underestimates the siloxanol formation in the RO2 + HO2 conditions (Figure 4). This formation of siloxanol could be achieved through a process such as the decomposition of a chemically activated alkoxy radical. Another possibility would be to have a faster rate constant for the isomerization in R8, as the siloxy radical formed after that isomerization could be a source of siloxanol. However, for this reaction to be important in all conditions (as the siloxanol is the largest product in high NOx experiments with D4 and D5), the rate constant would need to be increased by multiple orders of magnitude to compete with the RO2 + NO reaction. Overall, the siloxanol formation mechanism remains unclear and more investigation to better understand why RO2 fates do not significantly affect the products formed from D4 and D5 oxidation is required.
The model also fails to accurately capture the evolution of the largest product in the conditions that favor unimolecular reactions from D3 oxidation: the formate ester and siloxanol difunctional product (not shown in Figure 4). Because this difunctional product was the largest signal during the conditions that favor unimolecular reactions, it was anticipated that D3 R3SiO• radicals can readily undergo the auto-oxidation reaction shown in R10 after being formed from R8 and R9. Since siloxanol was formed in R10, the next steps need to preferentially make formate ester over another siloxanol. Therefore, R4″ needs to dominate over R8′, though to form the initial siloxanol, R8 needs to dominate over R4. This pathway requires that the ratio of the R4:R8 rate constants is less than R4″:R8′, which stands in contrast to theoretical calculations, suggesting that the isomerization rate increases as the molecules become more functionalized. (36) However, we did not detect any evidence in these experiments of the siloxanetriol product, which would likely be formed if the isomerization reactions R8 and R8′ both dominated over bimolecular reactions, as the next RO2 isomerization branching would likely not be different. Another possible explanation is that concentrations of NO or HO2 were higher later in the experiment, pushing the mechanism down R4″. The model, however, indicates that NO and HO2 concentrations do not change enough to alter the product distribution.
Overall, it is evident that the oxidation mechanism is unable to replicate our experimental results, particularly for D4 and D5. The finding that D3 oxidation produces different products than D4 and D5 suggests that results, either from laboratory-based experiments or theoretical calculations, from smaller VMS and Si-containing molecules (i.e., D3, hexamethyldisiloxane [L2], and tetramethylsilane) may not hold for larger Si compounds. In particular, the lack of sensitivity on the oxidation product yields on RO2 fate for D4 and D5 is unusual. As siloxanol was the most abundant product for D4 and D5 under all conditions, our results suggest that there is more unique chemistry occurring that requires further investigation.

3.4. Possible Fates of the First-Generation Products

As cVMS are globally distributed due to their long atmospheric lifetimes, their oxidation products will also be globally distributed. Previous research has focused on the parent compounds, though the oxidation products should not be discounted. Here, we use various structure–activity relationships (SAR) to investigate possible loss pathways for the main cVMS oxidation products we measured and discussed (siloxanol, formate ester, multisubstituted siloxanols/formate esters, hydroperoxides, and organic nitrates). The estimated values of vapor pressure, water solubility, and Henry’s solubility (M atm–1) constants of all of the products are presented in Table S6 of the Supporting Information. (58) Note that the Henry’s law constant (atm M–1) is the inverse of the Henry solubility.
Using the estimated vapor pressures, and assuming that absorption into organic aerosol is the main gas-particle partitioning method for these compounds, less than 1% of the least volatile oxidation product (the D5 siloxanediol) mass would partition to aerosol at a moderate organic aerosol loading of 10 μg m–3. (59) This value was determined using the vapor pressure estimated by MPBPWIN, which predicts vapor pressures about 1 order of magnitude lower than TEST, thus giving an upper limit to aerosol partitioning. It has been determined that cVMS heterogeneous reactions with components of mineral dust aerosol can lead to significant removal of the cVMS and its oxidation products, but due to the low typical dust loadings, the total loss to dust is expected to be minimal. (32)
The oxidation products have estimated Henry’s solubility constants that can be up to ∼8 orders of magnitude higher (higher partitioning into the aqueous phase) than the parent cVMS. Out of the various methods compared, HenryWIN gave the largest estimated Henry’s solubility coefficient of ∼105 M atm–1 for the D3 siloxanediol. Even using this Henry’s solubility constant, at most 30% of the siloxanediol will partition to cloud droplets, while there is no significant partitioning into an aqueous aerosol, as shown in Figure 5, adapted from Daumit et al. (60) We note that this estimate is uncertain as Henry’s solubility and law constants measured in the laboratory vary by orders of magnitude between studies for the parent cVMS, (61) and the Henry’s law constants of the different oxidation products have not been measured. (58,62,63) However, as we use the most extreme values from the structure–activity relationships, we assume these to be the upper limits for partitioning and thus conclude that absorptive and aqueous partitioning will have minor impacts on the mixing ratios of the oxidation products.

Figure 5

Figure 5. Estimated Henry’s solubility, estimated with HenryWIN v3.21, for D3 and the siloxanol oxidation products (dashed lines). The mass concentration labels on the solid lines correspond to the amount of liquid water in clouds or ambient aerosol. Only siloxanol products are shown as the hydroxyl groups change the Henry’s solubility most significantly. Larger cVMS oxidation products are estimated to have lower Henry’s solubility than D3. All values are listed in Table S6 of the Supporting Information. Adapted with permission from Daumit, K. E.; Carrasquillo, A. J.; Hunter, J. F.; Kroll, J. H. Laboratory Studies of the Aqueous-Phase Oxidation of Polyols: Submicron Particles vs Bulk Aqueous Solution. Atmos. Chem. Phys. 2014, 14 (19), 10773–10784. https://doi.org/10.5194/acp-14-10773-2014.

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The oxidized products may also be transformed in the atmosphere via oxidation. Atkinson et al. (4) measured the rate constants of tetramethylsilane and trimethylsiloxanol reactions with Cl atoms and OH radicals, which showed that the rate constant for the siloxanol oxidation reactions were 2 times faster than the parent compounds for Cl and 10 times faster for OH. However, it is uncertain how the rate constants will change with larger cVMS. Using the Atmospheric Oxidation Program for Microsoft Windows (AOPWIN) in EPIWIN with the adjustments to Si-containing group contributions suggested by Alton and Browne (2020), (5) the rate constant for reaction with OH is estimated to be approximately a factor of 5 times faster for D3OH compared to D3, and 3 times faster for D5OH compared to D5. This lowers the lifetime of D3OH and D5OH to approximately 2 days for both of the products, compared to 11 and 4.4 days. (5) Based on this work, it is probable that, like the parent VMS compounds, these first-generation oxidation products will have atmospheric lifetimes of days, and thus, it is necessary to better understand multiple-generation oxidation products and their potential chemistry and deposition to completely understand cVMS environmental fates.

4. Conclusions

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As ∼90% of cVMS emitted into the environment partitions into the atmosphere, (3) understanding the atmospheric degradation of these compounds is critical for understanding their environmental impacts. In this work, we oxidized three cVMS under conditions of different RO2 fates and measured the oxidation products to gain insight into the cVMS oxidation mechanism. We observed that the main oxidation product for D4 and D5 is siloxanol, regardless of the fate of the peroxy radical. As D4 and D5 are the most abundant cVMS in the atmosphere, we suggest that in chemical transport models cVMS oxidation products can be adequately represented as siloxanol, similar to previous representations of this chemistry. (30) Due to the high vapor pressure and low water solubility of the cVMS and oxidation products, it is predicted that not only is the parent cVMS globally distributed, (13,64) the oxidation products are likely also globally present, requiring multiple generations of oxidation before significant removal will occur. Because the oxidation products are also likely to be long lived in the atmosphere, more measurements of the oxidation products in the atmosphere and environmental matrices are necessary to better understand the environmental processing of these anthropogenic chemicals.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenvironau.1c00043.

  • Detailed information of the peak fitting in TOFWARE, KinSim mechanism and inputs, isotopically labeled D18L2 oxidation experiment, instrument response in different conditions, and the estimated physical parameters of cVMS and the oxidation products (PDF)

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Author Information

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  • Corresponding Author
    • Eleanor C. Browne - Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States;  Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States;  Orcidhttps://orcid.org/0000-0002-8076-9455;  Email: [email protected]
  • Author
    • Mitchell W. Alton - Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States;  Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States;  Orcidhttps://orcid.org/0000-0002-7119-3706
  • Author Contributions

    M.W.A. and E.C.B. designed the experiments. M.W.A. performed all of the experiments that contributed to this work. The analysis of the data was performed mainly by M.W.A. with support and guidance from E.C.B. The manuscript was written through the contributions of both authors. Both authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

    The experimental data is publicly available through the Index of Chamber Atmospheric Research in the United States website (https://icarus.ucdavis.edu/experimentset/242, https://icarus.ucdavis.edu/experimentset/243, https://icarus.ucdavis.edu/experimentset/244).


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Acknowledgments

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This research was supported by the National Science Foundation under Grant CHE-1808606. Additional funding to support MWA was provided by the Cooperative Institute for Research in Environmental Sciences Graduate Student Research Fellowship Grant.

References

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    International Journal of Chemical Kinetics (2015), 47 (7), 420-428CODEN: IJCKBO; ISSN:0538-8066. (John Wiley & Sons, Inc.)
    Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile Me siloxanes (cVMS) from air. We present new measurements of second-order rate consts. for reactions of the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) with OH detd. at temps. between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a ref. substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry anal. When extrapolated to 298 K, our measured reaction rate consts. of D4 and D5 with the OH radical are 1.9 × 10-12 (95% confidence interval (CI): (1.7-2.2) × 10-12) and 2.6 × 10-12 (CI: (2.3-2.9) × 10-12) cm3 mol.-1 s-1, resp., which are 1.9× and 1.7× faster than previous measurements. Our measured rate const. for D6 is 2.8 × 10-12 (CI: (2.5-3.2) × 10-12) cm3 mol.-1 s-1 and to our knowledge there are no comparable lab. measurements in the literature. Reaction rates for D5 were 33% higher than for D4 (CI: 30-37%), whereas the rates for D6 were only 8% higher than for D5 (CI: 5-10%). The activation energies of the reactions of D4, D5, and D6 with OH were not statistically different and had a value of 4300 ± 2800 J/mol.
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  10. 10
    Bernard, F.; Papanastasiou, D. K.; Papadimitriou, V. C.; Burkholder, J. B. Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes. J. Phys. Chem. A 2018, 122, 4252– 4264,  DOI: 10.1021/acs.jpca.8b01908
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    10
    Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes
    Bernard, Francois; Papanastasiou, Dimitrios K.; Papadimitriou, Vassileios C.; Burkholder, James B.
    Journal of Physical Chemistry A (2018), 122 (17), 4252-4264CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Permethylsiloxanes are emitted into the atm. during prodn. and use as personal care products, lubricants, and cleaning agents. The predominate atm. loss process for permethylsiloxanes is expected to be via gas-phase reaction with the OH radical. In this study, rate coeffs., k(T), for the OH radical gas-phase reaction with the two simplest linear and cyclic permethylsiloxanes were measured using a pulsed laser photolysis-laser induced fluorescence technique over the temp. range of 240-370 K and a relative rate method at 294 K: hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; hexamethylcyclotrisiloxane ([-Si(CH3)2O-]3, D3), k3; and octamethylcyclotetrasiloxane ([-Si(CH3)2O-]4, D4), k4. The obtained k(294 K) values and temp.-dependence expressions for the 240-370 K temp. range are (cm3 mol.-1 s-1, 2σ abs. uncertainties): k1(294 K) = (1.28 ± 0.08) × 10-12, k1(T) = (1.87 ± 0.18) × 10-11 exp(-(791 ± 27)/T); k2(294 K) = (1.72 ± 0.10) × 10-12, k2(T) = 1.96 × 10-13 (T/298)4.34 exp(657/T); k3(294 K) = (0.82 ± 0.05) × 10-12, k3(T) = (1.29 ± 0.19) × 10-11 exp(-(805 ± 43)/T); and k4(294 K) = (1.12 ± 0.10) × 10-12, k4(T) = (1.80 ± 0.26) × 10-11 exp(-(816 ± 43)/T). The cyclic mols. were found to be less reactive than the analogous linear mol. with the same no. of -CH3 groups, while the linear and cyclic permethylsiloxane reactivity both increase with the increasing no. of CH3- groups. The present results are compared with previous rate coeff. detns. where available. The permethylsiloxanes included in this study are atmospherically short-lived compds. with estd. atm. lifetimes of 11, 8, 17, and 13 days, resp.
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  11. 11
    Kim, J.; Xu, S. Quantitative Structure-Reactivity Relationships of Hydroxyl Radical Rate Constants for Linear and Cyclic Volatile Methylsiloxanes. Environ. Toxicol. Chem. 2017, 36, 3240– 3245,  DOI: 10.1002/etc.3914
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    11
    Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes
    Kim, Jaeshin; Xu, Shihe
    Environmental Toxicology and Chemistry (2017), 36 (12), 3240-3245CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
    An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial to det. their environmental persistence and concns. Although oxidn. by atm. OH- is considered a major airborne VMS degrdn. mechanism, the existing bimol. rate consts. with OH- measured and modeled for any given VMS compd. varied greatly, depending on the approach used to generate the data. This work measured OH- reaction rate consts. for four cyclic and four linear VMS, based on a relative rate method using a newly designed atm. chamber, and established structure-reactivity relationships for the kinetics of same. In the past, VMS reaction rate consts. were generally recognized to increase with the no. of Me groups/mol., the only differential factor in existing models; however, new measurements indicated that mol. structure should also be considered to predict reaction rates. Better empirical models were developed by simple and multiple linear regressions of measured values from this work and the literature. A high correlation existed for reaction rates with the no. of the Me group attached at two distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or two mol. descriptors which were directly related to VMS size, which, in turn, depended on the no. of Me groups and linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;9999:1-6. © 2017 SETAC.
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  12. 12
    Tuazon, E. C.; Aschmann, S. M.; Atkinson, R. Atmospheric Degradation of Volatile Methyl-Silicon Compounds. Environ. Sci. Technol. 2000, 34, 1970– 1976,  DOI: 10.1021/es9910053
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    12
    Atmospheric degradation of volatile methyl-silicon compounds
    Tuazon, Ernesto C.; Aschmann, Sara M.; Atkinson, Roger
    Environmental Science and Technology (2000), 34 (10), 1970-1976CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The kinetics of the gas-phase reactions of dimethylsilanediol, trimethylsilanol, and tetramethylsilane (degrdn. products of poly(dimethylsiloxane)s (PDMS)) with the OH radical were measured using a relative rate method which employed the N2H4 + O3 reaction as a nonphotolytic source of OH radicals, with anal. by FT-IR spectroscopy in a 5870 L chamber. The measured values of the OH radical reaction rate consts. (cm3 mol.-1 s-1) at 298 ± 2 K are as follows: dimethylsilanediol 8.1 × 10-13; trimethylsilanol 7.2 × 10-13; and tetramethylsilane 8.5 × 10-13. These values lead to an est. of tropospheric lifetimes with respect to reaction with the OH radical of ∼15 days for these organosilicon compds. FT-IR spectroscopy and atm. pressure ionization mass spectrometry (API-MS) were employed to analyze the products of OH radical- and Cl atom-initiated photooxidns. of dimethylsilanediol and trimethylsilanol. IR signatures of the probable formate ester intermediate products from both silanols were detected. API-MS analyses indicated the formation of methylsilanetriol from dimethylsilanediol, of both dimethylsilanediol and methylsilanetriol from trimethylsilanol, and of the corresponding intermediate formate esters. Possible reaction mechanisms are discussed.
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  13. 13
    Genualdi, S.; Harner, T.; Cheng, Y.; MacLeod, M.; Hansen, K. M.; Van Egmond, R.; Shoeib, M.; Lee, S. C. Global Distribution of Linear and Cyclic Volatile Methyl Siloxanes in Air. Environ. Sci. Technol. 2011, 45, 3349– 3354,  DOI: 10.1021/es200301j
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    13
    Global Distribution of Linear and Cyclic Volatile Methyl Siloxanes in Air
    Genualdi, Susie; Harner, Tom; Cheng, Yu; MacLeod, Matthew; Hansen, Kaj M.; van Egmond, Roger; Shoeib, Mahiba; Lee, Sum Chi
    Environmental Science & Technology (2011), 45 (8), 3349-3354CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The global distribution of linear and cyclic volatile Me siloxanes (VMS) was examd. at 20 sites worldwide, including 5 sites in the Arctic, using sorbent-impregnated polyurethane foam (SIP) disk passive air samplers. Cyclic VMS are currently being considered for regulation because they are high prodn. vol. chems. which are potentially persistent, bioaccumulative, and toxic. Linear and cyclic VMS (L3, L4, L5, D3, D4, D5, D6) were analyzed for in air at all urban, background, and Arctic sites. D3 and D4 concns. were significantly correlated, as were D5 and D6, suggesting different sources for these 2 pair of compds. Elevated D3 and D4 concns. on the west North America coast and at high elevation sites suggested these sites are affected by trans-Pacific transport; D5 and D6 had elevated concns. in urban areas, most likely due to personal care product use. Measured D5 concns. were compared to modeled concns. generated by the Danish Eulerian Hemispheric Model (DEHM) and the Berkeley-Trent Global Contaminant Fate Model (BETR Global). Correlation coeffs. (r) between measured and modeled results were 0.73 and 0.58 for the DEHM and BETR models, resp. Agreement between measurements and models indicated global atm. D5 sources, transport pathways, and sinks are fairly well understood.
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  14. 14
    Xu, S.; Warner, N.; Bohlin-Nizzetto, P.; Durham, J.; McNett, D. Long-Range Transport Potential and Atmospheric Persistence of Cyclic Volatile Methylsiloxanes Based on Global Measurements. Chemosphere 2019, 228, 460– 468,  DOI: 10.1016/j.chemosphere.2019.04.130
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    14
    Long-range transport potential and atmospheric persistence of cyclic volatile methylsiloxanes based on global measurements
    Xu, Shihe; Warner, Nicholas; Bohlin-Nizzetto, Pernilla; Durham, Jeremy; McNett, Debra
    Chemosphere (2019), 228 (), 460-468CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    This study investigates persistence (P) and long-range transport potential (LRTP) of cyclic volatile methylsiloxanes (cVMS) based on the field measurements in the Northern Hemisphere. The field data consisted of published outdoor air concns. of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) at urban, suburban, rural and remote locations excluding the point sources. Three major trends were obsd. First, D4 and D6 concns. were correlated with measured concns. for D5 at the same times and locations in the majority of the datasets, reflecting the common sources and similar removal mechanism(s) for these compds. Second, as the sampling sites changed from the source to remote locations along a south-to-north transect, av. cVMS concns. in air decreased in an exponential manner. The empirical characteristic travel distances (eCTD) extd. from these spatial patterns were smaller than model estd. values and differed in order among individual compds. (D4 ∼ D5 < D6). Finally, D5/D6 concn. ratios were also found to decrease exponentially along the same spatial gradient, contrary to model predictions of an increase based on current knowledge of mechanisms controlling atm. cVMS degrdn. These findings suggest that there may be addnl. removal process(es) for airborne cVMS, currently not accounted for, that requires further elucidation.
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  15. 15
    Warner, N. A.; Evenset, A.; Christensen, G.; Gabrielsen, G. W.; Borgä, K.; Leknes, H. Volatile Siloxanes in the European Arctic: Assessment of Sources and Spatial Distribution. Environ. Sci. Technol. 2010, 44, 7705– 7710,  DOI: 10.1021/es101617k
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    15
    Volatile Siloxanes in the European Arctic: Assessment of Sources and Spatial Distribution
    Warner, Nicholas A.; Evenset, Anita; Christensen, Guttorm; Gabrielsen, Geir W.; Borga, Katrine; Leknes, Henriette
    Environmental Science & Technology (2010), 44 (19), 7705-7710CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The purpose of this study was to investigate presence and potential accumulation of cyclic volatile Me siloxanes (cVMS) in the Arctic environment. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) were analyzed in sediment, zooplankton, Atlantic cod (Gadus morhua), shorthorn sculpin (Myoxocephalus scorpius), and bearded seal (Erignathus barbatus) collected from the Svalbard archipelago within the European Arctic in July 2009. Highest levels were found for D5 in fish collected from Adventfjorden, with av. concns. of 176 and 531 ng/g lipid in Atlantic cod and shorthorn sculpin, resp. Decreasing concn. of D5 in sediment collected away from waste water outlet in Adventfjorden indicates that the local settlement of Longyearbyen is a point source to the local aquatic environment. Median biota sediment accumulation factors (BSAFs) calcd. for D5 in Adventfjorden were 2.1 and 1.5 for Atlantic cod and shorthorn sculpin, resp. Biota concns. of D5 were lower or below detection limits in remote and sparsely populated regions (Kongsfjorden and Liefdefjorden) compared to Adventfjorden. The levels of cVMS were found to be low or below detection limits in bearded seal blubber and indicate a low risk for cVMS accumulation within mammals. Accumulation of cVMS in fish appears to be influenced by local exposure from human settlements within the Arctic.
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    Brooke, D.; Crookes, M.; Gray, D.; Robertson, S. Environmental Risk Assessment Report: Decamethylcyclopentasiloxane; Environment Agency, 2009.
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    Brooke, D. N.; Brooke, M. J.; Gray, D.; Robertson, S.; Crookes, M.; Gray, D.; Robertson, S. Environmental Risk Assessment Report: Octamethylcyclotetrasiloxane; Environment Agency, 2009.
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    Allen, R. B.; Annelin, R. B.; Atkinson, R.; Carpenter, J. C.; Carter, W. L. P.; Chandra, G.; Fendinger, N. J.; Gerhards, R.; Grigoras, S.; Hatcher, J. A.; Hobson, J. F.; Kochs, P.; Lehmann, R. G.; Maxim, L. D.; Mazzoni, S. M.; Mihaich, E. M.; Miyakawa, Y.; Pohl, E. R.; Powell, D. E.; Roy, S.; Sawano, T.; Slater, G. S.; Spivack, J. L.; Stevens, C.; Wischer, D. Organosilicon Materials. In The Handbook of Environmental Chemistry; Chandra, G., Ed.; Springer: Berlin, Heidelberg, 1997; Vol. 3.
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    European Chemicals Agency. Recommendation of the European Chemicals Agency of 20 December 2011 for the Inclusion of Substances in Annex XIV to REACH (List of Substances Subject to Authorisation) of Regulation (EC) No 1907/2006; European Chemicals Agency, 2011; Vol. 1, pp 1– 7.
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    United Kingdom Health & Safety Executive. Annex XV Restriction Report Proposal for a Restriction; United Kingdom Health & Safety Executive, 2015; pp 1– 89.
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    There is no corresponding record for this reference.
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    European Chemicals Agency. Recommendation of the European Chemicals Agency of 14 April 2021 for the Inclusion of Substances in Annex XIV to REACH (List of Substances Subject to Authorisation); EPA, 2021; Vol. 1, pp 1– 7.
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    There is no corresponding record for this reference.
  22. 22
    Wu, Y.; Johnston, M. V. Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes. J. Am. Soc. Mass Spectrom. 2016, 27, 402– 409,  DOI: 10.1007/s13361-015-1300-1
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    Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes
    Wu, Yue; Johnston, Murray V.
    Journal of the American Society for Mass Spectrometry (2016), 27 (3), 402-409CODEN: JAMSEF; ISSN:1044-0305. (Springer)
    Cyclic volatile methylsiloxanes (cVMS) have been identified as important gas-phase atm. pollutants, but knowledge of the mol. compn. of secondary aerosol derived from cVMS oxidn. is incomplete. This work characterized the chem. compn. of secondary aerosols produced from OH--initiated oxidn. of decamethylcyclopentasiloxane (D5, C10H30O5Si5) by high performance mass spectrometry. Electro-spray ionization mass spectrometry (ESI-MS) showed a large no. of monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) oxidn. products. High resoln. and tandem mass spectrometry showed that oxidn. leads mainly to a CH3 group substitution by OH or CH2OH; a single mol. can undergo many CH3 group substitutions. Dimers also exhibit OH and CH2OH substitutions and can be linked by O, CH2, and CH2CH2 groups. Gas chromatog.-mass spectrometry confirmed ESI-MS results. D4 (C8H24O4Si4) oxidn. exhibited similar substitutions and oligomerizations to D5, though the degree of oxidn. was greater under the same conditions, and there was direct evidence for peroxy group (CH2OOH) formation in addn. to OH and CH2OH.
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  23. 23
    Yucuis, R. A.; Stanier, C. O.; Hornbuckle, K. C. Cyclic Siloxanes in Air, Including Identification of High Levels in Chicago and Distinct Diurnal Variation. Chemosphere 2013, 92, 905– 910,  DOI: 10.1016/j.chemosphere.2013.02.051
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    23
    Cyclic siloxanes in air, including identification of high levels in Chicago and distinct diurnal variation
    Yucuis, Rachel A.; Stanier, Charles O.; Hornbuckle, Keri C.
    Chemosphere (2013), 92 (8), 905-910CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    The organosilicon compds. octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are high prodn. vol. chems. that are widely used in household goods and personal care products. Due to their prevalence and chem. characteristics, cyclic siloxanes are being assessed as possible persistent org. pollutants. D4, D5, and D6 were measured in indoor and outdoor air to quantify and compare siloxane concns. and compd. ratios depending on location type. Indoor air samples had a median concn. of 2200 ng m-3 for the sum of D4, D5, and D6. Outdoor sampling locations included downtown Chicago, Cedar Rapids, IA, and West Branch, IA, and had median sum siloxane levels of 280, 73, and 29 ng m-3 resp. A diurnal trend is apparent in the samples taken in downtown Chicago. Nighttime samples had a median 2.7 times higher on av. than daytime samples, which is due, in part, to the fluctuations of the planetary boundary layer. D5 was the dominant siloxane in both indoor and outdoor air. Ratios of D5 to D4 averaged 91 and 3.2 for indoor and outdoor air resp.
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    Milani, A.; Al-Naiema, I. M.; Stone, E. A. Detection of a Secondary Organic Aerosol Tracer Derived from Personal Care Products. Atmos. Environ. 2021, 246, 118078  DOI: 10.1016/j.atmosenv.2020.118078
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    Detection of a secondary organic aerosol tracer derived from personal care products
    Milani, Alissia; Al-Naiema, Ibrahim M.; Stone, Elizabeth A.
    Atmospheric Environment (2021), 246 (), 118078CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    Decamethylcyclopentasiloxane (D5) is frequently used in personal care products (PCPs). In the gas phase, D5 is oxidized to form 1-hydroxynonamethylcyclopentasiloxane (D4TOH), which can partition to the particle phase. Numerous studies have reported secondary org. aerosol (SOA) formation via hydroxyl (OH) radical-initiated oxidn. of D5. It is expected that PCPs have a significant impact on SOA, but the extent has not yet been investigated. To date, no studies have reported the occurrence of PCP-derived SOA in ambient particulate matter. This study examd. fine particulate matter (PM2.5) collected in Atlanta, GA and Houston, TX and detd. D4TOH was present in 28 of 29 and 33 of 46 ambient PM2.5 samples, resp. Gas chromatog. retention data in the form of the Kova´ts index is reported for the first time to aid others in identifying this compd. in order to assess the impact of PCPs on SOA formation. The estd. concn. of D4TOH ranged from 16 to 185 pg m-3 in Atlanta and 19-206 pg m-3 in Houston. Synthetic musks were also detected in Atlanta and Houston PM2.5 samples, which is consistent with PCPs impacting urban air quality. Because of its specificity and demonstrated detectability, D4TOH may be useful as a tracer of PCP-derived secondary org. aerosol.
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  25. 25
    Ahrens, L.; Harner, T.; Shoeib, M. Temporal Variations of Cyclic and Linear Volatile Methylsiloxanes in the Atmosphere Using Passive Samplers and High-Volume Air Samplers. Environ. Sci. Technol. 2014, 48, 9374– 9381,  DOI: 10.1021/es502081j
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    25
    Temporal variations of cyclic and linear volatile methylsiloxanes in the atmosphere using passive samplers and high-volume air samplers
    Ahrens, Lutz; Harner, Tom; Shoeib, Mahiba
    Environmental Science & Technology (2014), 48 (16), 9374-9381CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Cyclic and linear volatile methylsiloxanes (cVMSs and lVMSs, resp.) were measured in ambient air over a period of over one year in Toronto, Canada. Air samples were collected using passive air samplers (PAS) consisting of sorbent-impregnated polyurethane foam (SIP) disks in parallel with high vol. active air samplers (HV-AAS). The av. difference between the SIP-PAS derived concns. in air for the individual VMSs and those measured using HV-AAS was within a factor of 2. The air concns. (HV-AAS) ranged 22-351 ng m-3 and 1.3-15 ng m-3 for ΣcVMSs (D3, D4, D5, D6) and ΣlVMSs (L3, L4, L5), resp., with decamethylcyclopentasiloxane (D5) as the dominant compd. (∼75% of the ΣVMSs). Air masses arriving from north to northwest (i.e., less populated areas) were significantly less contaminated with VMSs compared to air arriving from the south that are impacted by major urban and industrial areas in Canada and the U.S. (p < 0.05). In addn., air concns. of ΣcVMSs were lower during major snowfall events (on av., 73 ng m-3) in comparison to the other sampling periods (121 ng m-3). Ambient temp. had a small influence on the seasonal trend of VMS concns. in air, except for dodecamethylcyclohexasiloxane (D6), which was pos. correlated with the ambient temp. (p < 0.001).
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    Gallego, E.; Perales, J. F.; Roca, F. J.; Guardino, X.; Gadea, E. Volatile Methyl Siloxanes (VMS) Concentrations in Outdoor Air of Several Catalan Urban Areas. Atmos. Environ. 2017, 155, 108– 118,  DOI: 10.1016/j.atmosenv.2017.02.013
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    26
    Volatile methyl siloxanes (VMS) concentrations in outdoor air of several Catalan urban areas
    Gallego, E.; Perales, J. F.; Roca, F. J.; Guardino, X.; Gadea, E.
    Atmospheric Environment (2017), 155 (), 108-118CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    Volatile Me siloxanes (VMS) were evaluated in ten Catalan urban areas with different industrial impacts, such as petrochem. industry, elec. and mech. equipment, metallurgical and chem. industries, municipal solid waste treatment plant and cement and food industries, during 2013-2015. 24 H samples were taken with LCMA-UPC pump samplers specially designed in our lab., with a flow range of 70 mL min-1. A sorbent-based sampling method, successfully developed to collect a wide-range of VOC, was used. The anal. was performed by automatic thermal desorption coupled with capillary gas chromatog./mass spectrometry detector. The presented methodol. allows the evaluation of VMS together with a wide range of other VOC, increasing the no. of compds. that can be detd. in outdoor air quality assessment of urban areas. This aspect is esp. relevant as a restriction of several VMS (D4 and D5) in consumer products has been made by the European Chems. Agency and US EPA is evaluating to include D4 in the Toxic Substances Control Act, regarding the concern of the possible effects of these compds. in human health and the environment. ΣVMS concns. (L2-L5, D3-D6 and trimethylsilanol) varied between 0.3 ± 0.2μg m-3 and 18 ± 12μg m-3, detd. in a hotspot area. Obsd. VMS concns. were generally of the same order of magnitude than the previously detd. in Barcelona, Chicago and Zurich urban areas, but higher than the published from suburban sites and Arctic locations. Cyclic siloxanes concns. were up to two-three orders of magnitude higher than those of linear siloxanes, accounting for av. contributions to the total concns. of 97 ± 6% for all samples except for the hotspot area, where cyclic VMS accounted for 99.9 ± 0.1%. D5 was the most abundant siloxane in 5 sampling points; however, differing from the generally obsd. in previous studies, D3 was the most abundant compd. in the other 5 sampling points.
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  27. 27
    US EPA. Enforceable Consent Agreement for Environmental Testing for Octamethylcyclotetrasiloxane (D4) (CASRN 556-67-2) Docket No. EPA-HQ-OPPT-2012-0209; EPA, 2014.
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  28. 28
    Dudzina, T.; Von Goetz, N.; Bogdal, C.; Biesterbos, J. W. H.; Hungerbühler, K. Concentrations of Cyclic Volatile Methylsiloxanes in European Cosmetics and Personal Care Products: Prerequisite for Human and Environmental Exposure Assessment. Environ. Int. 2014, 62, 86– 94,  DOI: 10.1016/j.envint.2013.10.002
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    28
    Concentrations of cyclic volatile methylsiloxanes in European cosmetics and personal care products: Prerequisite for human and environmental exposure assessment
    Dudzina, Tatsiana; von Goetz, Natalie; Bogdal, Christian; Biesterbos, Jacqueline W. H.; Hungerbuhler, Konrad
    Environment International (2014), 62 (), 86-94CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
    Low mol. wt. cyclic volatile methylsiloxanes (cVMSs) are widely employed as emollients and carrier solvents in personal care formulations in order to acquire desired performance benefits owing to their distinctive physicochem. properties. Under current European legislation cosmetic ingredients such as cVMSs are required to be labeled on the product package only qual., while for the assessment of environmental and consumer exposure quant. information is needed. The aim of this study was therefore to measure concns. of three cVMSs, namely octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) in 51 cosmetics and personal care products (C&PCPs) that are currently available on the European market. The list of selected articles comprised a variety of hair and sun care products, skin creams and lotions, deodorants including antiperspirants, liq. foundations and a toothpaste. The target compds. were extd. from the products with different org. solvents dependent on the product matrix, followed by gas chromatog. anal. with flame ionization detection (GC-FID). D5 was the predominant cVMS with the highest mean and median concns. in all the C&PCP categories. The median concns. of D5, D6 and D4 were 142, 2.3 and 0.053 mg/g in deodorants/antiperspirants (n = 11); 44.6, 30.0 mg/g and below the limit of quantification (< LOQ; LOQ for D4 = 0.00071 mg/g) in cosmetics (n = 5); 8.4, 0.32 mg/g and < LOQ in skin care (n = 16); 9.6, 0.18 and 0.0055 mg/g in hair care (n = 10); and, 34.8, 0.53 and 0.0085 mg/g in sun care (n = 8) products, resp. The calcd. median aggregate daily dermal exposure to D4 and D5 from multiple C&PCPs was approx. 100 times lower than the current NOAEL derived from chronic inhalation rat studies.
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  29. 29
    Bzdek, B. R.; Horan, A. J.; Pennington, M. R.; Janechek, N. J.; Baek, J.; Stanier, C. O.; Johnston, M. V. Silicon Is a Frequent Component of Atmospheric Nanoparticles. Environ. Sci. Technol. 2014, 48, 11137– 11145,  DOI: 10.1021/es5026933
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    29
    Silicon is a Frequent Component of Atmospheric Nanoparticles
    Bzdek, Bryan R.; Horan, Andrew J.; Pennington, M. Ross; Janechek, Nathan J.; Baek, Jaemeen; Stanier, Charles O.; Johnston, Murray V.
    Environmental Science & Technology (2014), 48 (19), 11137-11145CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Nanoparticles are the largest fraction of aerosol loading by no. Knowledge of the chem. components of nanoparticulate matter is needed to understand nanoparticle health and climate impacts. This work presents nano aerosol mass spectrometer (NAMS) field measurements which provide quant. elemental compn. of ∼20 nm diam. nanoparticles. NAMS measurements indicated the element, Si, was a frequent nanoparticle component. Nanoparticulate Si was most abundant at sites heavily impacted by anthropogenic activity. Wind direction correlations suggested Si sources are diffuse; diurnal trends suggested nanoparticulate Si may result from photochem. processing of gaseous Si-contg. compds., e.g., cyclic siloxanes. Atm. modeling of oxidized cyclic siloxanes was consistent with a diffuse photochem. source of aerosol Si. More broadly, the observations indicated a previously overlooked anthropogenic source of nanoaerosol mass. Further investigation is needed to fully resolve its atm. role.
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  30. 30
    Janechek, N. J.; Hansen, K. M.; Stanier, C. O. Comprehensive Atmospheric Modeling of Reactive Cyclic Siloxanes and Their Oxidation Products. Atmos. Chem. Phys. 2017, 17, 8357– 8370,  DOI: 10.5194/acp-17-8357-2017
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    Comprehensive atmospheric modeling of reactive cyclic siloxanes and their oxidation products
    Janechek, Nathan J.; Hansen, Kaj M.; Stanier, Charles O.
    Atmospheric Chemistry and Physics (2017), 17 (13), 8357-8370CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Cyclic volatile Me siloxanes (cVMSs) are important components in personal care products that transport and react in the atm. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and their gas-phase oxidn. products have been incorporated into the Community Multiscale Air Quality (CMAQ) model. Gas-phase oxidn. products, as the precursor to secondary org. aerosol from this compd. class, were included to quantify the max. potential for aerosol formation from gas-phase reactions with OH. Four 1-mo periods were modeled to quantify typical concns., seasonal variability, spatial patterns, and vertical profiles. Typical model concns. showed parent compds. were highly dependent on population d. as cities had monthly averaged peak D5 concns. up to 432 ngm-3. Peak oxidized D5 concns. were significantly less, up to 9 ngm-3, and were located downwind of major urban areas. Model results were compared to available measurements and previous simulation results. Seasonal variation was analyzed and differences in seasonal influences were obsd. between urban and rural locations. Parent compd. concns. in urban and peri-urban locations were sensitive to transport factors, while parent compds. in rural areas and oxidized product concns. were influenced by large-scale seasonal variability in OH.
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  31. 31
    Chandramouli, B.; Kamens, R. M. The Photochemical Formation and Gas-Particle Partitioning of Oxidation Products of Decamethyl Cyclopentasiloxane and Decamethyl Tetrasiloxane in the Atmosphere. Atmos. Environ. 2001, 35, 87– 95,  DOI: 10.1016/S1352-2310(00)00289-2
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    Navea, J. G.; Xu, S.; Stanier, C. O.; Young, M. A.; Grassian, V. H. Heterogeneous Uptake of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) onto Mineral Dust Aerosol under Variable RH Conditions. Atmos. Environ. 2009, 43, 4060– 4069,  DOI: 10.1016/j.atmosenv.2009.05.012
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    32
    Heterogeneous uptake of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) onto mineral dust aerosol under variable RH conditions
    Navea, Juan G.; Xu, Shihe; Stanier, Charles O.; Young, Mark A.; Grassian, Vicki H.
    Atmospheric Environment (2009), 43 (26), 4060-4069CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    We have carried out kinetic studies to characterize the heterogeneous decay of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in the presence of representative mineral dust aerosol in order to obtain a better understanding of the atm. fate of these siloxanes. The heterogeneous chem. of D4 and D5 with various mineral dusts was studied in an environmental aerosol reaction chamber using FTIR absorption spectroscopy to monitor the reaction. The apparent heterogeneous uptake coeff., γapp, for D4 and D5 with various mineral dusts was measured under dry conditions and as a function of relative humidity (RH). In addn., the effect of initial D4 and D5 concn. on the rate and yield of the reaction was examd. The uptake coeff., γapp, for D4 and D5 was similar for the most reactive aerosols tested, with kaolinite ≈ hematite > silica. Limited uptake onto carbon black and calcite surfaces was obsd. for either siloxane. Reaction with hematite and kaolinite resulted in multilayer coverages, suggesting extensive polymn. of D4 and D5 on the aerosol surface.
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  33. 33
    Janechek, N. J.; Marek, R. F.; Bryngelson, N.; Singh, A.; Bullard, R. L.; Brune, W. H.; Stanier, C. O. Physical Properties of Secondary Photochemical Aerosol from OH Oxidation of a Cyclic Siloxane. Atmos. Chem. Phys. 2019, 19, 1649– 1664,  DOI: 10.5194/acp-19-1649-2019
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    Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane
    Janechek, Nathan J.; Marek, Rachel F.; Bryngelson, Nathan; Singh, Ashish; Bullard, Robert L.; Brune, William H.; Stanier, Charles O.
    Atmospheric Chemistry and Physics (2019), 19 (3), 1649-1664CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Cyclic volatile Me siloxanes (cVMS) are high prodn. chems. present in many personal care products. They are volatile, hydrophobic, and relatively long lived due to slow oxidn. kinetics. Evidence from chamber and ambient studies indicates that oxidn. products may be found in the condensed aerosol phase. In this work, we use an oxidn. flow reactor to produce ∼ 100 μgm-3 of organosilicon aerosol from OH oxidn. of decamethylcyclopentasiloxane (D5) with aerosol mass fractions (i.e.,yields) of 0.2-0.5. The aerosols were assessed for concn., size distribution, morphol., sensitivity to seed aerosol, hygroscopicity, volatility and chem. compn. through a combination of aerosol size distribution measurement, tandem differential mobility anal., and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidn. conditions and chamber operation. The D5 oxidn. aerosol products were relatively non-hygroscopic, with an av. hygroscopicity kappa of ∼0.01, and nearly non-volatile up to 190 °C temp. Parameters for exploratory treatment as a semi-volatile org. aerosol in atm. models are provided.
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    Fairbrother, A.; Woodburn, K. B. Assessing the Aquatic Risks of the Cyclic Volatile Methyl Siloxane D4. Environ. Sci. Technol. Lett. 2016, 3, 359– 363,  DOI: 10.1021/acs.estlett.6b00341
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    Assessing the Aquatic Risks of the Cyclic Volatile Methyl Siloxane D4
    Fairbrother, Anne; Woodburn, Kent B.
    Environmental Science & Technology Letters (2016), 3 (10), 359-363CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Assessing the ecol. risks of the widely used cyclic volatile Me siloxane D4 (octamethylcyclotetrasiloxane, CAS Registry No. 556-67-2) to aquatic systems is difficult because of its high volatility and low water soly., but the potential for long distance atm. transport and persistence in the sediments has placed D4 under intense regulatory scrutiny. This paper explores the difficulties inherent in detg. the toxicity of D4 to aquatic species and reveals the increased sensitivity of aquatic species tested within artificially closed systems compared to that of similar tests conducted in open systems that allow natural volatilization to occur. The concepts of narcosis mode of action and chem. activity explain the apparent lack of toxicity of D4 to aquatic species under environmentally realistic conditions. Discharge levels for the past 30 yr during which D4 has been in use have produced field-measured concns. that pose negligible risk to aquatic organisms.
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  35. 35
    Tang, X.; Misztal, P. K.; Nazaroff, W. W.; Goldstein, A. H. Siloxanes Are the Most Abundant Volatile Organic Compound Emitted from Engineering Students in a Classroom. Environ. Sci. Technol. Lett. 2015, 2, 303– 307,  DOI: 10.1021/acs.estlett.5b00256
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    35
    Siloxanes Are the Most Abundant Volatile Organic Compound Emitted from Engineering Students in a Classroom
    Tang, Xiaochen; Misztal, Pawel K.; Nazaroff, William W.; Goldstein, Allen H.
    Environmental Science & Technology Letters (2015), 2 (11), 303-307CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Direct human emissions are known to contribute volatile org. compds. (VOC) to indoor air by various mechanisms; however, few measurements which det. emissions of a full suite of occupant-assocd. VOC are available. This work measured occupant-related VOC emissions from engineering students in a classroom using a proton transfer reaction, time-of-flight mass spectrometry (PTR/TOF-MS). The dominant compd. emitted was a cyclic volatile methylsiloxane (cVMS), decamethylcyclopentasiloxane (D5), a major inactive ingredient in some personal care products, e.g., antiperspirant. D5 contributed ∼30% of the total indoor VOC mass concn. measured by the PTR/TOF-MS. Octamethylcyclotetrasiloxane (D4) and dodecamethylcyclohexasiloxane (D6) were detected at abundances 1-2 orders of magnitude lower. The per-person emission rate of these 3 cVMS declined monotonically from morning to afternoon, consistent with expectations for emissions from daily morning application of personal care products.
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  36. 36
    Fu, Z.; Xie, H.-B.; Elm, J.; Guo, X.; Fu, Z.; Chen, J. Formation of Low-Volatile Products and Unexpected High Formaldehyde Yield from the Atmospheric Oxidation of Methylsiloxanes. Environ. Sci. Technol. 2020, 54, 7136– 7145,  DOI: 10.1021/acs.est.0c01090
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    Formation of Low-Volatile Products and Unexpected High Formaldehyde Yield from the Atmospheric Oxidation of Methylsiloxanes
    Fu, Zihao; Xie, Hong-Bin; Elm, Jonas; Guo, Xirui; Fu, Zhiqiang; Chen, Jingwen
    Environmental Science & Technology (2020), 54 (12), 7136-7145CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    With more strict regulation of atm. volatile org. compds. originating from fossil fuel-based vehicles and industries, the use of volatile chem. products (VCP) and the VCP transformation mechanism have become increasingly important to quantify air quality. Volatile methylsiloxanes (VMS) are an important class of VCP and high-prodn. chems. Using quantum chem. calcns. and kinetics modeling, the authors examd. the reaction mechanism of peroxy radicals with VMS, key intermediates in detg. VMS atm. chem. L2-RSiCH2O2- and D3-RSiCH2O2- derived from hexamethyldisiloxane and hexamethylcyclotrisiloxane, resp., were selected as representative model systems. Results indicated L2-RSiCH2O2- and D3-RSiCH2O2- followed a novel Si-C-O rearrangement-driven autoxidn. mechanism, leading to formation of low volatile silanols and high yield of formaldehyde at low NO:HO2- conditions. At high NO/HO2- conditions, L2-RSiCH2O2- and D3-RSiCH2O2- reacted with NO/HO2- to form org. nitrate, hydroperoxide, and active alkoxy radicals. Alkoxy radicals further followed a Si-C-O rearrangement step to form formate esters. The novel Si-C-O rearrangement mechanism of peroxy and alkoxy radicals was supported by available exptl. studies on VMS oxidn. The high formaldehyde yield was estd. to significantly contribute to formaldehyde indoor air pollution, esp. during cleaning.
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  37. 37
    Ren, Z.; da Silva, G. Auto-Oxidation of a Volatile Silicon Compound: A Theoretical Study of the Atmospheric Chemistry of Tetramethylsilane. J. Phys. Chem. A 2020, 124, 6544– 6551,  DOI: 10.1021/acs.jpca.0c02922
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    37
    Auto-Oxidation of a Volatile Silicon Compound: A Theoretical Study of the Atmospheric Chemistry of Tetramethylsilane
    Ren, Zhonghua; da Silva, Gabriel
    Journal of Physical Chemistry A (2020), 124 (32), 6544-6551CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Volatile Si compds. (VOSiC) are air pollutants which occur in indoor and outdoor environments. Tetramethylsilane (TMS) was selected as a model to examine photochem. oxidn. mechanisms for VOSiC using ab-initio and RRKM theory/master equation kinetic modeling. Under tropospheric conditions, the radical, (CH3)3SiCH2- reacts with O2 to produce a stabilized peroxyl radical, which is expected to ultimately yield the alkoxyl radical, (CH3)3SiCH2O-. However, at combustion-relevant temps., a well-skipping reaction to (CH3)3SiO- + HCHO dominates. The (CH3)3SiCH2O- radical is predicted to rearrange to (CH3)3SiOCH2- with a very low reaction barrier, enabling an auto-oxidn. process involving adding a second O2. Subsequent (CH3)3SiOCH2- oxidn. reaction mechanisms have been developed, with major product predicted to be trimethylsilyl formate, (CH3)3SiOCHO, an exptl. obsd. TMS oxidn. product. Prodn. of substantially oxygenated compds. following a single radical initiation reaction had implications for the VOSiC ability to contribute to O3 and particle formation in outdoor and indoor environments.
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  38. 38
    Coggon, M. M.; McDonald, B. C.; Vlasenko, A.; Veres, P. R.; Bernard, F.; Koss, A. R.; Yuan, B.; Gilman, J. B.; Peischl, J.; Aikin, K. C.; DuRant, J.; Warneke, C.; Li, S.; de Gouw, J. A. Diurnal Variability and Emission Pattern of Decamethylcyclopentasiloxane (D5) from the Application of Personal Care Products in Two North American Cities. Environ. Sci. Technol. 2018, 52, 5610– 5618,  DOI: 10.1021/acs.est.8b00506
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    Diurnal Variability and Emission Pattern of Decamethylcyclopentasiloxane (D5) from the Application of Personal Care Products in Two North American Cities
    Coggon, Matthew M.; McDonald, Brian C.; Vlasenko, Alexander; Veres, Patrick R.; Bernard, Francois; Koss, Abigail R.; Yuan, Bin; Gilman, Jessica B.; Peischl, Jeff; Aikin, Kenneth C.; DuRant, Justin; Warneke, Carsten; Li, Shao-Meng; de Gouw, Joost A.
    Environmental Science & Technology (2018), 52 (10), 5610-5618CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Decamethylcyclopentasiloxane (D5) is a cyclic volatile Me siloxane (cVMS) that is widely used in consumer products and commonly obsd. in urban air. This study quantifies the ambient mixing ratios of D5 from ground sites in two North American cities (Boulder, CO, USA, and Toronto, ON, CA). From these data, we est. the diurnal emission profile of D5 in Boulder, CO. Ambient mixing ratios were consistent with those measured at other urban locations; however, the diurnal pattern exhibited similarities with those of traffic-related compds. such as benzene. Mobile measurements and vehicle expts. demonstrate that emissions of D5 from personal care products are coincident in time and place with emissions of benzene from motor vehicles. During peak commuter times, the D5/benzene ratio (wt./wt.) is in excess of 0.3, suggesting that the mass emission rate of D5 from personal care product usage is comparable to that of benzene due to traffic. The diurnal emission pattern of D5 is estd. using the measured D5/benzene ratio and inventory ests. of benzene emission rates in Boulder. The hourly D5 emission rate is obsd. to peak between 6:00 and 7:00 AM and subsequently follow an exponential decay with a time const. of 9.2 h. This profile could be used by models to constrain temporal emission patterns of personal care products.
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  39. 39
    Tran, T. M.; Abualnaja, K. O.; Asimakopoulos, A. G.; Covaci, A.; Gevao, B.; Johnson-Restrepo, B.; Kumosani, T. A.; Malarvannan, G.; Minh, T. B.; Moon, H. B.; Nakata, H.; Sinha, R. K.; Kannan, K. A Survey of Cyclic and Linear Siloxanes in Indoor Dust and Their Implications for Human Exposures in Twelve Countries. Environ. Int. 2015, 78, 39– 44,  DOI: 10.1016/j.envint.2015.02.011
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    A survey of cyclic and linear siloxanes in indoor dust and their implications for human exposures in twelve countries
    Tran, Tri Manh; Abualnaja, Khalid O.; Asimakopoulos, Alexandros G.; Covaci, Adrian; Gevao, Bondi; Johnson-Restrepo, Boris; Kumosani, Taha A.; Malarvannan, Govindan; Minh, Tu Binh; Moon, Hyo-Bang; Nakata, Haruhiko; Sinha, Ravindra K.; Kannan, Kurunthachalam
    Environment International (2015), 78 (), 39-44CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
    Siloxanes are used widely in a variety of consumer products, including cosmetics, personal care products, medical and elec. devices, cookware, and building materials. Nevertheless, little is known on the occurrence of siloxanes in indoor dust. In this survey, five cyclic (D3-D7) and 11 linear (L4-L14) siloxanes were detd. in 310 indoor dust samples collected from 12 countries. Dust samples collected from Greece contained the highest concns. of total cyclic siloxanes (TCSi), ranging from 118 to 25,100 ng/g (median: 1380), and total linear siloxanes (TLSi), ranging from 129 to 4990 ng/g (median: 772). The median total siloxane (TSi) concns. in dust samples from 12 countries were in the following decreasing order: Greece (2970 ng/g), Kuwait (2400), South Korea (1810), Japan (1500), the USA (1220), China (1070), Romania (538), Colombia (230), Vietnam (206), Saudi Arabia (132), India (116), and Pakistan (68.3). TLSi concns. as high as 42,800 ng/g (Kuwait) and TCSi concns. as high as 25,000 ng/g (Greece) were found in indoor dust samples. Among the 16 siloxanes detd., decamethylcyclopentasiloxane (D5) was found at the highest concn. in dust samples from all countries, except for Japan and South Korea, with a predominance of L11; Kuwait, with L10; and Pakistan and Romania, with L12. The compn. profiles of 16 siloxanes in dust samples varied by country. TCSi accounted for a major proportion of TSi concns. in dust collected from Colombia (90%), India (80%) and Saudi Arabia (70%), whereas TLSi predominated in samples collected from Japan (89%), Kuwait (85%), and South Korea (78%). Based on the measured median TSi concns. in indoor dust, we estd. human exposure doses through indoor dust ingestion for various age groups. The exposure doses ranged from 0.27 to 11.9 ng/kg-bw/d for toddlers and 0.06 to 2.48 ng/kg-bw/d for adults.
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  40. 40
    Praske, E.; Otkjær, R. V.; Crounse, J. D.; Hethcox, J. C.; Stoltz, B. M.; Kjaergaard, H. G.; Wennberg, P. O. Atmospheric Autoxidation Is Increasingly Important in Urban and Suburban North America. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 64– 69,  DOI: 10.1073/pnas.1715540115
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    Atmospheric autoxidation is increasingly important in urban and suburban North America
    Praske, Eric; Otkjaer, Rasmus V.; Crounse, John D.; Hethcox, J. Caleb; Stoltz, Brian M.; Kjaergaard, Henrik G.; Wennberg, Paul O.
    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (1), 64-69CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Gas-phase autoxidn.-regenerative peroxy radical formation following intramol. hydrogen shifts-is known to be important in the combustion of org. materials. The relevance of this chem. in the oxidn. of orgs. in the atm. has received less attention due, in part, to the lack of kinetic data at relevant temps. Here, we combine computational and exptl. approaches to investigate the rate of autoxidn. for org. peroxy radicals (RO2) produced in the oxidn. of a prototypical atm. pollutant, n-hexane. We find that the reaction rate depends critically on the mol. configuration of the RO2 radical undergoing hydrogen transfer (H-shift). RO2 H-shift rate coeffs. via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, resp.) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1/s at 296 K, while the 1,4 H-shift is calcd. to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coeffs. of these reactions increase rapidly with temp. and exhibit a large, primary, kinetic isotope effect. The obsd. H-shift rate coeffs. are sufficiently fast that, as a result of ongoing NOx emission redns., autoxidn. is now competing with bimol. chem. even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temps. are elevated.
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  41. 41
    Wu, Y.; Johnston, M. V. Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (CVMS) Decamethylcyclopentasiloxane. Environ. Sci. Technol. 2017, 51, 4445– 4451,  DOI: 10.1021/acs.est.7b00655
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    41
    Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS) Decamethylcyclopentasiloxane
    Wu, Yue; Johnston, Murray V.
    Environmental Science & Technology (2017), 51 (8), 4445-4451CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Aerosol formation from OH- oxidn. of decamethylcyclopentasiloxane (D5, C10H30O5Si5), a cyclic volatile Me siloxane (cVMS) in consumer products, was examd. in a flow-through photooxidn. chamber with and without the presence of (NH4)2SO4 seed aerosols. In unseeded expts., high performance mass spectrometry chem. characterization showed that mol. compn. changed substantially with aerosol mass loads in the 1-12 μg/m3 range. Monomers (5 Si atoms/mol.) and dimers (10 Si atoms/mol.) dominated the mass spectra of aerosols at higher mass loads; ring-opened species (neither 5 nor 10 Si atoms/mol.) dominated the mass spectra of aerosols at lower mass loads. Mol. signal intensity dependencies on aerosol vol.:surface area ratio suggested non-volatile ring-opened species are formed in the gas phase and assist particle formation via condensation; dimers are formed by accretion reactions within the particle phase as particles grow. These conclusions were supported by expts. in the presence of seed aerosols with a similar siloxane aerosol mass loading but higher vol.:surface area ratio, where ring-opened species were much less prevalent than monomers or dimers and the aerosol yield was higher. Due to the importance of accretion chem., the aerosol yield from D5 oxidn. is likely to strongly depend on particle size and morphol.
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  42. 42
    Cheng, Z.; Qiu, X.; Shi, X.; Zhu, T. Identification of Organosiloxanes in Ambient Fine Particulate Matters Using an Untargeted Strategy via Gas Chromatography and Time-of-Flight Mass Spectrometry. Environ. Pollut. 2021, 271, 116128  DOI: 10.1016/j.envpol.2020.116128
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    42
    Identification of organosiloxanes in ambient fine particulate matters using an untargeted strategy via gas chromatography and time-of-flight mass spectrometry
    Cheng, Zhen; Qiu, Xinghua; Shi, Xiaodi; Zhu, Tong
    Environmental Pollution (Oxford, United Kingdom) (2021), 271 (), 116128CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
    Organosilicons are widely used in consumer products and are ubiquitous in living environments. However, there is little systemic information on this group of pollutants in ambient particles. This study proposes a novel untargeted strategy based mainly on the mass difference of three silicon isotopes to screen organosilicon compds. from 2-yr PM2.5 samples of Beijing using gas chromatog. and high-resoln. time-of-flight mass spectrometry. 61 organosilicons were filtered from 1019 peaks, and 35 ones were identified as organosiloxanes including 17 methylsiloxanes and 18 phenylmethylsiloxanes, of which 6 and 3 species were confirmed using ref. stds., resp. These organosiloxanes could be clustered into three groups: low-silicon-no. methylsiloxanes, high-silicon-no. methylsiloxanes, and phenylmethylsiloxanes. Low-silicon-no. methylsiloxanes showed high abundance in the heating season but low abundance in the non-heating season, whereas high-silicon-no. methylsiloxanes showed the opposite seasonal variation. This study provides a promising strategy for screening organosilicon compds. through an untargeted approach and gives insights for further investigation of the sources and health risks of organosiloxanes.
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  43. 43
    Lu, D.; Tan, J.; Yang, X.; Sun, X.; Liu, Q.; Jiang, G.; Tan, J.; Sun, X.; Lu, D.; Jiang, G.; Yang, X. Unraveling the Role of Silicon in Atmospheric Aerosol Secondary Formation: A New Conservative Tracer for Aerosol Chemistry. Atmos. Chem. Phys. Discuss. 2018, 19, 2861– 2870,  DOI: 10.5194/acp-2018-914
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    Lu, D.; Liu, Q.; Yu, M.; Yang, X.; Fu, Q.; Zhang, X.; Mu, Y.; Jiang, G. Natural Silicon Isotopic Signatures Reveal the Sources of Airborne Fine Particulate Matter. Environ. Sci. Technol. 2018, 52, 1088– 1095,  DOI: 10.1021/acs.est.7b06317
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    Natural Silicon Isotopic Signatures Reveal the Sources of Airborne Fine Particulate Matter
    Lu, Dawei; Liu, Qian; Yu, Miao; Yang, Xuezhi; Fu, Qiang; Zhang, Xiaoshan; Mu, Yujing; Jiang, Guibin
    Environmental Science & Technology (2018), 52 (3), 1088-1095CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Airborne particulate pollution is a crit. environmental problem affecting human health and sustainable development. Understanding aerosol particle sources is extremely important for regional air pollution control. This work showed that a natural Si isotopic signature can be used as a tool to elucidate fine particulate matter (PM2.5) sources. By analyzing the PM2/5 Si isotopic compn. (δ30Si) and its primary sources collected in a typical polluted region (Beijing, China) the authors recognized the direct source tracing ability of Si isotopes for PM2.5. Different primary PM2.5 sources had different Si isotopic signatures. The PM2.5 δ30Si value was -1.99 ‰ to -0.01 ‰ and displayed a distinct seasonal trend (isotopically lighter in spring/winter and heavier in summer/autumn). PM2.5 δ30Si variations showed Si-isotopically light sources were important sources for the Beijing severe haze pollution; coal combustion was a major cause for the aggregating haze spring/winter weather in Beijing. Several typical haze events were also analyzed using Si isotopic signatures. As the first study on natural Si isotopes in the atm. environment, this work may reveal an important tool to advance particulate pollution research and control.
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    Charan, S.; Huang, Y.; Buenconsejo, R.; Li, Q.; Cocker, D., III; Seinfeld, J. Secondary Organic Aerosol Formation from the Oxidation of Decamethylcyclopentasiloxane at Atmospherically Relevant OH Concentrations. Atmos. Chem. Phys. Discuss. 2021, 917– 928,  DOI: 10.5194/acp-2021-353
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    Ziemann, P. J.; Atkinson, R. Kinetics, Products, and Mechanisms of Secondary Organic Aerosol Formation. Chem. Soc. Rev. 2012, 41, 6582– 6605,  DOI: 10.1039/c2cs35122f
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    Kinetics, products, and mechanisms of secondary organic aerosol formation
    Ziemann, Paul J.; Atkinson, Roger
    Chemical Society Reviews (2012), 41 (19), 6582-6605CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Secondary org. aerosol (SOA) is formed in the atm. when volatile org. compds. (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O3, NO3 radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these org. compds. can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atm. aerosol mass and can have significant effects on atm. chem., visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atm. VOC reactions and the general chem. and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chem. as they apply to SOA formation, with a focus on the effects of VOC mol. structure on the kinetics of initial reactions with the major atm. oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the compn. of the resulting products. Structural features of reactants and products discussed include compd. carbon no.; linear, branched, and cyclic configurations; the presence of C:C bonds and arom. rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atm. chemists with sufficient information to understand the dominant pathways by which the major classes of atm. VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on mol. structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NOx concns. as well as temp., humidity, and particle acidity. Such knowledge should be useful for interpreting the results of lab. and field studies and for developing atm. chem. models. A no. of recommendations for future research are also presented.
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    Peng, Z.; Jimenez, J. L. KinSim: A Research-Grade, User-Friendly, Visual Kinetics Simulator for Chemical-Kinetics and Environmental-Chemistry Teaching. J. Chem. Educ. 2019, 96, 806– 811,  DOI: 10.1021/acs.jchemed.9b00033
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    KinSim: A Research-Grade, User-Friendly, Visual Kinetics Simulator for Chemical-Kinetics and Environmental-Chemistry Teaching
    Peng, Zhe; Jimenez, Jose L.
    Journal of Chemical Education (2019), 96 (4), 806-811CODEN: JCEDA8; ISSN:0021-9584. (American Chemical Society and Division of Chemical Education, Inc.)
    KinSim is a research-grade, interactive, user-friendly, open-source, and visual software for kinetics modeling of environmental chem. and other applications. Students without any computer-programming background and limited knowledge of environmental chem. can use KinSim, which also includes multiple features and functionality dedicated to reducing users' workload and preventing users from creating errors in modeling; thus, KinSim is particularly suitable for in-classroom and homework teaching applications. Students can choose from several preprogrammed mechanisms and initial conditions for important environmental-chem. problems, and only clicking a few buttons is needed to perform a simulation and obtain graphs with concns. and chem. fluxes. The mechanism and initial conditions can be edited very easily to study other cases. Feedback from its use in courses shows the effectiveness of KinSim in helping students conduct computer expts. to gain familiarity with environmental chem. systems and helping them gain deeper understanding of the complex emergent behaviors of the systems. KinSim's accuracy and speed are similar to those of other software packages used in research, and at least 15 published peer-reviewed papers have used it.
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    Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. Atmos. Chem. Phys. 2004, 4, 1461– 1738,  DOI: 10.5194/acp-4-1461-2004
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    Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of Ox, HOx, NOx and SOx species
    Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J.
    Atmospheric Chemistry and Physics (2004), 4 (6), 1461-1738CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
    This review article, the first in the series, presents kinetic and photochem. data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atm. Chem. It covers the gas phase and photochem. reactions of Ox, HOx, NOx and SOx species, which were last published in 1997, and were updated on the IUPAC website in late 2001. The article consists of a summary sheet, contg. the recommended kinetic parameters for the evaluated reactions, and five appendices contg. the data sheets, which provide information upon which the recommendations are made.
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  49. 49
    Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume III─Gas Phase Reactions of Inorganic Halogens. Atmos. Chem. Phys. 2007, 7, 981– 1191,  DOI: 10.5194/acp-7-981-2007
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    Evaluated kinetic and photochemical data for atmospheric chemistry: volume III - gas phase reactions of inorganic halogens
    Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J.
    Atmospheric Chemistry and Physics (2007), 7 (4), 981-1191CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
    A review. This article, the third in the series, presents kinetic and photochem. data evaluated by the IUPAC Sub-committee on Gas Kinetic Data Evaluation for Atm. Chem. It covers the gas phase and photochem. reactions of inorg. halogen species, which were last published in J. Phys. Chem. Ref. Data, in 2000 were updated on the IUPAC website in 2003 and are updated again in the present evaluation. The article consists of a summary sheet, contg. the recommended kinetic parameters for the evaluated reactions, and five appendices contg. the data sheets, which provide information upon which the recommendations were made.
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    Liu, X.; Day, D. A.; Krechmer, J. E.; Brown, W.; Peng, Z.; Ziemann, P. J.; Jimenez, J. L. Direct Measurements of Semi-Volatile Organic Compound Dynamics Show near-Unity Mass Accommodation Coefficients for Diverse Aerosols. Commun. Chem. 2019, 2, 98  DOI: 10.1038/s42004-019-0200-x
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    Krechmer, J. E.; Pagonis, D.; Ziemann, P. J.; Jimenez, J. L. Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ. Environ. Sci. Technol. 2016, 50, 5757– 5765,  DOI: 10.1021/acs.est.6b00606
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    Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ
    Krechmer, Jordan E.; Pagonis, Demetrios; Ziemann, Paul J.; Jimenez, Jose L.
    Environmental Science & Technology (2016), 50 (11), 5757-5765CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Partitioning gas-phase org. compds. to the walls of Teflon environmental chambers is a recently reported phenomenon which can affect yields of reaction products and secondary org. aerosols measured in lab. expts. Reported time scales to reach gas-wall partitioning (GWP) equil. (τGWE) differ by up to 3 orders of magnitude; however, leading to predicted effects which vary from substantial to negligible. A method is demonstrated in which semi- and low-volatility oxidized org. compds. (satn. concn. [c*] <100 μg/m3) were photochem. generated by rapid, in-situ bursts in an 8 m3 environmental chamber, then their decay in the absence of aerosol was measured by a high resoln. chem. ionization mass spectrometer (CIMS) equipped with an inlet-less NO3- source. Measured τGWE were 7-13 min with a 33% relative std. deviation for all compds. The fraction of each compd. which partitioned to the walls at equil. followed absorptive partitioning theory with an equiv. wall mass concn. of 0.3-10 mg/m3. CIMS measurements, including a std. ion-mol. reaction region showed large biases due to compd. contact with chamber walls. Based on these results, a set of parameters is proposed to modeling GWP in chamber expts.
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    Boethling, R.; Meylan, W. How Accurate Are Physical Property Estimation Programs for Organosilicon Compounds?. Environ. Toxicol. Chem. 2013, 8, 2433– 2440,  DOI: 10.1002/etc.2326
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    Carter, W. L. P.; Pierce, J. A.; Malkina, I. L.; Luo, D. Investigation of the Ozone Formation Potential of Selected Volatile Silicone Compounds; Final Report to Dow Corning Corporation: Midland, MI, 1992.
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    Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2006.
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    Orlando, J. J.; Tyndall, G. S. Laboratory Studies of Organic Peroxy Radical Chemistry: An Overview with Emphasis on Recent Issues of Atmospheric Significance. Chem. Soc. Rev. 2012, 41, 6294– 6317,  DOI: 10.1039/c2cs35166h
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    Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance
    Orlando, John J.; Tyndall, Geoffrey S.
    Chemical Society Reviews (2012), 41 (19), 6294-6317CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Org. peroxy radicals (often abbreviated RO2) play a central role in the chem. of the Earth's lower atm. Formed in the atm. oxidn. of essentially every org. species emitted, their chem. is part of the radical cycles that control the oxidative capacity of the atm. and lead to the formation of ozone, org. nitrates, org. acids, particulate matter and other so-called secondary pollutants. In this review, lab. studies of this peroxy radical chem. are detailed, as they pertain to the chem. of the atm. First, a brief discussion of methods used to detect the peroxy radicals in the lab. is presented. Then, the basic reaction pathways - involving RO2 unimol. reactions and bimol. reactions with atm. constituents such as NO, NO2, NO3, O3, halogen oxides, HO2, and other RO2 species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chem. assocd. with the atm. oxidn. of biogenic hydrocarbons.
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    Jenkin, M. E.; Valorso, R.; Aumont, B.; Rickard, A. R. Estimation of Rate Coefficients and Branching Ratios for Reactions of Organic Peroxy Radicals for Use in Automated Mechanism Construction. Atmos. Chem. Phys. Discuss. 2019, 19, 7691– 7717,  DOI: 10.5194/acp-19-7691-2019
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    Narayanasamy, J.; Kubicki, J. D. Mechanism of Hydroxyl Radical Generation from a Silica Surface: Molecular Orbital Calculations. J. Phys. Chem. B 2005, 109, 21796– 21807,  DOI: 10.1021/jp0543025
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    Mechanism of Hydroxyl Radical Generation from a Silica Surface: Molecular Orbital Calculations
    Narayanasamy, Jayakumar; Kubicki, James D.
    Journal of Physical Chemistry B (2005), 109 (46), 21796-21807CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The interaction of an H2O mol. with cluster models of fractured silica surfaces was studied by means of quantum mech. calcns. Two clusters representing homolytic cleavage (≡Si• and ≡SiO•) and two representing heterolytic cleavage (≡Si+ and ≡Si-O-) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H2O have been calcd. and compare favorably with expt. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approx. -70 to -85 kJ/mol, depending on the computational methodol. These calcns. suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H2O. The reaction of H2O with ≡SiO• was predicted to generate OH•. Rate consts. for these reactions were also calcd. and predict a rapid equil. for the reaction ≡SiO• + H2O → ≡SiOH + OH•. Stability of a finite no. of ≡SiO• sites at equil. in the above reaction with H2O was also predicted, which implies a long-term ability of silica surfaces to produce OH• radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.
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    Xu, S.; Kropscott, B. Evaluation of the Three-Phase Equilibrium Method for Measuring Temperature Dependence of Internally Consistent Partition Coefficients (KOW, KOA, and KAW) for Volatile Methylsiloxanes and Trimethylsilanol. Environ. Toxicol. Chem. 2014, 33, 2702– 2710,  DOI: 10.1002/etc.2754
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    Evaluation of the three-phase equilibrium method for measuring temperature dependence of internally consistent partition coefficients (KOW, KOA, and KAW) for volatile methylsiloxanes and trimethylsilanol
    Xu, Shihe; Kropscott, Bruce
    Environmental Toxicology and Chemistry (2014), 33 (12), 2702-2710CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
    Partitioning equil. and their temp. dependence of chems. between different environmental media are important in detg. the fate, transport, and distribution of contaminants. Unfortunately, internally consistent air/water (KAW), 1-octanol/air (KOA), and 1-octanol/water (KOW) partition coeffs., as well as information on their temp. dependence, are scarce for organosilicon compds. because of the reactivity of these compds. in water and octanol and their extreme partition coeffs. A newly published 3-phase equil. method was evaluated for simultaneous detn. of the temp. dependence of KAW, KOA, and KOW of 5 volatile methylsiloxanes (VMS) and trimethylsilanol (TMS) in a temp. range from 4° to 35°. The measured partition coeffs. at the different temps. for any given compd., and the enthalpy and entropy changes for the corresponding partition processes, were all internally consistent, suggesting that the 3-phase equil. method is suitable for this type of measurement. Compared with common environmental contaminants reported in the literature, VMS have enthalpy and entropy relationships similar to those of alkanes for air/water partitioning and similar to those of polyfluorinated compds. for octanol/air partitioning, but more like those for benzoates and phenolic compds. for octanol/water partitioning. The temp. dependence of the partition coeffs. of TMS is different from those of VMS and is more like that of alcs., phenols, and sulfonamides.
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    Donahue, N. M.; Robinson, A. L.; Pandis, S. N. Atmospheric Organic Particulate Matter: From Smoke to Secondary Organic Aerosol. Atmos. Environ. 2009, 43, 94– 106,  DOI: 10.1016/j.atmosenv.2008.09.055
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    Atmospheric organic particulate matter: From smoke to secondary organic aerosol
    Donahue, Neil M.; Robinson, Allen L.; Pandis, Spyros N.
    Atmospheric Environment (2008), 43 (1), 94-106CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    A review. We present an overview of the development of our understanding of the sources, formation mechanisms, phys. and chem. transformations of atm. org. aerosol (OA) during the last thirty years. Until recently, org. particulate material was simply classified as either primary or secondary with the primary component being treated in models as nonvolatile and inert. However, this oversimplified view fails to explain the highly oxygenated nature of ambient OA, the relatively small OA concn. gradients between urban areas and their surroundings, and the concns. of OA during periods of high photochem. activity. A unifying framework for the description of all components based on their volatility distribution (the volatility basis set) can be used for the treatment of a wide range of processes affecting org. aerosol loadings and compn. in the atm. These processes include direct org. particle and vapor emissions, chem. prodn. of org. PM from volatile precursors, chem. reactions (aging) in all phases, as well as deposition of both particles and vapors and chem. losses to volatile products. The combination of this new framework with the recent results of lab. studies can resolve some of the discrepancies between OA observations and lab. results. The mass balance of the org. material as a function of its volatility is investigated and used to frame the corresponding constraints on the system. Finally we revisit the traditional definitions of primary and secondary org. aerosol and propose a new set of terms and definitions based on the improvements of our understanding.
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    A crit. review of methods to det. and interpret the best available values and environmental data for volatile methylsiloxanes (VMS), particularly partitioning properties, to accurately predict their environmental fate, distribution, transport, exposure and potential effects is given. Measured partition properties included air/water (KAW), octanol/water (KOW), and octanol/air partitioning coeffs. (KOA), soil org. C/water distribution coeff. (KOC), biol. medium/fluid partition coeffs., and their temp. dependence. Based on the results, organosilicon compds. (VMS) are expected to behave differently in the environment vs. conventional hydrophobic environmental pollutants due to their inherent properties related to mol. size and capacity for different types of mol. interactions which control partitioning. Topics discussed include: introduction; background (partition coeffs. and their concn. and temp. dependence, internal consistency of environmental partition coeffs., methods to quantify VMS in partition media, solvolysis of organosiloxanes and organosilanols in partition media); methods to det. partition properties (sep. partition coeff. detn. [air/water, octanol/water, octanol/air]; simultaneous partition coeff. detn.); room temp. partition coeff.; temp. difference; soil/water distribution coeffs.; biol. medium/fluid partition coeffs.; environmental implications (risks of using single parameter quant. structure-property relationships outside domain, exposure limits in soil and sediment, VMS distribution and overall persistence as predicted by partition coeffs., potential for VMS long-range transport as predicted by partition coeffs.); and supplementary information (VMS exposure limit calcn. for soil and sediment ad estg. VMS deposition to remote surface media).
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  2. Christopher E. Brunet, Rachel F. Marek, Charles O. Stanier, Keri C. Hornbuckle. Concentrations of Volatile Methyl Siloxanes in New York City Reflect Emissions from Personal Care and Industrial Use. Environmental Science & Technology 2024, 58 (20) , 8835-8845. https://doi.org/10.1021/acs.est.3c10752
  3. Jeewani N. Meepage, Josie K. Welker, Claire M. Meyer, Saeideh Mohammadi, Charles O. Stanier, Elizabeth A. Stone. Advances in the Separation and Detection of Secondary Organic Aerosol Produced by Decamethylcyclopentasiloxane (D5) in Laboratory-Generated and Ambient Aerosol. ACS ES&T Air 2024, 1 (5) , 365-375. https://doi.org/10.1021/acsestair.3c00073
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  • Abstract

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    Figure 1

    Figure 1. Example time series of D3 oxidation with H2O2 as oxidant precursors. The shading signifies when the lights are on, and the dashed vertical line signifies when 10% of D3 had been oxidized. Signals averaged to 1 min time resolution are shown.

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    Figure 2

    Figure 2. Example cVMS oxidation product structures.

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    Figure 3

    Figure 3. Signal yield of different cVMS oxidation products when RO2 was most likely to react with (a, d, g) NO, (b, e, h) HO2, or (c, f, i) when unimolecular reactions are favored, divided by the signal of the siloxane lost at that point. Panels (a–c) are for D3, (d–f) for D4, and (g–i) for D5 oxidation. The signals for siloxanediol, ether hydroperoxide, and ether nitrate are multiplied by 10 and striped for visual clarity.

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    Scheme 1

    Scheme 1. Potential Reactions of cVMS in High NOx/HO2 Conditions
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    Scheme 2

    Scheme 2. Potential Reactions of cVMS in Low NOx/HO2 Condition
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    Figure 4

    Figure 4. Signal yields of formate ester (a–c) and siloxanol (d–f) for all three cVMS from the experimental results and kinetic modeling using the mechanism presented, normalized to the results from the RO2 + NO conditions. Panels (a) and (d) are for D3 oxidation, (b) and (e) are for D4 oxidation, and (c) and (f) for D5. Note the log axis on siloxanol results. Only the rate constants for reactions with OH/Cl and wall loss constants were changed between cVMS. In the model results that had a source of siloxanol from the RO2 + NO reaction, 5% of the reaction product made siloxanol directly, and the other 95% made RO.

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    Figure 5

    Figure 5. Estimated Henry’s solubility, estimated with HenryWIN v3.21, for D3 and the siloxanol oxidation products (dashed lines). The mass concentration labels on the solid lines correspond to the amount of liquid water in clouds or ambient aerosol. Only siloxanol products are shown as the hydroxyl groups change the Henry’s solubility most significantly. Larger cVMS oxidation products are estimated to have lower Henry’s solubility than D3. All values are listed in Table S6 of the Supporting Information. Adapted with permission from Daumit, K. E.; Carrasquillo, A. J.; Hunter, J. F.; Kroll, J. H. Laboratory Studies of the Aqueous-Phase Oxidation of Polyols: Submicron Particles vs Bulk Aqueous Solution. Atmos. Chem. Phys. 2014, 14 (19), 10773–10784. https://doi.org/10.5194/acp-14-10773-2014.

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      Volatile Me siloxanes (VMS) are ubiquitous anthropogenic pollutants which have recently been scrutinized for their potential toxicity and environmental persistence. This work detd. rate consts. for oxidn. by OH- and Cl atoms at 297 ± 3° K and atm. pressure over Boulder, Colorado (∼860 mbar) of hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5). Measured rate consts. for OH- reactions were (1.20 ± 0.09) x 10-12, (1.7 ± 0.1) x 10-12, (2.5 ± 0.2) x 10-12, (3.4 ± 0.5) x 10-12, (0.86 ± 0.09) x 10-12, (1.3 ± 0.1) x 10-12, and (2.1 ± 0.1) x 10-12 cm3/mol-s for L2, L3, L4, L5, D3, D4, and D5, resp.; measured rate consts. for Cl atom reactions for the same compds. were (1.44 ± 0.05) x 10-10, (1.85 ± 0.05) x 10-10, (2.2 ± 0.1) x 10-10, (2.9 ± 0.1) x 10-10, (0.56 ± 0.05) x 10-10, (1.16 ± 0.08) x 10-10, and (1.8 ± 0.1) x 10-10 cm3/mol-s, resp. Substituent factors of F(-Si(CH3)2OR) and F(-SiCH3(OR)2) were proposed for use in AOPWIN, a common model for OH- rate const. estns. Cl atoms can globally remove VMS percentage levels with potentially increased importance in urban areas.
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      Markgraf, Stewart J.; Wells, J. R.
      International Journal of Chemical Kinetics (1997), 29 (6), 445-451CODEN: IJCKBO; ISSN:0538-8066. (Wiley)
      The relative rate technique was used to measure the hydroxyl radical (OH) reaction rate const. of hexamethyldisiloxane (Me3SiOSiMe3), octamethyltrisiloxane (Me3SiOSiMe2OSiMe3) and decamethyltetrasiloxane (Me3SiOSiMe2OSiMe2OSiMe3). Hexamethyldisiloxane, octamethyltrisiloxane, and decamethyltetrasiloxane react with OH with bimol. rate consts. of 1.32 ± 0.05 × 10-12 cm3mol.-1s-1, 1.83 ± 0.09 x 10-12cm3mol.-1s-1 and 2.66 ± 0.13 × 10-12 cm3mol.-1s-1 resp. Study of the OH + siloxane reaction products yielded trimethylsilanol, pentamethyldisiloxanol, heptamethyltetrasiloxanol, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, and other compds. Several of these products were not reported before because these siloxanes and the proposed reaction mechanisms yielding these products are complicated. Some unusual cyclic siloxane products were obsd. and their formation pathways are discussed in light of current understanding of siloxane atm. chem.
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    8. 8
      Sommerlade, R.; Parlar, H.; Wrobel, D.; Kochs, P. Product Analysis and Kinetics of the Gas-Phase Reactions of Selected Organosilicon Compounds with OH Radicals Using a Smog Chamber-Mass Spectrometer System. Environ. Sci. Technol. 1993, 27, 2435– 2440,  DOI: 10.1021/es00048a019
      8
      Product analysis and kinetics of the gas-phase reactions of selected organosilicon compounds with OH radicals using a smog chamber-mass spectrometer system
      Sommerlade, Ronald; Parlar, Harun; Wrobel, Dieter; Kochs, Peter
      Environmental Science and Technology (1993), 27 (12), 2435-40CODEN: ESTHAG; ISSN:0013-936X.
      Relative rate consts. for the gas-phase reactions of tetramethylsilane (I), hexamethyldisiloxane (II), trimethylsilanol (III), and octamethylcyclotetrasiloxane (IV) with OH radicals have been detd. at 297 ± 2 K and 70 Torr in a 20-L smog chamber coupled to a quadrupole mass spectrometer. Some of the reaction products of IV could be detected and their structure elucidated by GC-MS and GC-FTIR. The rate consts. obtained for the OH-radical reactions were 1.28 ± 0.46 × 10-12 for I, 1.19 ± 0.30 × 10-12 for II, 3.95 ± 0.95 × 10-12 for III, and 1.26 ± 0.40 × 10-12 cm3/mol. s for IV. Indicated errors are 2-least-squares std. deviations. The calcd. lifetimes of these organosilicon compds. in the troposphere, due to reactions with the OH radicals, range from ca. 2 to 9 days by use of tropospheric concns. of OH radicals of 7.7 × 105 mol./cm3 over a 24-h period.
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    9. 9
      Safron, A.; Strandell, M.; Kierkegaard, A.; Macleod, M. Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical. Int. J. Chem. Kinet. 2015, 47, 420– 428,  DOI: 10.1002/kin.20919
      9
      Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical
      Safron, Andreas; Strandell, Michael; Kierkegaard, Amelie; MacLeod, Matthew
      International Journal of Chemical Kinetics (2015), 47 (7), 420-428CODEN: IJCKBO; ISSN:0538-8066. (John Wiley & Sons, Inc.)
      Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile Me siloxanes (cVMS) from air. We present new measurements of second-order rate consts. for reactions of the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) with OH detd. at temps. between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a ref. substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry anal. When extrapolated to 298 K, our measured reaction rate consts. of D4 and D5 with the OH radical are 1.9 × 10-12 (95% confidence interval (CI): (1.7-2.2) × 10-12) and 2.6 × 10-12 (CI: (2.3-2.9) × 10-12) cm3 mol.-1 s-1, resp., which are 1.9× and 1.7× faster than previous measurements. Our measured rate const. for D6 is 2.8 × 10-12 (CI: (2.5-3.2) × 10-12) cm3 mol.-1 s-1 and to our knowledge there are no comparable lab. measurements in the literature. Reaction rates for D5 were 33% higher than for D4 (CI: 30-37%), whereas the rates for D6 were only 8% higher than for D5 (CI: 5-10%). The activation energies of the reactions of D4, D5, and D6 with OH were not statistically different and had a value of 4300 ± 2800 J/mol.
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    10. 10
      Bernard, F.; Papanastasiou, D. K.; Papadimitriou, V. C.; Burkholder, J. B. Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes. J. Phys. Chem. A 2018, 122, 4252– 4264,  DOI: 10.1021/acs.jpca.8b01908
      10
      Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes
      Bernard, Francois; Papanastasiou, Dimitrios K.; Papadimitriou, Vassileios C.; Burkholder, James B.
      Journal of Physical Chemistry A (2018), 122 (17), 4252-4264CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Permethylsiloxanes are emitted into the atm. during prodn. and use as personal care products, lubricants, and cleaning agents. The predominate atm. loss process for permethylsiloxanes is expected to be via gas-phase reaction with the OH radical. In this study, rate coeffs., k(T), for the OH radical gas-phase reaction with the two simplest linear and cyclic permethylsiloxanes were measured using a pulsed laser photolysis-laser induced fluorescence technique over the temp. range of 240-370 K and a relative rate method at 294 K: hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; hexamethylcyclotrisiloxane ([-Si(CH3)2O-]3, D3), k3; and octamethylcyclotetrasiloxane ([-Si(CH3)2O-]4, D4), k4. The obtained k(294 K) values and temp.-dependence expressions for the 240-370 K temp. range are (cm3 mol.-1 s-1, 2σ abs. uncertainties): k1(294 K) = (1.28 ± 0.08) × 10-12, k1(T) = (1.87 ± 0.18) × 10-11 exp(-(791 ± 27)/T); k2(294 K) = (1.72 ± 0.10) × 10-12, k2(T) = 1.96 × 10-13 (T/298)4.34 exp(657/T); k3(294 K) = (0.82 ± 0.05) × 10-12, k3(T) = (1.29 ± 0.19) × 10-11 exp(-(805 ± 43)/T); and k4(294 K) = (1.12 ± 0.10) × 10-12, k4(T) = (1.80 ± 0.26) × 10-11 exp(-(816 ± 43)/T). The cyclic mols. were found to be less reactive than the analogous linear mol. with the same no. of -CH3 groups, while the linear and cyclic permethylsiloxane reactivity both increase with the increasing no. of CH3- groups. The present results are compared with previous rate coeff. detns. where available. The permethylsiloxanes included in this study are atmospherically short-lived compds. with estd. atm. lifetimes of 11, 8, 17, and 13 days, resp.
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    11. 11
      Kim, J.; Xu, S. Quantitative Structure-Reactivity Relationships of Hydroxyl Radical Rate Constants for Linear and Cyclic Volatile Methylsiloxanes. Environ. Toxicol. Chem. 2017, 36, 3240– 3245,  DOI: 10.1002/etc.3914
      11
      Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes
      Kim, Jaeshin; Xu, Shihe
      Environmental Toxicology and Chemistry (2017), 36 (12), 3240-3245CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial to det. their environmental persistence and concns. Although oxidn. by atm. OH- is considered a major airborne VMS degrdn. mechanism, the existing bimol. rate consts. with OH- measured and modeled for any given VMS compd. varied greatly, depending on the approach used to generate the data. This work measured OH- reaction rate consts. for four cyclic and four linear VMS, based on a relative rate method using a newly designed atm. chamber, and established structure-reactivity relationships for the kinetics of same. In the past, VMS reaction rate consts. were generally recognized to increase with the no. of Me groups/mol., the only differential factor in existing models; however, new measurements indicated that mol. structure should also be considered to predict reaction rates. Better empirical models were developed by simple and multiple linear regressions of measured values from this work and the literature. A high correlation existed for reaction rates with the no. of the Me group attached at two distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or two mol. descriptors which were directly related to VMS size, which, in turn, depended on the no. of Me groups and linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;9999:1-6. © 2017 SETAC.
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    12. 12
      Tuazon, E. C.; Aschmann, S. M.; Atkinson, R. Atmospheric Degradation of Volatile Methyl-Silicon Compounds. Environ. Sci. Technol. 2000, 34, 1970– 1976,  DOI: 10.1021/es9910053
      12
      Atmospheric degradation of volatile methyl-silicon compounds
      Tuazon, Ernesto C.; Aschmann, Sara M.; Atkinson, Roger
      Environmental Science and Technology (2000), 34 (10), 1970-1976CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The kinetics of the gas-phase reactions of dimethylsilanediol, trimethylsilanol, and tetramethylsilane (degrdn. products of poly(dimethylsiloxane)s (PDMS)) with the OH radical were measured using a relative rate method which employed the N2H4 + O3 reaction as a nonphotolytic source of OH radicals, with anal. by FT-IR spectroscopy in a 5870 L chamber. The measured values of the OH radical reaction rate consts. (cm3 mol.-1 s-1) at 298 ± 2 K are as follows: dimethylsilanediol 8.1 × 10-13; trimethylsilanol 7.2 × 10-13; and tetramethylsilane 8.5 × 10-13. These values lead to an est. of tropospheric lifetimes with respect to reaction with the OH radical of ∼15 days for these organosilicon compds. FT-IR spectroscopy and atm. pressure ionization mass spectrometry (API-MS) were employed to analyze the products of OH radical- and Cl atom-initiated photooxidns. of dimethylsilanediol and trimethylsilanol. IR signatures of the probable formate ester intermediate products from both silanols were detected. API-MS analyses indicated the formation of methylsilanetriol from dimethylsilanediol, of both dimethylsilanediol and methylsilanetriol from trimethylsilanol, and of the corresponding intermediate formate esters. Possible reaction mechanisms are discussed.
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    13. 13
      Genualdi, S.; Harner, T.; Cheng, Y.; MacLeod, M.; Hansen, K. M.; Van Egmond, R.; Shoeib, M.; Lee, S. C. Global Distribution of Linear and Cyclic Volatile Methyl Siloxanes in Air. Environ. Sci. Technol. 2011, 45, 3349– 3354,  DOI: 10.1021/es200301j
      13
      Global Distribution of Linear and Cyclic Volatile Methyl Siloxanes in Air
      Genualdi, Susie; Harner, Tom; Cheng, Yu; MacLeod, Matthew; Hansen, Kaj M.; van Egmond, Roger; Shoeib, Mahiba; Lee, Sum Chi
      Environmental Science & Technology (2011), 45 (8), 3349-3354CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The global distribution of linear and cyclic volatile Me siloxanes (VMS) was examd. at 20 sites worldwide, including 5 sites in the Arctic, using sorbent-impregnated polyurethane foam (SIP) disk passive air samplers. Cyclic VMS are currently being considered for regulation because they are high prodn. vol. chems. which are potentially persistent, bioaccumulative, and toxic. Linear and cyclic VMS (L3, L4, L5, D3, D4, D5, D6) were analyzed for in air at all urban, background, and Arctic sites. D3 and D4 concns. were significantly correlated, as were D5 and D6, suggesting different sources for these 2 pair of compds. Elevated D3 and D4 concns. on the west North America coast and at high elevation sites suggested these sites are affected by trans-Pacific transport; D5 and D6 had elevated concns. in urban areas, most likely due to personal care product use. Measured D5 concns. were compared to modeled concns. generated by the Danish Eulerian Hemispheric Model (DEHM) and the Berkeley-Trent Global Contaminant Fate Model (BETR Global). Correlation coeffs. (r) between measured and modeled results were 0.73 and 0.58 for the DEHM and BETR models, resp. Agreement between measurements and models indicated global atm. D5 sources, transport pathways, and sinks are fairly well understood.
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    14. 14
      Xu, S.; Warner, N.; Bohlin-Nizzetto, P.; Durham, J.; McNett, D. Long-Range Transport Potential and Atmospheric Persistence of Cyclic Volatile Methylsiloxanes Based on Global Measurements. Chemosphere 2019, 228, 460– 468,  DOI: 10.1016/j.chemosphere.2019.04.130
      14
      Long-range transport potential and atmospheric persistence of cyclic volatile methylsiloxanes based on global measurements
      Xu, Shihe; Warner, Nicholas; Bohlin-Nizzetto, Pernilla; Durham, Jeremy; McNett, Debra
      Chemosphere (2019), 228 (), 460-468CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
      This study investigates persistence (P) and long-range transport potential (LRTP) of cyclic volatile methylsiloxanes (cVMS) based on the field measurements in the Northern Hemisphere. The field data consisted of published outdoor air concns. of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) at urban, suburban, rural and remote locations excluding the point sources. Three major trends were obsd. First, D4 and D6 concns. were correlated with measured concns. for D5 at the same times and locations in the majority of the datasets, reflecting the common sources and similar removal mechanism(s) for these compds. Second, as the sampling sites changed from the source to remote locations along a south-to-north transect, av. cVMS concns. in air decreased in an exponential manner. The empirical characteristic travel distances (eCTD) extd. from these spatial patterns were smaller than model estd. values and differed in order among individual compds. (D4 ∼ D5 < D6). Finally, D5/D6 concn. ratios were also found to decrease exponentially along the same spatial gradient, contrary to model predictions of an increase based on current knowledge of mechanisms controlling atm. cVMS degrdn. These findings suggest that there may be addnl. removal process(es) for airborne cVMS, currently not accounted for, that requires further elucidation.
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    15. 15
      Warner, N. A.; Evenset, A.; Christensen, G.; Gabrielsen, G. W.; Borgä, K.; Leknes, H. Volatile Siloxanes in the European Arctic: Assessment of Sources and Spatial Distribution. Environ. Sci. Technol. 2010, 44, 7705– 7710,  DOI: 10.1021/es101617k
      15
      Volatile Siloxanes in the European Arctic: Assessment of Sources and Spatial Distribution
      Warner, Nicholas A.; Evenset, Anita; Christensen, Guttorm; Gabrielsen, Geir W.; Borga, Katrine; Leknes, Henriette
      Environmental Science & Technology (2010), 44 (19), 7705-7710CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The purpose of this study was to investigate presence and potential accumulation of cyclic volatile Me siloxanes (cVMS) in the Arctic environment. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) were analyzed in sediment, zooplankton, Atlantic cod (Gadus morhua), shorthorn sculpin (Myoxocephalus scorpius), and bearded seal (Erignathus barbatus) collected from the Svalbard archipelago within the European Arctic in July 2009. Highest levels were found for D5 in fish collected from Adventfjorden, with av. concns. of 176 and 531 ng/g lipid in Atlantic cod and shorthorn sculpin, resp. Decreasing concn. of D5 in sediment collected away from waste water outlet in Adventfjorden indicates that the local settlement of Longyearbyen is a point source to the local aquatic environment. Median biota sediment accumulation factors (BSAFs) calcd. for D5 in Adventfjorden were 2.1 and 1.5 for Atlantic cod and shorthorn sculpin, resp. Biota concns. of D5 were lower or below detection limits in remote and sparsely populated regions (Kongsfjorden and Liefdefjorden) compared to Adventfjorden. The levels of cVMS were found to be low or below detection limits in bearded seal blubber and indicate a low risk for cVMS accumulation within mammals. Accumulation of cVMS in fish appears to be influenced by local exposure from human settlements within the Arctic.
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    16. 16
      Brooke, D.; Crookes, M.; Gray, D.; Robertson, S. Environmental Risk Assessment Report: Decamethylcyclopentasiloxane; Environment Agency, 2009.
      There is no corresponding record for this reference.
    17. 17
      Brooke, D. N.; Brooke, M. J.; Gray, D.; Robertson, S.; Crookes, M.; Gray, D.; Robertson, S. Environmental Risk Assessment Report: Octamethylcyclotetrasiloxane; Environment Agency, 2009.
      There is no corresponding record for this reference.
    18. 18
      Allen, R. B.; Annelin, R. B.; Atkinson, R.; Carpenter, J. C.; Carter, W. L. P.; Chandra, G.; Fendinger, N. J.; Gerhards, R.; Grigoras, S.; Hatcher, J. A.; Hobson, J. F.; Kochs, P.; Lehmann, R. G.; Maxim, L. D.; Mazzoni, S. M.; Mihaich, E. M.; Miyakawa, Y.; Pohl, E. R.; Powell, D. E.; Roy, S.; Sawano, T.; Slater, G. S.; Spivack, J. L.; Stevens, C.; Wischer, D. Organosilicon Materials. In The Handbook of Environmental Chemistry; Chandra, G., Ed.; Springer: Berlin, Heidelberg, 1997; Vol. 3.
      There is no corresponding record for this reference.
    19. 19
      European Chemicals Agency. Recommendation of the European Chemicals Agency of 20 December 2011 for the Inclusion of Substances in Annex XIV to REACH (List of Substances Subject to Authorisation) of Regulation (EC) No 1907/2006; European Chemicals Agency, 2011; Vol. 1, pp 1– 7.
      There is no corresponding record for this reference.
    20. 20
      United Kingdom Health & Safety Executive. Annex XV Restriction Report Proposal for a Restriction; United Kingdom Health & Safety Executive, 2015; pp 1– 89.
      There is no corresponding record for this reference.
    21. 21
      European Chemicals Agency. Recommendation of the European Chemicals Agency of 14 April 2021 for the Inclusion of Substances in Annex XIV to REACH (List of Substances Subject to Authorisation); EPA, 2021; Vol. 1, pp 1– 7.
      There is no corresponding record for this reference.
    22. 22
      Wu, Y.; Johnston, M. V. Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes. J. Am. Soc. Mass Spectrom. 2016, 27, 402– 409,  DOI: 10.1007/s13361-015-1300-1
      22
      Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes
      Wu, Yue; Johnston, Murray V.
      Journal of the American Society for Mass Spectrometry (2016), 27 (3), 402-409CODEN: JAMSEF; ISSN:1044-0305. (Springer)
      Cyclic volatile methylsiloxanes (cVMS) have been identified as important gas-phase atm. pollutants, but knowledge of the mol. compn. of secondary aerosol derived from cVMS oxidn. is incomplete. This work characterized the chem. compn. of secondary aerosols produced from OH--initiated oxidn. of decamethylcyclopentasiloxane (D5, C10H30O5Si5) by high performance mass spectrometry. Electro-spray ionization mass spectrometry (ESI-MS) showed a large no. of monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) oxidn. products. High resoln. and tandem mass spectrometry showed that oxidn. leads mainly to a CH3 group substitution by OH or CH2OH; a single mol. can undergo many CH3 group substitutions. Dimers also exhibit OH and CH2OH substitutions and can be linked by O, CH2, and CH2CH2 groups. Gas chromatog.-mass spectrometry confirmed ESI-MS results. D4 (C8H24O4Si4) oxidn. exhibited similar substitutions and oligomerizations to D5, though the degree of oxidn. was greater under the same conditions, and there was direct evidence for peroxy group (CH2OOH) formation in addn. to OH and CH2OH.
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    23. 23
      Yucuis, R. A.; Stanier, C. O.; Hornbuckle, K. C. Cyclic Siloxanes in Air, Including Identification of High Levels in Chicago and Distinct Diurnal Variation. Chemosphere 2013, 92, 905– 910,  DOI: 10.1016/j.chemosphere.2013.02.051
      23
      Cyclic siloxanes in air, including identification of high levels in Chicago and distinct diurnal variation
      Yucuis, Rachel A.; Stanier, Charles O.; Hornbuckle, Keri C.
      Chemosphere (2013), 92 (8), 905-910CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
      The organosilicon compds. octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are high prodn. vol. chems. that are widely used in household goods and personal care products. Due to their prevalence and chem. characteristics, cyclic siloxanes are being assessed as possible persistent org. pollutants. D4, D5, and D6 were measured in indoor and outdoor air to quantify and compare siloxane concns. and compd. ratios depending on location type. Indoor air samples had a median concn. of 2200 ng m-3 for the sum of D4, D5, and D6. Outdoor sampling locations included downtown Chicago, Cedar Rapids, IA, and West Branch, IA, and had median sum siloxane levels of 280, 73, and 29 ng m-3 resp. A diurnal trend is apparent in the samples taken in downtown Chicago. Nighttime samples had a median 2.7 times higher on av. than daytime samples, which is due, in part, to the fluctuations of the planetary boundary layer. D5 was the dominant siloxane in both indoor and outdoor air. Ratios of D5 to D4 averaged 91 and 3.2 for indoor and outdoor air resp.
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    24. 24
      Milani, A.; Al-Naiema, I. M.; Stone, E. A. Detection of a Secondary Organic Aerosol Tracer Derived from Personal Care Products. Atmos. Environ. 2021, 246, 118078  DOI: 10.1016/j.atmosenv.2020.118078
      24
      Detection of a secondary organic aerosol tracer derived from personal care products
      Milani, Alissia; Al-Naiema, Ibrahim M.; Stone, Elizabeth A.
      Atmospheric Environment (2021), 246 (), 118078CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      Decamethylcyclopentasiloxane (D5) is frequently used in personal care products (PCPs). In the gas phase, D5 is oxidized to form 1-hydroxynonamethylcyclopentasiloxane (D4TOH), which can partition to the particle phase. Numerous studies have reported secondary org. aerosol (SOA) formation via hydroxyl (OH) radical-initiated oxidn. of D5. It is expected that PCPs have a significant impact on SOA, but the extent has not yet been investigated. To date, no studies have reported the occurrence of PCP-derived SOA in ambient particulate matter. This study examd. fine particulate matter (PM2.5) collected in Atlanta, GA and Houston, TX and detd. D4TOH was present in 28 of 29 and 33 of 46 ambient PM2.5 samples, resp. Gas chromatog. retention data in the form of the Kova´ts index is reported for the first time to aid others in identifying this compd. in order to assess the impact of PCPs on SOA formation. The estd. concn. of D4TOH ranged from 16 to 185 pg m-3 in Atlanta and 19-206 pg m-3 in Houston. Synthetic musks were also detected in Atlanta and Houston PM2.5 samples, which is consistent with PCPs impacting urban air quality. Because of its specificity and demonstrated detectability, D4TOH may be useful as a tracer of PCP-derived secondary org. aerosol.
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    25. 25
      Ahrens, L.; Harner, T.; Shoeib, M. Temporal Variations of Cyclic and Linear Volatile Methylsiloxanes in the Atmosphere Using Passive Samplers and High-Volume Air Samplers. Environ. Sci. Technol. 2014, 48, 9374– 9381,  DOI: 10.1021/es502081j
      25
      Temporal variations of cyclic and linear volatile methylsiloxanes in the atmosphere using passive samplers and high-volume air samplers
      Ahrens, Lutz; Harner, Tom; Shoeib, Mahiba
      Environmental Science & Technology (2014), 48 (16), 9374-9381CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Cyclic and linear volatile methylsiloxanes (cVMSs and lVMSs, resp.) were measured in ambient air over a period of over one year in Toronto, Canada. Air samples were collected using passive air samplers (PAS) consisting of sorbent-impregnated polyurethane foam (SIP) disks in parallel with high vol. active air samplers (HV-AAS). The av. difference between the SIP-PAS derived concns. in air for the individual VMSs and those measured using HV-AAS was within a factor of 2. The air concns. (HV-AAS) ranged 22-351 ng m-3 and 1.3-15 ng m-3 for ΣcVMSs (D3, D4, D5, D6) and ΣlVMSs (L3, L4, L5), resp., with decamethylcyclopentasiloxane (D5) as the dominant compd. (∼75% of the ΣVMSs). Air masses arriving from north to northwest (i.e., less populated areas) were significantly less contaminated with VMSs compared to air arriving from the south that are impacted by major urban and industrial areas in Canada and the U.S. (p < 0.05). In addn., air concns. of ΣcVMSs were lower during major snowfall events (on av., 73 ng m-3) in comparison to the other sampling periods (121 ng m-3). Ambient temp. had a small influence on the seasonal trend of VMS concns. in air, except for dodecamethylcyclohexasiloxane (D6), which was pos. correlated with the ambient temp. (p < 0.001).
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    26. 26
      Gallego, E.; Perales, J. F.; Roca, F. J.; Guardino, X.; Gadea, E. Volatile Methyl Siloxanes (VMS) Concentrations in Outdoor Air of Several Catalan Urban Areas. Atmos. Environ. 2017, 155, 108– 118,  DOI: 10.1016/j.atmosenv.2017.02.013
      26
      Volatile methyl siloxanes (VMS) concentrations in outdoor air of several Catalan urban areas
      Gallego, E.; Perales, J. F.; Roca, F. J.; Guardino, X.; Gadea, E.
      Atmospheric Environment (2017), 155 (), 108-118CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      Volatile Me siloxanes (VMS) were evaluated in ten Catalan urban areas with different industrial impacts, such as petrochem. industry, elec. and mech. equipment, metallurgical and chem. industries, municipal solid waste treatment plant and cement and food industries, during 2013-2015. 24 H samples were taken with LCMA-UPC pump samplers specially designed in our lab., with a flow range of 70 mL min-1. A sorbent-based sampling method, successfully developed to collect a wide-range of VOC, was used. The anal. was performed by automatic thermal desorption coupled with capillary gas chromatog./mass spectrometry detector. The presented methodol. allows the evaluation of VMS together with a wide range of other VOC, increasing the no. of compds. that can be detd. in outdoor air quality assessment of urban areas. This aspect is esp. relevant as a restriction of several VMS (D4 and D5) in consumer products has been made by the European Chems. Agency and US EPA is evaluating to include D4 in the Toxic Substances Control Act, regarding the concern of the possible effects of these compds. in human health and the environment. ΣVMS concns. (L2-L5, D3-D6 and trimethylsilanol) varied between 0.3 ± 0.2μg m-3 and 18 ± 12μg m-3, detd. in a hotspot area. Obsd. VMS concns. were generally of the same order of magnitude than the previously detd. in Barcelona, Chicago and Zurich urban areas, but higher than the published from suburban sites and Arctic locations. Cyclic siloxanes concns. were up to two-three orders of magnitude higher than those of linear siloxanes, accounting for av. contributions to the total concns. of 97 ± 6% for all samples except for the hotspot area, where cyclic VMS accounted for 99.9 ± 0.1%. D5 was the most abundant siloxane in 5 sampling points; however, differing from the generally obsd. in previous studies, D3 was the most abundant compd. in the other 5 sampling points.
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    27. 27
      US EPA. Enforceable Consent Agreement for Environmental Testing for Octamethylcyclotetrasiloxane (D4) (CASRN 556-67-2) Docket No. EPA-HQ-OPPT-2012-0209; EPA, 2014.
      There is no corresponding record for this reference.
    28. 28
      Dudzina, T.; Von Goetz, N.; Bogdal, C.; Biesterbos, J. W. H.; Hungerbühler, K. Concentrations of Cyclic Volatile Methylsiloxanes in European Cosmetics and Personal Care Products: Prerequisite for Human and Environmental Exposure Assessment. Environ. Int. 2014, 62, 86– 94,  DOI: 10.1016/j.envint.2013.10.002
      28
      Concentrations of cyclic volatile methylsiloxanes in European cosmetics and personal care products: Prerequisite for human and environmental exposure assessment
      Dudzina, Tatsiana; von Goetz, Natalie; Bogdal, Christian; Biesterbos, Jacqueline W. H.; Hungerbuhler, Konrad
      Environment International (2014), 62 (), 86-94CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
      Low mol. wt. cyclic volatile methylsiloxanes (cVMSs) are widely employed as emollients and carrier solvents in personal care formulations in order to acquire desired performance benefits owing to their distinctive physicochem. properties. Under current European legislation cosmetic ingredients such as cVMSs are required to be labeled on the product package only qual., while for the assessment of environmental and consumer exposure quant. information is needed. The aim of this study was therefore to measure concns. of three cVMSs, namely octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) in 51 cosmetics and personal care products (C&PCPs) that are currently available on the European market. The list of selected articles comprised a variety of hair and sun care products, skin creams and lotions, deodorants including antiperspirants, liq. foundations and a toothpaste. The target compds. were extd. from the products with different org. solvents dependent on the product matrix, followed by gas chromatog. anal. with flame ionization detection (GC-FID). D5 was the predominant cVMS with the highest mean and median concns. in all the C&PCP categories. The median concns. of D5, D6 and D4 were 142, 2.3 and 0.053 mg/g in deodorants/antiperspirants (n = 11); 44.6, 30.0 mg/g and below the limit of quantification (< LOQ; LOQ for D4 = 0.00071 mg/g) in cosmetics (n = 5); 8.4, 0.32 mg/g and < LOQ in skin care (n = 16); 9.6, 0.18 and 0.0055 mg/g in hair care (n = 10); and, 34.8, 0.53 and 0.0085 mg/g in sun care (n = 8) products, resp. The calcd. median aggregate daily dermal exposure to D4 and D5 from multiple C&PCPs was approx. 100 times lower than the current NOAEL derived from chronic inhalation rat studies.
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    29. 29
      Bzdek, B. R.; Horan, A. J.; Pennington, M. R.; Janechek, N. J.; Baek, J.; Stanier, C. O.; Johnston, M. V. Silicon Is a Frequent Component of Atmospheric Nanoparticles. Environ. Sci. Technol. 2014, 48, 11137– 11145,  DOI: 10.1021/es5026933
      29
      Silicon is a Frequent Component of Atmospheric Nanoparticles
      Bzdek, Bryan R.; Horan, Andrew J.; Pennington, M. Ross; Janechek, Nathan J.; Baek, Jaemeen; Stanier, Charles O.; Johnston, Murray V.
      Environmental Science & Technology (2014), 48 (19), 11137-11145CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Nanoparticles are the largest fraction of aerosol loading by no. Knowledge of the chem. components of nanoparticulate matter is needed to understand nanoparticle health and climate impacts. This work presents nano aerosol mass spectrometer (NAMS) field measurements which provide quant. elemental compn. of ∼20 nm diam. nanoparticles. NAMS measurements indicated the element, Si, was a frequent nanoparticle component. Nanoparticulate Si was most abundant at sites heavily impacted by anthropogenic activity. Wind direction correlations suggested Si sources are diffuse; diurnal trends suggested nanoparticulate Si may result from photochem. processing of gaseous Si-contg. compds., e.g., cyclic siloxanes. Atm. modeling of oxidized cyclic siloxanes was consistent with a diffuse photochem. source of aerosol Si. More broadly, the observations indicated a previously overlooked anthropogenic source of nanoaerosol mass. Further investigation is needed to fully resolve its atm. role.
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    30. 30
      Janechek, N. J.; Hansen, K. M.; Stanier, C. O. Comprehensive Atmospheric Modeling of Reactive Cyclic Siloxanes and Their Oxidation Products. Atmos. Chem. Phys. 2017, 17, 8357– 8370,  DOI: 10.5194/acp-17-8357-2017
      30
      Comprehensive atmospheric modeling of reactive cyclic siloxanes and their oxidation products
      Janechek, Nathan J.; Hansen, Kaj M.; Stanier, Charles O.
      Atmospheric Chemistry and Physics (2017), 17 (13), 8357-8370CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Cyclic volatile Me siloxanes (cVMSs) are important components in personal care products that transport and react in the atm. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and their gas-phase oxidn. products have been incorporated into the Community Multiscale Air Quality (CMAQ) model. Gas-phase oxidn. products, as the precursor to secondary org. aerosol from this compd. class, were included to quantify the max. potential for aerosol formation from gas-phase reactions with OH. Four 1-mo periods were modeled to quantify typical concns., seasonal variability, spatial patterns, and vertical profiles. Typical model concns. showed parent compds. were highly dependent on population d. as cities had monthly averaged peak D5 concns. up to 432 ngm-3. Peak oxidized D5 concns. were significantly less, up to 9 ngm-3, and were located downwind of major urban areas. Model results were compared to available measurements and previous simulation results. Seasonal variation was analyzed and differences in seasonal influences were obsd. between urban and rural locations. Parent compd. concns. in urban and peri-urban locations were sensitive to transport factors, while parent compds. in rural areas and oxidized product concns. were influenced by large-scale seasonal variability in OH.
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    31. 31
      Chandramouli, B.; Kamens, R. M. The Photochemical Formation and Gas-Particle Partitioning of Oxidation Products of Decamethyl Cyclopentasiloxane and Decamethyl Tetrasiloxane in the Atmosphere. Atmos. Environ. 2001, 35, 87– 95,  DOI: 10.1016/S1352-2310(00)00289-2
      There is no corresponding record for this reference.
    32. 32
      Navea, J. G.; Xu, S.; Stanier, C. O.; Young, M. A.; Grassian, V. H. Heterogeneous Uptake of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) onto Mineral Dust Aerosol under Variable RH Conditions. Atmos. Environ. 2009, 43, 4060– 4069,  DOI: 10.1016/j.atmosenv.2009.05.012
      32
      Heterogeneous uptake of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) onto mineral dust aerosol under variable RH conditions
      Navea, Juan G.; Xu, Shihe; Stanier, Charles O.; Young, Mark A.; Grassian, Vicki H.
      Atmospheric Environment (2009), 43 (26), 4060-4069CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      We have carried out kinetic studies to characterize the heterogeneous decay of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in the presence of representative mineral dust aerosol in order to obtain a better understanding of the atm. fate of these siloxanes. The heterogeneous chem. of D4 and D5 with various mineral dusts was studied in an environmental aerosol reaction chamber using FTIR absorption spectroscopy to monitor the reaction. The apparent heterogeneous uptake coeff., γapp, for D4 and D5 with various mineral dusts was measured under dry conditions and as a function of relative humidity (RH). In addn., the effect of initial D4 and D5 concn. on the rate and yield of the reaction was examd. The uptake coeff., γapp, for D4 and D5 was similar for the most reactive aerosols tested, with kaolinite ≈ hematite > silica. Limited uptake onto carbon black and calcite surfaces was obsd. for either siloxane. Reaction with hematite and kaolinite resulted in multilayer coverages, suggesting extensive polymn. of D4 and D5 on the aerosol surface.
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    33. 33
      Janechek, N. J.; Marek, R. F.; Bryngelson, N.; Singh, A.; Bullard, R. L.; Brune, W. H.; Stanier, C. O. Physical Properties of Secondary Photochemical Aerosol from OH Oxidation of a Cyclic Siloxane. Atmos. Chem. Phys. 2019, 19, 1649– 1664,  DOI: 10.5194/acp-19-1649-2019
      33
      Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane
      Janechek, Nathan J.; Marek, Rachel F.; Bryngelson, Nathan; Singh, Ashish; Bullard, Robert L.; Brune, William H.; Stanier, Charles O.
      Atmospheric Chemistry and Physics (2019), 19 (3), 1649-1664CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Cyclic volatile Me siloxanes (cVMS) are high prodn. chems. present in many personal care products. They are volatile, hydrophobic, and relatively long lived due to slow oxidn. kinetics. Evidence from chamber and ambient studies indicates that oxidn. products may be found in the condensed aerosol phase. In this work, we use an oxidn. flow reactor to produce ∼ 100 μgm-3 of organosilicon aerosol from OH oxidn. of decamethylcyclopentasiloxane (D5) with aerosol mass fractions (i.e.,yields) of 0.2-0.5. The aerosols were assessed for concn., size distribution, morphol., sensitivity to seed aerosol, hygroscopicity, volatility and chem. compn. through a combination of aerosol size distribution measurement, tandem differential mobility anal., and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidn. conditions and chamber operation. The D5 oxidn. aerosol products were relatively non-hygroscopic, with an av. hygroscopicity kappa of ∼0.01, and nearly non-volatile up to 190 °C temp. Parameters for exploratory treatment as a semi-volatile org. aerosol in atm. models are provided.
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    34. 34
      Fairbrother, A.; Woodburn, K. B. Assessing the Aquatic Risks of the Cyclic Volatile Methyl Siloxane D4. Environ. Sci. Technol. Lett. 2016, 3, 359– 363,  DOI: 10.1021/acs.estlett.6b00341
      34
      Assessing the Aquatic Risks of the Cyclic Volatile Methyl Siloxane D4
      Fairbrother, Anne; Woodburn, Kent B.
      Environmental Science & Technology Letters (2016), 3 (10), 359-363CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      Assessing the ecol. risks of the widely used cyclic volatile Me siloxane D4 (octamethylcyclotetrasiloxane, CAS Registry No. 556-67-2) to aquatic systems is difficult because of its high volatility and low water soly., but the potential for long distance atm. transport and persistence in the sediments has placed D4 under intense regulatory scrutiny. This paper explores the difficulties inherent in detg. the toxicity of D4 to aquatic species and reveals the increased sensitivity of aquatic species tested within artificially closed systems compared to that of similar tests conducted in open systems that allow natural volatilization to occur. The concepts of narcosis mode of action and chem. activity explain the apparent lack of toxicity of D4 to aquatic species under environmentally realistic conditions. Discharge levels for the past 30 yr during which D4 has been in use have produced field-measured concns. that pose negligible risk to aquatic organisms.
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    35. 35
      Tang, X.; Misztal, P. K.; Nazaroff, W. W.; Goldstein, A. H. Siloxanes Are the Most Abundant Volatile Organic Compound Emitted from Engineering Students in a Classroom. Environ. Sci. Technol. Lett. 2015, 2, 303– 307,  DOI: 10.1021/acs.estlett.5b00256
      35
      Siloxanes Are the Most Abundant Volatile Organic Compound Emitted from Engineering Students in a Classroom
      Tang, Xiaochen; Misztal, Pawel K.; Nazaroff, William W.; Goldstein, Allen H.
      Environmental Science & Technology Letters (2015), 2 (11), 303-307CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      Direct human emissions are known to contribute volatile org. compds. (VOC) to indoor air by various mechanisms; however, few measurements which det. emissions of a full suite of occupant-assocd. VOC are available. This work measured occupant-related VOC emissions from engineering students in a classroom using a proton transfer reaction, time-of-flight mass spectrometry (PTR/TOF-MS). The dominant compd. emitted was a cyclic volatile methylsiloxane (cVMS), decamethylcyclopentasiloxane (D5), a major inactive ingredient in some personal care products, e.g., antiperspirant. D5 contributed ∼30% of the total indoor VOC mass concn. measured by the PTR/TOF-MS. Octamethylcyclotetrasiloxane (D4) and dodecamethylcyclohexasiloxane (D6) were detected at abundances 1-2 orders of magnitude lower. The per-person emission rate of these 3 cVMS declined monotonically from morning to afternoon, consistent with expectations for emissions from daily morning application of personal care products.
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    36. 36
      Fu, Z.; Xie, H.-B.; Elm, J.; Guo, X.; Fu, Z.; Chen, J. Formation of Low-Volatile Products and Unexpected High Formaldehyde Yield from the Atmospheric Oxidation of Methylsiloxanes. Environ. Sci. Technol. 2020, 54, 7136– 7145,  DOI: 10.1021/acs.est.0c01090
      36
      Formation of Low-Volatile Products and Unexpected High Formaldehyde Yield from the Atmospheric Oxidation of Methylsiloxanes
      Fu, Zihao; Xie, Hong-Bin; Elm, Jonas; Guo, Xirui; Fu, Zhiqiang; Chen, Jingwen
      Environmental Science & Technology (2020), 54 (12), 7136-7145CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      With more strict regulation of atm. volatile org. compds. originating from fossil fuel-based vehicles and industries, the use of volatile chem. products (VCP) and the VCP transformation mechanism have become increasingly important to quantify air quality. Volatile methylsiloxanes (VMS) are an important class of VCP and high-prodn. chems. Using quantum chem. calcns. and kinetics modeling, the authors examd. the reaction mechanism of peroxy radicals with VMS, key intermediates in detg. VMS atm. chem. L2-RSiCH2O2- and D3-RSiCH2O2- derived from hexamethyldisiloxane and hexamethylcyclotrisiloxane, resp., were selected as representative model systems. Results indicated L2-RSiCH2O2- and D3-RSiCH2O2- followed a novel Si-C-O rearrangement-driven autoxidn. mechanism, leading to formation of low volatile silanols and high yield of formaldehyde at low NO:HO2- conditions. At high NO/HO2- conditions, L2-RSiCH2O2- and D3-RSiCH2O2- reacted with NO/HO2- to form org. nitrate, hydroperoxide, and active alkoxy radicals. Alkoxy radicals further followed a Si-C-O rearrangement step to form formate esters. The novel Si-C-O rearrangement mechanism of peroxy and alkoxy radicals was supported by available exptl. studies on VMS oxidn. The high formaldehyde yield was estd. to significantly contribute to formaldehyde indoor air pollution, esp. during cleaning.
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    37. 37
      Ren, Z.; da Silva, G. Auto-Oxidation of a Volatile Silicon Compound: A Theoretical Study of the Atmospheric Chemistry of Tetramethylsilane. J. Phys. Chem. A 2020, 124, 6544– 6551,  DOI: 10.1021/acs.jpca.0c02922
      37
      Auto-Oxidation of a Volatile Silicon Compound: A Theoretical Study of the Atmospheric Chemistry of Tetramethylsilane
      Ren, Zhonghua; da Silva, Gabriel
      Journal of Physical Chemistry A (2020), 124 (32), 6544-6551CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Volatile Si compds. (VOSiC) are air pollutants which occur in indoor and outdoor environments. Tetramethylsilane (TMS) was selected as a model to examine photochem. oxidn. mechanisms for VOSiC using ab-initio and RRKM theory/master equation kinetic modeling. Under tropospheric conditions, the radical, (CH3)3SiCH2- reacts with O2 to produce a stabilized peroxyl radical, which is expected to ultimately yield the alkoxyl radical, (CH3)3SiCH2O-. However, at combustion-relevant temps., a well-skipping reaction to (CH3)3SiO- + HCHO dominates. The (CH3)3SiCH2O- radical is predicted to rearrange to (CH3)3SiOCH2- with a very low reaction barrier, enabling an auto-oxidn. process involving adding a second O2. Subsequent (CH3)3SiOCH2- oxidn. reaction mechanisms have been developed, with major product predicted to be trimethylsilyl formate, (CH3)3SiOCHO, an exptl. obsd. TMS oxidn. product. Prodn. of substantially oxygenated compds. following a single radical initiation reaction had implications for the VOSiC ability to contribute to O3 and particle formation in outdoor and indoor environments.
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    38. 38
      Coggon, M. M.; McDonald, B. C.; Vlasenko, A.; Veres, P. R.; Bernard, F.; Koss, A. R.; Yuan, B.; Gilman, J. B.; Peischl, J.; Aikin, K. C.; DuRant, J.; Warneke, C.; Li, S.; de Gouw, J. A. Diurnal Variability and Emission Pattern of Decamethylcyclopentasiloxane (D5) from the Application of Personal Care Products in Two North American Cities. Environ. Sci. Technol. 2018, 52, 5610– 5618,  DOI: 10.1021/acs.est.8b00506
      38
      Diurnal Variability and Emission Pattern of Decamethylcyclopentasiloxane (D5) from the Application of Personal Care Products in Two North American Cities
      Coggon, Matthew M.; McDonald, Brian C.; Vlasenko, Alexander; Veres, Patrick R.; Bernard, Francois; Koss, Abigail R.; Yuan, Bin; Gilman, Jessica B.; Peischl, Jeff; Aikin, Kenneth C.; DuRant, Justin; Warneke, Carsten; Li, Shao-Meng; de Gouw, Joost A.
      Environmental Science & Technology (2018), 52 (10), 5610-5618CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Decamethylcyclopentasiloxane (D5) is a cyclic volatile Me siloxane (cVMS) that is widely used in consumer products and commonly obsd. in urban air. This study quantifies the ambient mixing ratios of D5 from ground sites in two North American cities (Boulder, CO, USA, and Toronto, ON, CA). From these data, we est. the diurnal emission profile of D5 in Boulder, CO. Ambient mixing ratios were consistent with those measured at other urban locations; however, the diurnal pattern exhibited similarities with those of traffic-related compds. such as benzene. Mobile measurements and vehicle expts. demonstrate that emissions of D5 from personal care products are coincident in time and place with emissions of benzene from motor vehicles. During peak commuter times, the D5/benzene ratio (wt./wt.) is in excess of 0.3, suggesting that the mass emission rate of D5 from personal care product usage is comparable to that of benzene due to traffic. The diurnal emission pattern of D5 is estd. using the measured D5/benzene ratio and inventory ests. of benzene emission rates in Boulder. The hourly D5 emission rate is obsd. to peak between 6:00 and 7:00 AM and subsequently follow an exponential decay with a time const. of 9.2 h. This profile could be used by models to constrain temporal emission patterns of personal care products.
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    39. 39
      Tran, T. M.; Abualnaja, K. O.; Asimakopoulos, A. G.; Covaci, A.; Gevao, B.; Johnson-Restrepo, B.; Kumosani, T. A.; Malarvannan, G.; Minh, T. B.; Moon, H. B.; Nakata, H.; Sinha, R. K.; Kannan, K. A Survey of Cyclic and Linear Siloxanes in Indoor Dust and Their Implications for Human Exposures in Twelve Countries. Environ. Int. 2015, 78, 39– 44,  DOI: 10.1016/j.envint.2015.02.011
      39
      A survey of cyclic and linear siloxanes in indoor dust and their implications for human exposures in twelve countries
      Tran, Tri Manh; Abualnaja, Khalid O.; Asimakopoulos, Alexandros G.; Covaci, Adrian; Gevao, Bondi; Johnson-Restrepo, Boris; Kumosani, Taha A.; Malarvannan, Govindan; Minh, Tu Binh; Moon, Hyo-Bang; Nakata, Haruhiko; Sinha, Ravindra K.; Kannan, Kurunthachalam
      Environment International (2015), 78 (), 39-44CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
      Siloxanes are used widely in a variety of consumer products, including cosmetics, personal care products, medical and elec. devices, cookware, and building materials. Nevertheless, little is known on the occurrence of siloxanes in indoor dust. In this survey, five cyclic (D3-D7) and 11 linear (L4-L14) siloxanes were detd. in 310 indoor dust samples collected from 12 countries. Dust samples collected from Greece contained the highest concns. of total cyclic siloxanes (TCSi), ranging from 118 to 25,100 ng/g (median: 1380), and total linear siloxanes (TLSi), ranging from 129 to 4990 ng/g (median: 772). The median total siloxane (TSi) concns. in dust samples from 12 countries were in the following decreasing order: Greece (2970 ng/g), Kuwait (2400), South Korea (1810), Japan (1500), the USA (1220), China (1070), Romania (538), Colombia (230), Vietnam (206), Saudi Arabia (132), India (116), and Pakistan (68.3). TLSi concns. as high as 42,800 ng/g (Kuwait) and TCSi concns. as high as 25,000 ng/g (Greece) were found in indoor dust samples. Among the 16 siloxanes detd., decamethylcyclopentasiloxane (D5) was found at the highest concn. in dust samples from all countries, except for Japan and South Korea, with a predominance of L11; Kuwait, with L10; and Pakistan and Romania, with L12. The compn. profiles of 16 siloxanes in dust samples varied by country. TCSi accounted for a major proportion of TSi concns. in dust collected from Colombia (90%), India (80%) and Saudi Arabia (70%), whereas TLSi predominated in samples collected from Japan (89%), Kuwait (85%), and South Korea (78%). Based on the measured median TSi concns. in indoor dust, we estd. human exposure doses through indoor dust ingestion for various age groups. The exposure doses ranged from 0.27 to 11.9 ng/kg-bw/d for toddlers and 0.06 to 2.48 ng/kg-bw/d for adults.
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    40. 40
      Praske, E.; Otkjær, R. V.; Crounse, J. D.; Hethcox, J. C.; Stoltz, B. M.; Kjaergaard, H. G.; Wennberg, P. O. Atmospheric Autoxidation Is Increasingly Important in Urban and Suburban North America. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 64– 69,  DOI: 10.1073/pnas.1715540115
      40
      Atmospheric autoxidation is increasingly important in urban and suburban North America
      Praske, Eric; Otkjaer, Rasmus V.; Crounse, John D.; Hethcox, J. Caleb; Stoltz, Brian M.; Kjaergaard, Henrik G.; Wennberg, Paul O.
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (1), 64-69CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Gas-phase autoxidn.-regenerative peroxy radical formation following intramol. hydrogen shifts-is known to be important in the combustion of org. materials. The relevance of this chem. in the oxidn. of orgs. in the atm. has received less attention due, in part, to the lack of kinetic data at relevant temps. Here, we combine computational and exptl. approaches to investigate the rate of autoxidn. for org. peroxy radicals (RO2) produced in the oxidn. of a prototypical atm. pollutant, n-hexane. We find that the reaction rate depends critically on the mol. configuration of the RO2 radical undergoing hydrogen transfer (H-shift). RO2 H-shift rate coeffs. via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, resp.) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1/s at 296 K, while the 1,4 H-shift is calcd. to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coeffs. of these reactions increase rapidly with temp. and exhibit a large, primary, kinetic isotope effect. The obsd. H-shift rate coeffs. are sufficiently fast that, as a result of ongoing NOx emission redns., autoxidn. is now competing with bimol. chem. even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temps. are elevated.
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    41. 41
      Wu, Y.; Johnston, M. V. Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (CVMS) Decamethylcyclopentasiloxane. Environ. Sci. Technol. 2017, 51, 4445– 4451,  DOI: 10.1021/acs.est.7b00655
      41
      Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS) Decamethylcyclopentasiloxane
      Wu, Yue; Johnston, Murray V.
      Environmental Science & Technology (2017), 51 (8), 4445-4451CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Aerosol formation from OH- oxidn. of decamethylcyclopentasiloxane (D5, C10H30O5Si5), a cyclic volatile Me siloxane (cVMS) in consumer products, was examd. in a flow-through photooxidn. chamber with and without the presence of (NH4)2SO4 seed aerosols. In unseeded expts., high performance mass spectrometry chem. characterization showed that mol. compn. changed substantially with aerosol mass loads in the 1-12 μg/m3 range. Monomers (5 Si atoms/mol.) and dimers (10 Si atoms/mol.) dominated the mass spectra of aerosols at higher mass loads; ring-opened species (neither 5 nor 10 Si atoms/mol.) dominated the mass spectra of aerosols at lower mass loads. Mol. signal intensity dependencies on aerosol vol.:surface area ratio suggested non-volatile ring-opened species are formed in the gas phase and assist particle formation via condensation; dimers are formed by accretion reactions within the particle phase as particles grow. These conclusions were supported by expts. in the presence of seed aerosols with a similar siloxane aerosol mass loading but higher vol.:surface area ratio, where ring-opened species were much less prevalent than monomers or dimers and the aerosol yield was higher. Due to the importance of accretion chem., the aerosol yield from D5 oxidn. is likely to strongly depend on particle size and morphol.
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    42. 42
      Cheng, Z.; Qiu, X.; Shi, X.; Zhu, T. Identification of Organosiloxanes in Ambient Fine Particulate Matters Using an Untargeted Strategy via Gas Chromatography and Time-of-Flight Mass Spectrometry. Environ. Pollut. 2021, 271, 116128  DOI: 10.1016/j.envpol.2020.116128
      42
      Identification of organosiloxanes in ambient fine particulate matters using an untargeted strategy via gas chromatography and time-of-flight mass spectrometry
      Cheng, Zhen; Qiu, Xinghua; Shi, Xiaodi; Zhu, Tong
      Environmental Pollution (Oxford, United Kingdom) (2021), 271 (), 116128CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
      Organosilicons are widely used in consumer products and are ubiquitous in living environments. However, there is little systemic information on this group of pollutants in ambient particles. This study proposes a novel untargeted strategy based mainly on the mass difference of three silicon isotopes to screen organosilicon compds. from 2-yr PM2.5 samples of Beijing using gas chromatog. and high-resoln. time-of-flight mass spectrometry. 61 organosilicons were filtered from 1019 peaks, and 35 ones were identified as organosiloxanes including 17 methylsiloxanes and 18 phenylmethylsiloxanes, of which 6 and 3 species were confirmed using ref. stds., resp. These organosiloxanes could be clustered into three groups: low-silicon-no. methylsiloxanes, high-silicon-no. methylsiloxanes, and phenylmethylsiloxanes. Low-silicon-no. methylsiloxanes showed high abundance in the heating season but low abundance in the non-heating season, whereas high-silicon-no. methylsiloxanes showed the opposite seasonal variation. This study provides a promising strategy for screening organosilicon compds. through an untargeted approach and gives insights for further investigation of the sources and health risks of organosiloxanes.
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    43. 43
      Lu, D.; Tan, J.; Yang, X.; Sun, X.; Liu, Q.; Jiang, G.; Tan, J.; Sun, X.; Lu, D.; Jiang, G.; Yang, X. Unraveling the Role of Silicon in Atmospheric Aerosol Secondary Formation: A New Conservative Tracer for Aerosol Chemistry. Atmos. Chem. Phys. Discuss. 2018, 19, 2861– 2870,  DOI: 10.5194/acp-2018-914
      There is no corresponding record for this reference.
    44. 44
      Lu, D.; Liu, Q.; Yu, M.; Yang, X.; Fu, Q.; Zhang, X.; Mu, Y.; Jiang, G. Natural Silicon Isotopic Signatures Reveal the Sources of Airborne Fine Particulate Matter. Environ. Sci. Technol. 2018, 52, 1088– 1095,  DOI: 10.1021/acs.est.7b06317
      44
      Natural Silicon Isotopic Signatures Reveal the Sources of Airborne Fine Particulate Matter
      Lu, Dawei; Liu, Qian; Yu, Miao; Yang, Xuezhi; Fu, Qiang; Zhang, Xiaoshan; Mu, Yujing; Jiang, Guibin
      Environmental Science & Technology (2018), 52 (3), 1088-1095CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Airborne particulate pollution is a crit. environmental problem affecting human health and sustainable development. Understanding aerosol particle sources is extremely important for regional air pollution control. This work showed that a natural Si isotopic signature can be used as a tool to elucidate fine particulate matter (PM2.5) sources. By analyzing the PM2/5 Si isotopic compn. (δ30Si) and its primary sources collected in a typical polluted region (Beijing, China) the authors recognized the direct source tracing ability of Si isotopes for PM2.5. Different primary PM2.5 sources had different Si isotopic signatures. The PM2.5 δ30Si value was -1.99 ‰ to -0.01 ‰ and displayed a distinct seasonal trend (isotopically lighter in spring/winter and heavier in summer/autumn). PM2.5 δ30Si variations showed Si-isotopically light sources were important sources for the Beijing severe haze pollution; coal combustion was a major cause for the aggregating haze spring/winter weather in Beijing. Several typical haze events were also analyzed using Si isotopic signatures. As the first study on natural Si isotopes in the atm. environment, this work may reveal an important tool to advance particulate pollution research and control.
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    45. 45
      Charan, S.; Huang, Y.; Buenconsejo, R.; Li, Q.; Cocker, D., III; Seinfeld, J. Secondary Organic Aerosol Formation from the Oxidation of Decamethylcyclopentasiloxane at Atmospherically Relevant OH Concentrations. Atmos. Chem. Phys. Discuss. 2021, 917– 928,  DOI: 10.5194/acp-2021-353
      There is no corresponding record for this reference.
    46. 46
      Ziemann, P. J.; Atkinson, R. Kinetics, Products, and Mechanisms of Secondary Organic Aerosol Formation. Chem. Soc. Rev. 2012, 41, 6582– 6605,  DOI: 10.1039/c2cs35122f
      46
      Kinetics, products, and mechanisms of secondary organic aerosol formation
      Ziemann, Paul J.; Atkinson, Roger
      Chemical Society Reviews (2012), 41 (19), 6582-6605CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Secondary org. aerosol (SOA) is formed in the atm. when volatile org. compds. (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O3, NO3 radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these org. compds. can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atm. aerosol mass and can have significant effects on atm. chem., visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atm. VOC reactions and the general chem. and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chem. as they apply to SOA formation, with a focus on the effects of VOC mol. structure on the kinetics of initial reactions with the major atm. oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the compn. of the resulting products. Structural features of reactants and products discussed include compd. carbon no.; linear, branched, and cyclic configurations; the presence of C:C bonds and arom. rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atm. chemists with sufficient information to understand the dominant pathways by which the major classes of atm. VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on mol. structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NOx concns. as well as temp., humidity, and particle acidity. Such knowledge should be useful for interpreting the results of lab. and field studies and for developing atm. chem. models. A no. of recommendations for future research are also presented.
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    47. 47
      Peng, Z.; Jimenez, J. L. KinSim: A Research-Grade, User-Friendly, Visual Kinetics Simulator for Chemical-Kinetics and Environmental-Chemistry Teaching. J. Chem. Educ. 2019, 96, 806– 811,  DOI: 10.1021/acs.jchemed.9b00033
      47
      KinSim: A Research-Grade, User-Friendly, Visual Kinetics Simulator for Chemical-Kinetics and Environmental-Chemistry Teaching
      Peng, Zhe; Jimenez, Jose L.
      Journal of Chemical Education (2019), 96 (4), 806-811CODEN: JCEDA8; ISSN:0021-9584. (American Chemical Society and Division of Chemical Education, Inc.)
      KinSim is a research-grade, interactive, user-friendly, open-source, and visual software for kinetics modeling of environmental chem. and other applications. Students without any computer-programming background and limited knowledge of environmental chem. can use KinSim, which also includes multiple features and functionality dedicated to reducing users' workload and preventing users from creating errors in modeling; thus, KinSim is particularly suitable for in-classroom and homework teaching applications. Students can choose from several preprogrammed mechanisms and initial conditions for important environmental-chem. problems, and only clicking a few buttons is needed to perform a simulation and obtain graphs with concns. and chem. fluxes. The mechanism and initial conditions can be edited very easily to study other cases. Feedback from its use in courses shows the effectiveness of KinSim in helping students conduct computer expts. to gain familiarity with environmental chem. systems and helping them gain deeper understanding of the complex emergent behaviors of the systems. KinSim's accuracy and speed are similar to those of other software packages used in research, and at least 15 published peer-reviewed papers have used it.
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    48. 48
      Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. Atmos. Chem. Phys. 2004, 4, 1461– 1738,  DOI: 10.5194/acp-4-1461-2004
      48
      Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of Ox, HOx, NOx and SOx species
      Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J.
      Atmospheric Chemistry and Physics (2004), 4 (6), 1461-1738CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
      This review article, the first in the series, presents kinetic and photochem. data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atm. Chem. It covers the gas phase and photochem. reactions of Ox, HOx, NOx and SOx species, which were last published in 1997, and were updated on the IUPAC website in late 2001. The article consists of a summary sheet, contg. the recommended kinetic parameters for the evaluated reactions, and five appendices contg. the data sheets, which provide information upon which the recommendations are made.
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    49. 49
      Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume III─Gas Phase Reactions of Inorganic Halogens. Atmos. Chem. Phys. 2007, 7, 981– 1191,  DOI: 10.5194/acp-7-981-2007
      49
      Evaluated kinetic and photochemical data for atmospheric chemistry: volume III - gas phase reactions of inorganic halogens
      Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J.
      Atmospheric Chemistry and Physics (2007), 7 (4), 981-1191CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
      A review. This article, the third in the series, presents kinetic and photochem. data evaluated by the IUPAC Sub-committee on Gas Kinetic Data Evaluation for Atm. Chem. It covers the gas phase and photochem. reactions of inorg. halogen species, which were last published in J. Phys. Chem. Ref. Data, in 2000 were updated on the IUPAC website in 2003 and are updated again in the present evaluation. The article consists of a summary sheet, contg. the recommended kinetic parameters for the evaluated reactions, and five appendices contg. the data sheets, which provide information upon which the recommendations were made.
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    50. 50
      Liu, X.; Day, D. A.; Krechmer, J. E.; Brown, W.; Peng, Z.; Ziemann, P. J.; Jimenez, J. L. Direct Measurements of Semi-Volatile Organic Compound Dynamics Show near-Unity Mass Accommodation Coefficients for Diverse Aerosols. Commun. Chem. 2019, 2, 98  DOI: 10.1038/s42004-019-0200-x
      There is no corresponding record for this reference.
    51. 51
      Krechmer, J. E.; Pagonis, D.; Ziemann, P. J.; Jimenez, J. L. Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ. Environ. Sci. Technol. 2016, 50, 5757– 5765,  DOI: 10.1021/acs.est.6b00606
      51
      Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ
      Krechmer, Jordan E.; Pagonis, Demetrios; Ziemann, Paul J.; Jimenez, Jose L.
      Environmental Science & Technology (2016), 50 (11), 5757-5765CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Partitioning gas-phase org. compds. to the walls of Teflon environmental chambers is a recently reported phenomenon which can affect yields of reaction products and secondary org. aerosols measured in lab. expts. Reported time scales to reach gas-wall partitioning (GWP) equil. (τGWE) differ by up to 3 orders of magnitude; however, leading to predicted effects which vary from substantial to negligible. A method is demonstrated in which semi- and low-volatility oxidized org. compds. (satn. concn. [c*] <100 μg/m3) were photochem. generated by rapid, in-situ bursts in an 8 m3 environmental chamber, then their decay in the absence of aerosol was measured by a high resoln. chem. ionization mass spectrometer (CIMS) equipped with an inlet-less NO3- source. Measured τGWE were 7-13 min with a 33% relative std. deviation for all compds. The fraction of each compd. which partitioned to the walls at equil. followed absorptive partitioning theory with an equiv. wall mass concn. of 0.3-10 mg/m3. CIMS measurements, including a std. ion-mol. reaction region showed large biases due to compd. contact with chamber walls. Based on these results, a set of parameters is proposed to modeling GWP in chamber expts.
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    52. 52
      Boethling, R.; Meylan, W. How Accurate Are Physical Property Estimation Programs for Organosilicon Compounds?. Environ. Toxicol. Chem. 2013, 8, 2433– 2440,  DOI: 10.1002/etc.2326
      There is no corresponding record for this reference.
    53. 53
      Carter, W. L. P.; Pierce, J. A.; Malkina, I. L.; Luo, D. Investigation of the Ozone Formation Potential of Selected Volatile Silicone Compounds; Final Report to Dow Corning Corporation: Midland, MI, 1992.
      There is no corresponding record for this reference.
    54. 54
      Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2006.
      There is no corresponding record for this reference.
    55. 55
      Orlando, J. J.; Tyndall, G. S. Laboratory Studies of Organic Peroxy Radical Chemistry: An Overview with Emphasis on Recent Issues of Atmospheric Significance. Chem. Soc. Rev. 2012, 41, 6294– 6317,  DOI: 10.1039/c2cs35166h
      55
      Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance
      Orlando, John J.; Tyndall, Geoffrey S.
      Chemical Society Reviews (2012), 41 (19), 6294-6317CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Org. peroxy radicals (often abbreviated RO2) play a central role in the chem. of the Earth's lower atm. Formed in the atm. oxidn. of essentially every org. species emitted, their chem. is part of the radical cycles that control the oxidative capacity of the atm. and lead to the formation of ozone, org. nitrates, org. acids, particulate matter and other so-called secondary pollutants. In this review, lab. studies of this peroxy radical chem. are detailed, as they pertain to the chem. of the atm. First, a brief discussion of methods used to detect the peroxy radicals in the lab. is presented. Then, the basic reaction pathways - involving RO2 unimol. reactions and bimol. reactions with atm. constituents such as NO, NO2, NO3, O3, halogen oxides, HO2, and other RO2 species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chem. assocd. with the atm. oxidn. of biogenic hydrocarbons.
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    56. 56
      Jenkin, M. E.; Valorso, R.; Aumont, B.; Rickard, A. R. Estimation of Rate Coefficients and Branching Ratios for Reactions of Organic Peroxy Radicals for Use in Automated Mechanism Construction. Atmos. Chem. Phys. Discuss. 2019, 19, 7691– 7717,  DOI: 10.5194/acp-19-7691-2019
      There is no corresponding record for this reference.
    57. 57
      Narayanasamy, J.; Kubicki, J. D. Mechanism of Hydroxyl Radical Generation from a Silica Surface: Molecular Orbital Calculations. J. Phys. Chem. B 2005, 109, 21796– 21807,  DOI: 10.1021/jp0543025
      57
      Mechanism of Hydroxyl Radical Generation from a Silica Surface: Molecular Orbital Calculations
      Narayanasamy, Jayakumar; Kubicki, James D.
      Journal of Physical Chemistry B (2005), 109 (46), 21796-21807CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The interaction of an H2O mol. with cluster models of fractured silica surfaces was studied by means of quantum mech. calcns. Two clusters representing homolytic cleavage (≡Si• and ≡SiO•) and two representing heterolytic cleavage (≡Si+ and ≡Si-O-) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H2O have been calcd. and compare favorably with expt. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approx. -70 to -85 kJ/mol, depending on the computational methodol. These calcns. suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H2O. The reaction of H2O with ≡SiO• was predicted to generate OH•. Rate consts. for these reactions were also calcd. and predict a rapid equil. for the reaction ≡SiO• + H2O → ≡SiOH + OH•. Stability of a finite no. of ≡SiO• sites at equil. in the above reaction with H2O was also predicted, which implies a long-term ability of silica surfaces to produce OH• radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.
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    58. 58
      Xu, S.; Kropscott, B. Evaluation of the Three-Phase Equilibrium Method for Measuring Temperature Dependence of Internally Consistent Partition Coefficients (KOW, KOA, and KAW) for Volatile Methylsiloxanes and Trimethylsilanol. Environ. Toxicol. Chem. 2014, 33, 2702– 2710,  DOI: 10.1002/etc.2754
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      Evaluation of the three-phase equilibrium method for measuring temperature dependence of internally consistent partition coefficients (KOW, KOA, and KAW) for volatile methylsiloxanes and trimethylsilanol
      Xu, Shihe; Kropscott, Bruce
      Environmental Toxicology and Chemistry (2014), 33 (12), 2702-2710CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      Partitioning equil. and their temp. dependence of chems. between different environmental media are important in detg. the fate, transport, and distribution of contaminants. Unfortunately, internally consistent air/water (KAW), 1-octanol/air (KOA), and 1-octanol/water (KOW) partition coeffs., as well as information on their temp. dependence, are scarce for organosilicon compds. because of the reactivity of these compds. in water and octanol and their extreme partition coeffs. A newly published 3-phase equil. method was evaluated for simultaneous detn. of the temp. dependence of KAW, KOA, and KOW of 5 volatile methylsiloxanes (VMS) and trimethylsilanol (TMS) in a temp. range from 4° to 35°. The measured partition coeffs. at the different temps. for any given compd., and the enthalpy and entropy changes for the corresponding partition processes, were all internally consistent, suggesting that the 3-phase equil. method is suitable for this type of measurement. Compared with common environmental contaminants reported in the literature, VMS have enthalpy and entropy relationships similar to those of alkanes for air/water partitioning and similar to those of polyfluorinated compds. for octanol/air partitioning, but more like those for benzoates and phenolic compds. for octanol/water partitioning. The temp. dependence of the partition coeffs. of TMS is different from those of VMS and is more like that of alcs., phenols, and sulfonamides.
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    59. 59
      Donahue, N. M.; Robinson, A. L.; Pandis, S. N. Atmospheric Organic Particulate Matter: From Smoke to Secondary Organic Aerosol. Atmos. Environ. 2009, 43, 94– 106,  DOI: 10.1016/j.atmosenv.2008.09.055
      59
      Atmospheric organic particulate matter: From smoke to secondary organic aerosol
      Donahue, Neil M.; Robinson, Allen L.; Pandis, Spyros N.
      Atmospheric Environment (2008), 43 (1), 94-106CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      A review. We present an overview of the development of our understanding of the sources, formation mechanisms, phys. and chem. transformations of atm. org. aerosol (OA) during the last thirty years. Until recently, org. particulate material was simply classified as either primary or secondary with the primary component being treated in models as nonvolatile and inert. However, this oversimplified view fails to explain the highly oxygenated nature of ambient OA, the relatively small OA concn. gradients between urban areas and their surroundings, and the concns. of OA during periods of high photochem. activity. A unifying framework for the description of all components based on their volatility distribution (the volatility basis set) can be used for the treatment of a wide range of processes affecting org. aerosol loadings and compn. in the atm. These processes include direct org. particle and vapor emissions, chem. prodn. of org. PM from volatile precursors, chem. reactions (aging) in all phases, as well as deposition of both particles and vapors and chem. losses to volatile products. The combination of this new framework with the recent results of lab. studies can resolve some of the discrepancies between OA observations and lab. results. The mass balance of the org. material as a function of its volatility is investigated and used to frame the corresponding constraints on the system. Finally we revisit the traditional definitions of primary and secondary org. aerosol and propose a new set of terms and definitions based on the improvements of our understanding.
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    60. 60
      Daumit, K. E.; Carrasquillo, A. J.; Hunter, J. F.; Kroll, J. H. Laboratory Studies of the Aqueous-Phase Oxidation of Polyols: Submicron Particles vs. Bulk Aqueous Solution. Atmos. Chem. Phys. 2014, 14, 10773– 10784,  DOI: 10.5194/acp-14-10773-2014
      60
      Laboratory studies of the aqueous-phase oxidation of polyols: submicron particles vs. bulk aqueous solution
      Daumit, K. E.; Carrasquillo, A. J.; Hunter, J. F.; Kroll, J. H.
      Atmospheric Chemistry and Physics (2014), 14 (19), 10773-10784, 12 pp.CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Oxidn. in the atm. aq. phase (cloud droplets and deliquesced particles) has received recent attention as a potential pathway for the formation of highly oxidized org. aerosol. Most lab. studies of aq.-phase oxidn., however, are carried out in bulk solns. rather than aq. droplets. Here, we describe expts. in which aq. oxidn. of polyols (water-sol. species with chem. formula CnH2n+2On) is carried out within submicron particles in an environmental chamber, allowing for significant gas-particle partitioning of reactants, intermediates, and products. Dark Fenton chem. is used as a source of hydroxyl radicals, and oxidn. is monitored using a high-resoln. aerosol mass spectrometer (AMS). Aq. oxidn. is rapid, and results in the formation of particulate oxalate; this is accompanied by substantial loss of carbon to the gas phase, indicating the formation of volatile products. Results are compared to those from analogous oxidn. reactions carried out in bulk soln. The bulk-phase chem. is similar to that in the particles, but with substantially less carbon loss. This is likely due to differences in partitioning of early-generation products, which evap. out of the aq. phase under chamber conditions (in which liq. water content is low), but remain in soln. for further aq. processing in the bulk phase. This work suggests that the product distributions from oxidn. in aq. aerosol may be substantially different from those in bulk oxidn. expts. This highlights the need for aq. oxidn. studies to be carried out under atmospherically relevant partitioning conditions, with liq. water contents mimicking those of cloud droplets or aq. aerosol.
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    61. 61
      Sander, R. Compilation of Henry’s Law Constants (Version 4.0) for Water as Solvent. Atmos. Chem. Phys. 2015, 15, 4399– 4981,  DOI: 10.5194/acp-15-4399-2015
      Compilation of Henry's law constants (version 4.0) for water as solvent
      Sander, R.
      Atmospheric Chemistry and Physics (2015), 15 (8), 4399-4981CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Many atm. chems. occur in the gas phase as well as in liq. cloud droplets and aerosol particles. Therefore, it is necessary to understand the distribution between the phases. According to Henry's law, the equil. ratio between the abundances in the gas phase and in the aq. phase is const. for a dil. soln. Henry's law consts. of trace gases of potential importance in environmental chem. have been collected and converted into a uniform format. The compilation contains 17350 values of Henry's law consts. for 4632 species, collected from 689 refs.
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    62. 62
      Xu, S.; Kozerski, G.; Mackay, D. Critical Review and Interpretation of Environmental Data for Volatile Methylsiloxanes: Partition Properties. Environ. Sci. Technol. 2014, 48, 11748– 11759,  DOI: 10.1021/es503465b
      62
      Critical Review and Interpretation of Environmental Data for Volatile Methylsiloxanes: Partition Properties
      Xu, Shihe; Kozerski, Gary; Mackay, Donald
      Environmental Science & Technology (2014), 48 (20), 11748-11759CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A crit. review of methods to det. and interpret the best available values and environmental data for volatile methylsiloxanes (VMS), particularly partitioning properties, to accurately predict their environmental fate, distribution, transport, exposure and potential effects is given. Measured partition properties included air/water (KAW), octanol/water (KOW), and octanol/air partitioning coeffs. (KOA), soil org. C/water distribution coeff. (KOC), biol. medium/fluid partition coeffs., and their temp. dependence. Based on the results, organosilicon compds. (VMS) are expected to behave differently in the environment vs. conventional hydrophobic environmental pollutants due to their inherent properties related to mol. size and capacity for different types of mol. interactions which control partitioning. Topics discussed include: introduction; background (partition coeffs. and their concn. and temp. dependence, internal consistency of environmental partition coeffs., methods to quantify VMS in partition media, solvolysis of organosiloxanes and organosilanols in partition media); methods to det. partition properties (sep. partition coeff. detn. [air/water, octanol/water, octanol/air]; simultaneous partition coeff. detn.); room temp. partition coeff.; temp. difference; soil/water distribution coeffs.; biol. medium/fluid partition coeffs.; environmental implications (risks of using single parameter quant. structure-property relationships outside domain, exposure limits in soil and sediment, VMS distribution and overall persistence as predicted by partition coeffs., potential for VMS long-range transport as predicted by partition coeffs.); and supplementary information (VMS exposure limit calcn. for soil and sediment ad estg. VMS deposition to remote surface media).
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    63. 63
      Xu, S.; Kropscott, B. Method for Simultaneous Determination of Partition Coefficients for Cyclic Volatile Methylsiloxanes and Dimethylsilanediol. Anal. Chem. 2012, 84, 1948– 1955,  DOI: 10.1021/ac202953t
      63
      Method for Simultaneous Determination of Partition Coefficients for Cyclic Volatile Methylsiloxanes and Dimethylsilanediol
      Xu, Shihe; Kropscott, Bruce
      Analytical Chemistry (Washington, DC, United States) (2012), 84 (4), 1948-1955CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Cyclic volatile Me siloxanes (cVMS) such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) may enter the environment through industrial activities and the use of various consumer products. Reliable air/water (KAW), 1-octanol/water (KOW), and octanol/air partition coeffs. (KOA) for those compds. and their common degrdn. product, dimethylsilanediol, are crit. for accurate prediction of the environmental fate, distribution, and transport of these materials. Challenges have been encountered in detg. these properties for cVMS and their degrdn. products mainly due to the extremely low water soly. of the organosiloxanes, low volatility of their degrdn. products, and reactivity of those compds. in the water/1-octanol system that can lead to inconsistent and inaccurate partition coeffs. A novel direct method is presented for the simultaneous detn. of KAW, KOW, and KOA of org. compds. and was applied to these organosilicon compds. It was tested in a range of log KAW values from -6.8 to 3.1, log KOW values from -0.4 to 8.9, and log KOA values up to 7. The advantages of the new direct method include the improved accuracy, a shortened measurement time, simultaneous measurement of three partition coeffs. of multiple compds., self-consistency among resultant partition coeffs., and a wide range of applicability including materials that may be slowly reactive in the water/1-octanol system.
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    64. 64
      Krogseth, I. S.; Kierkegaard, A.; McLachlan, M. S.; Breivik, K.; Hansen, K. M.; Schlabach, M. Occurrence and Seasonality of Cyclic Volatile Methyl Siloxanes in Arctic Air. Environ. Sci. Technol. 2013, 47, 502– 509,  DOI: 10.1021/es3040208
      64
      Occurrence and Seasonality of Cyclic Volatile Methyl Siloxanes in Arctic Air
      Krogseth, Ingjerd S.; Kierkegaard, Amelie; McLachlan, Michael S.; Breivik, Knut; Hansen, Kaj M.; Schlabach, Martin
      Environmental Science & Technology (2013), 47 (1), 502-509CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Cyclic volatile Me siloxanes (cVMS), present in tech. applications and personal care products, are predicted to undergo long-range atm. transport, but cVMS measurements in remote areas are scarce. An active air sampling method for decamethylcyclopentasiloxane (D5) was evaluated to include hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and dodecamethylcyclohexasiloxane (D6). Air samples were collected at the Zeppelin Observatory in the remote Arctic (79°N, 12°E) with an av. sampling time of 81 ± 23 h in late summer (Aug.-Oct.) and 25 ± 10 h in early winter (Nov.-Dec.) 2011. Av. D5 and D6 concns. in late summer were 0.73 ± 0.31 and 0.23 ± 0.17 ng/m3, resp., and 2.94 ± 0.46 and 0.45 ± 0.18 ng/m3 in early winter, resp. D5 and D6 detection in the Arctic atm. confirmed their long-range atm. transport. D5 measurements agreed well with Eulerian atm. chem.-transport model predictions; seasonal variability was explained by the seasonality of OH- concns. Results extend the understanding of the atm. fate of D5 at high latitudes, but questions D3 and D4 concns. previously measured at Zeppelin using passive air samplers.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenvironau.1c00043.

    • Detailed information of the peak fitting in TOFWARE, KinSim mechanism and inputs, isotopically labeled D18L2 oxidation experiment, instrument response in different conditions, and the estimated physical parameters of cVMS and the oxidation products (PDF)


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