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Anthropogenic Impacts on the Atmosphere

Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps
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Environmental Science & Technology Letters

Cite this: Environ. Sci. Technol. Lett. 2023, 10, 1, 6–13
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https://doi.org/10.1021/acs.estlett.2c00599
Published December 2, 2022

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Abstract

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Air disinfection using germicidal ultraviolet light (GUV) has received increasing attention during the COVID-19 pandemic. GUV uses UVC lamps to inactivate microorganisms, but it also initiates photochemistry in air. However, GUV’s indoor-air-quality impact has not been investigated in detail. Here, we model the chemistry initiated by GUV at 254 (“GUV254”) or 222 nm (“GUV222”) in a typical indoor setting for different ventilation levels. Our analysis shows that GUV254, usually installed in the upper room, can significantly photolyze O3, generating OH radicals that oxidize indoor volatile organic compounds (VOCs) into more oxidized VOCs. Secondary organic aerosol (SOA) is also formed as a VOC-oxidation product. GUV254-induced SOA formation is of the order of 0.1–1 μg/m3 for the cases studied here. GUV222 (described by some as harmless to humans and thus applicable for the whole room) with the same effective virus-removal rate makes a smaller indoor-air-quality impact at mid-to-high ventilation rates. This is mainly because of the lower UV irradiance needed and also less efficient OH-generating O3 photolysis than GUV254. GUV222 has a higher impact than GUV254 under poor ventilation due to a small but significant photochemical production of O3 at 222 nm, which does not occur with GUV254.

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Introduction

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Germicidal ultraviolet light (GUV) has been employed to disinfect air in indoor spaces since the 1930s. (1) It has been shown to effectively limit the airborne transmission of infectious diseases, e.g., measles and tuberculosis. (1−3) This is due to photon-induced dimerization of pyrimidines in the nucleic acids of airborne pathogens (and loss of their ability to replicate as a result). GUV fixtures use lamps that emit in the UVC range, most commonly at 254 nm (hereinafter “GUV254”). (4) As 254 nm UV can cause skin and eye irritation, (5) GUV254 is usually applied near the ceiling (Figure 1a) or inside ventilation ducts. Recently, 222 nm UV has been shown to not only have a strong capability of inactivating airborne viruses, (6) but also is claimed to be safe to humans (7) (although this is controversial (8)), potentially allowing whole-room GUV applications (“GUV222”) (Figure 1b). Ground-resting GUV-based air cleaners have also been commercialized, in which a fan continuously pulls air into a box and exposes it to UV light, from which the occupants are shielded. (9)

Figure 1

Figure 1. Schematics of a germicidal ultraviolet air disinfection system at (a) 254 nm and at (b) 222 nm in a room; (c) absorption cross sections of several important gas-phase species relevant to this study (a discontinuity in the spectrum of methacrolein is due to lack of data).

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During the COVID-19 pandemic, GUV has drawn renewed interest as a tool for airborne virus inactivation. Inhalation of an airborne virus is widely accepted as the main transmission route of COVID-19, (10−12) which explains the dominant indoor character of transmission. (13) An important component of the transmission is due to superspreading events, (14) which have been shown to be mostly explained by shared-room airborne transmission. (15) Much transmission also happens in close proximity due to short-range airborne transmission, but even in this situation a substantial fraction of the inhaled virus may come from well-mixed room air. (16,17) As COVID-19 remains widespread, and with the possible appearance of new variants, there is a pressing need to remove inhalable viruses from indoor environments. (3) Similar measures would be beneficial for other airborne diseases such as tuberculosis, measles, or a future pandemic virus.
Physical measures such as natural and/or mechanical ventilation and air filtration have been proven safe and effective. (18) Nevertheless, mechanical ventilation and air filtration usually can remove airborne pathogens only at a few effective air changes per hour (ACH) (19) and natural ventilation can be highly variable and impractical depending on weather, or when pollution, allergens or noise are present outdoors. When a high virus removal rate (e.g., >10 ACH) needs to be ensured (e.g., in high-risk environments), GUV emerges as a practical and potentially cost-effective way to achieve it. (3,18)
UVC light is known to generate strong oxidants (e.g., OH radicals, and sometimes also O3 depending on the wavelengths used), (20) which can subsequently oxidize volatile organic compounds (VOCs) indoors and initiate organic radical chemistry in indoor air. (21,22) Energetic UVC photons can also directly photolyze many VOCs, such as peroxides (23,24) and carbonyls, (25,26) and generate organic radicals. This radical chemistry is thought to lead to further oxidation of indoor VOCs and the formation of oxygenated VOCs (OVOCs) and secondary organic aerosol (SOA), both of which may have negative health effects. (27) Surveys of the concentration of total VOCs in indoor environments range from ∼0.1–4 mg m–3. (28−30) Thus, there is always a significant amount of VOC to react with any radicals and oxidants that are generated indoors, and any “air cleaning” technique that can create radicals and/or oxidants indoors has the potential to lead to secondary chemistry. (27) Very few studies on this topic have been conducted with state-of-the-art measurements or models. Recently, air cleaning devices based on chemistry induced by UV light (photocatalysis and OH generation, but not GUV), often also marketed as suitable for air disinfection, have been experimentally shown to produce significant amounts of OVOCs and SOA. (31,32)
Despite the potential of GUV to cause secondary chemistry, to our knowledge this topic has not been studied in detail to date. Some studies of GUV inactivation effectiveness have included measurements of ozone, to assess whether any was generated. (9) These studies report no production of ozone when mercury vapor lamps coated to limit emission from wavelengths nearer to the ozone generating wavelength of 185 nm are used, as expected. However, some uncoated or improperly coated lamps are commercially available, so ozone production can be a problem in some cases. In this study, we perform a first modeling evaluation of the impacts of GUV254 (assuming properly coated lamps) and GUV222 on indoor air quality. The amounts of OVOC and SOA that can be formed in typical indoor environments are investigated.

Materials and Methods

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We include the photochemistry due to GUV and subsequent radical, oxidation, and SOA formation chemistries. Given the complexity of the composition of indoor air, we simplify both the chemical species present indoors and the reaction scheme, while keeping them consistent with the state-of-the-art knowledge for indoor air. Surface reactions are neglected but could be important and should be investigated in future studies.
The chemical mechanism for this study is a combination of the inorganic radical chemistry in an oxidation flow reactor (OFR) model (20,33,34) and part of the Regional Atmospheric Chemistry Mechanism (RACM) (35) relevant to this study. Section S1 of the Supporting Information provides more details of the mechanism. The mechanisms are run within the open-source KinSim chemical kinetics simulator, (36) and are made available in GitLab (https://gitlab.com/JimenezGroup/KinSim_Cases_Mechs/). We perform all simulations until a steady state is reached.
We investigate a typical indoor space with representative indoor and urban outdoor air concentrations from the literature (Table S2). The initial concentrations of most VOCs are estimated based on McDonald et al., (37) with the total VOC concentration assumed to be 1.7 mg m–3, a typical value for US indoor spaces. (28) See Section S2 for details on species lumping and initial conditions.
The GUV254 fixture in our simulations is based on the AeroMed LEXUS L2.1 Open. (38) The space irradiated by this device is 45 m3, and is placed in a room of 300 m3 (volume of a typical classroom), thus the irradiated volume is 15% of the volume of the room, consistent with refs (39,40). The GUV254 model has two compartments, one for the irradiated zone and the other for the rest of the room, while that for GUV222 has only one compartment. Based on the UV inactivation rate constants at 222 nm for SARS-CoV-2, (6,7) the UV intensity for GUV222 is adjusted such that it provides the same whole-room effective virus-removal rate as GUV254 (see Section S3). Three levels of ventilation, i.e., a representative residential level (0.3 ACH, “low ventilation”), (41) a representative commercial level (3 ACH, “medium ventilation”), (19) and a representative medical level (9 ACH, “high ventilation”) (3) are simulated in this study. Indoor VOC emissions are set such that all VOC concentrations remain at their literature-constrained initial values at low ventilation without chemistry occurring. To test COVID-19 infection risk in different situations, we assume the presence of an infector shedding aerosolized SARS-CoV-2 at 16 quanta h–1 (roughly for light exercise while speaking 50% time), (42) which is consistent or lower than values constrained for literature superspreading events. (15) A quantum is an infectious dose that, if inhaled by a susceptible person, will lead to a probability of infection of 1–1/e. (15,43) The rate of SARS-CoV-2 loss apart from ventilation and GUV (i.e., due to intrinsic loss of infectivity, aerosol deposition etc.) is assumed to be 1 h–1.

Results and Discussion

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Disinfection

Figure 2 shows the amount of SARS-CoV-2 present in the room to be consistent with the steady-state prediction. In the absence of GUV, the emission rate of SARS-CoV-2 is 16 quanta h–1, and its total loss rate 1.3 h–1 (0.3 h–1 from ventilation and 1 h–1 from decay and deposition) for the low-ventilation case. The steady state SARS-CoV-2 quantity in the entire room is 12.3 quanta. It is lowered to 4 and 1.6 quanta by increasing the ventilation rate to 3 and 9 ACH, respectively. When GUV with a whole-room virus-removal rate of ∼30 h–1 is applied, SARS-CoV-2 decreases to ∼0.6, ∼0.5, and ∼0.4 quanta in the low-, medium-, and high-ventilation cases, respectively. The relative impact of GUV is higher at low ventilation, as expected.

Figure 2

Figure 2. Final quantity/concentration of the main (types of) species of interest in this study under different GUV and ventilation conditions. In the GUV254 cases, the volume-weighted average concentrations for the whole room are shown. The stable chemical species concentrations are similar between the irradiated and unirradiated zones, while the radical and SARS-CoV-2 concentrations in the unirradiated zone can be significantly lower and higher, respectively (Table S3). Note that some panels use a log scale for concentrations while other panels use a linear scale. SOA is assumed to have a molar weight of 200 g mol–1.

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The total quantity of SARS-CoV-2 does not directly reflect its infection risk, which also depends on the volume of the room and the inhalation by susceptible occupants. For an occupant with a breathing rate of 0.5 m3 h–1 (typical for light physical activities) (44) present in the 300 m3 room with low ventilation and no GUV fixture for 1 h, ∼0.02 quantum is inhaled. This corresponds to an infection probability of ∼2%, since the infection probability is approximately equal to the inhaled quanta if the latter is small. (45) For the cases studied in this work, infection risk is reduced by a factor of about 3 by medium ventilation, and by a factor of ∼22/8/4 when adding GUV to a low/medium/high ventilation situation.

Secondary Chemistry

For chemical species in the room, ventilation alone (without GUV) can make some difference (Figure 2). The differences in O3, NO, and ketone concentrations are largely due to these species being ventilated in or out. For other chemical species, secondary chemical processes also play a role. OH radicals can form even without UV, i.e., from limonene ozonolysis. As a result, OH radicals are higher at medium and high ventilation, which introduces more O3 from outdoors than at low ventilation. OH concentration at high ventilation is not higher than medium ventilation because high ventilation also decreases indoor limonene, reducing the overall limonene-O3 reaction rate. HO2 radical concentration is lower at higher ventilation because of higher NO being ventilated into the room, which reacts with HO2. All other organic radicals and stable products shown in Figure 2 (including SOA) have higher concentrations in the low-ventilation case due to higher VOC concentrations.
With GUV254, although the concentration of photolyzable O3 remains relatively stable (due to much stronger replenishment from outdoor air ventilation than photolytic destruction), the chemistry is significantly altered by UV (Figure 2). The fundamental cause of this change is OH production from O3 photolysis. (20) OH concentrations in the GUV254 cases are approximately a factor of 1.2–5 and 3–20 times those in the corresponding no-UV cases for the whole room average and for the irradiated zone, respectively. The difference in the higher-ventilation cases is larger, due to more O3 in the room from outside air (Figure 2 and Table S3). OH in the higher-ventilation cases is similar to daytime outdoor urban levels. (46) This OH level is high enough to drive substantial oxidation of VOCs, production of other radicals (e.g., HO2 and organic peroxy radicals (RO2)), and SOA formation. Organic peroxides (including hydroperoxides), carbonyls (aldehydes (excluding formaldehyde) and ketones (excluding acetone)), and organic nitrates (including peroxynitrates) are among common VOC oxidation products and all have ∼10% to several-fold concentration increases (Figure 2). The exceptions are ketones (excluding acetone), whose production is dominated in the model by UV-independent limonene ozonolysis (Figure 2). Doubling RH significantly increases OH concentration, but changes of product species are within 10% of the base RH results, likely due to nonlinear buffering from the reaction scheme (not shown). The exceptions are ketones, which are relatively unreactive and dominated in the model by the chemistry-independent acetone emission and its dilution by ventilation (Figure 2). In addition to VOC oxidation by OH, radicals (OH, HO2, and RO2) are also produced by active photolysis of carbonyls and peroxides at 254 nm, where both strongly absorb (Figure 1c). Due to higher peroxy radical concentrations, NO is lowered to ∼30 ppt at low ventilation (Figure 2). Such a low NO concentration leads to reactions of RO2 with both HO2 and NO being important, as estimated per ref (47) (Section S4).
SOA formation is estimated from the consumption of individual VOCs and SOA mass yields from the literature (Table S4). Significant SOA production (∼8 μg m–3 at low ventilation) occurs even without GUV irradiation, through limonene ozonolysis, as this reaction has a high SOA yield (20%). In the GUV254 cases, both limonene ozonolysis and VOC oxidation by OH contribute to SOA. The overall SOA mass yield from total VOC is of the order of 0.1%, because a large fraction of total organic molecules present are too small to form SOA through oxidation (e.g., ethanol or acetone). Given that 1.5 mg m–3 of total VOCs (excluding limonene) are present, that leads to ∼1.2 μg m–3 due to GUV254. The enhancement of SOA formation by OH oxidation relative to the no-UV cases decreases with increasing ventilation, as VOC concentrations are lowered by ventilation (Figure 2). The SOA precursors and mechanism used in this work are likely incomplete, given the incomplete scientific understanding of this topic. (48)
Due to the fast air exchange between the GUV254 irradiated and unirradiated spaces, the concentrations of stable species are similar between these two spaces (Table S3). In contrast, radicals are more rapidly consumed in the unirradiated space than supplied by the transport from the irradiated space, and thus have much lower concentrations (up to >1 order of magnitude for highly reactive ones such as OH and acylperoxy) in the unirradiated space.
The GUV222 cases assume irradiation of the entire room volume (Figure 1b). 222 nm photons can photolyze O2 and produce O3, albeit at a small rate, leading to higher O3 in all cases relative to GUV254. The amounts of organic products formed in the medium- and high-ventilation (low-ventilation) cases are lower (higher) than in the unirradiated zone in the GUV254 cases (Figure 2).
At medium and high ventilation rates, the main O3 source in the GUV222 cases is still outdoor O3 through ventilation. Despite some O3 production, the product enhancement is much smaller than in the corresponding GUV254 cases (Figure 2). These results indicate a weak OH-initiated VOC oxidation on top of the VOC ozonolysis chemistry that is active in the no-UV cases.
This difference from the active photochemistry in the GUV254 cases can be attributed to several factors. First, the UV irradiance of the 222 nm fixture is significantly lower, even in terms of the number of photons emitted per unit time. 222 nm photons are ∼40% more efficient in inactivating SARS-CoV-2 than 254 nm photons. (6) In addition the latter cannot be used in the most efficient fashion. Due to the need to protect humans from irradiation, all photons are concentrated in the small irradiated zone (15% of the room volume). Because of the limited rate of transport of virus-containing aerosol to the irradiated zone, the steady-state infectious virus concentration is ∼70% lower than in the unirradiated zone, where the infector and the susceptible individuals are present (Table S3). Even if the per-photon virus-inactivation efficiency was the same, GUV254 would need about 3 times the photons for GUV222 to reach the same effective GUV virus-removal rate for the occupied unirradiated space. Furthermore, the first step of OH photochemical production is O3 photolysis, whose corresponding absorption at 222 nm is about ∼5 times lower than at 254 nm (Figure 1c). Simple carbonyl compounds, the most abundant OVOCs in this study, also absorb much less efficiently at 222 nm than at 254 nm (Figure 1c), further reducing radical production. Although other products, such as peroxides and conjugated carbonyl species, can have stronger absorption at 222 nm, their relatively low concentrations (∼1 ppb or lower vs hundreds of ppb of ketones) limit their relative contributions to the radical budget.
Because of the small direct production of O3 by the GUV222 lights, O3 in the GUV222 low-ventilation case is substantially less depleted by limonene ozonolysis than for GUV254. As a result, compared to GUV254 SOA formation through limonene ozonolysis is substantially stronger and OH concentration is also higher (Figure 2). Other gas-phase stable organic products have comparable concentrations to those in the GUV254 case.

Implications

We have shown that GUV disinfection can induce active photochemistry producing OVOCs and SOA in typical indoor environments. Under the conditions simulated here, these products do not necessarily have significant negative effects on human health because of their relatively low concentrations. Among the VOCs (including OVOCs) modeled in this study, only formaldehyde has a concentration exceeding the minimal risk level (MRL) recommended by the CDC (49) due to strong indoor emissions. However, only a very limited number of species were explicitly modeled in this study, particularly aldehydes, whose toxicity is generally high. Future studies with higher chemical speciation are needed to better assess the toxicity of gas-phase products. In polluted indoor spaces and/or polluted outdoor air transported indoors, the concentrations of the VOCs of interest can be much higher, (28) while the GUV-induced photochemistry can still be active (Sections S5 and S6). In this case, OVOC products might exceed the MRLs and SOA formation might reach tens of μg m–3.
The risk of GUV254 due to secondary photochemical products is not negligible but also not dominant under typical indoor conditions. The risk for GUV222 appears to be substantially lower when ventilation is not poor, but comparable or slightly higher in case of poor ventilation. We note that many indoor environments, in particular homes and schools, have ventilation rates similar to the definition of “poor” in this paper, even in high-income countries. If GUV222 is confirmed to be safe for direct human exposure, it would have an advantage over GUV254 at mid-to-high ventilation rates in terms of indoor chemistry, in addition to more efficient air disinfection. When GUV254 is used, a strong air exchange between the irradiated and unirradiated zones (e.g., by fans) is preferable, as recommended by the CDC/NIOSH. (50) It can lower the UV irradiance needed for a given virus inactivation rate, (51) and hence limit the induced photochemistry. Good ventilation can not only remove airborne pathogens, but also limit the production of secondary indoor pollutants, and is thus also recommended when outdoor air is relatively clean. (19,52) Similarly, particulate air filtration is also recommended as it removes virus-containing aerosol, indoor-formed SOA, and particulate pollution from other indoor and outdoor sources. Gas filtration with sorbent materials such as activated carbon is also useful for reducing VOC, NOx, and ozone levels indoors. (53,54) The findings of this study are limited due to modeling assumptions, e.g., the simplified chemical mechanism, limited data on photolysis parameters in the UVC range, the assumed indoor and outdoor air pollutant compositions, uncertainties over precursors and yields of SOA formation, and surface reactions. Experimental studies in both simplified laboratory settings and real indoor conditions are needed to fully constrain the impacts of GUV in indoor chemistry.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.estlett.2c00599.

  • Details about the model setup (reaction scheme, species lumping, indoor emission, outdoor air composition, initial conditions, effective virus-removal rates of GUV), RO2 fates in the GUV254 cases, concentrations in the irradiated and unirradiated zones in the GUV254 cases, and sensitivity cases for highly polluted conditions (PDF)

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Author Information

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  • Corresponding Authors
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  • Notes
    The authors declare no competing financial interest.

    This paper has been previously submitted to medRxiv, a preprint server for health sciences. The preprint can be cited as Peng, Z.; Miller, S. L.; Jimenez, J. L. Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps. 2022. medRxiv 10.1101/2022.08.25.22279238 (accessed November 23, 2022).

Acknowledgments

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Z.P. and J.L.J. were supported by the CIRES Innovative Research Program and the Balvi Filantropic Fund. We thank Vito Ilacqua, Zachary Finewax, Donald Milton, and Edward Nardell for valuable discussions.

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    UV germicidal tools have recently gained attention as a disinfection strategy against the COVID-19 pandemic, but the safety profile arising from their exposure has been controversial and impeded larger-scale implementation. We compare the emerging 222-nm far UVC and 277-nm UVC LED disinfection modules with the traditional UVC mercury lamp emitting at 254 nm to understand their effects on human retinal cell line ARPE-19 and HEK-A keratinocytes. Cells illuminated with 222-nm far UVC survived, while those treated with 254-nm and 277-nm wavelengths underwent apoptosis via the JNK/ATF2 pathway. However, cells exposed to 222-nm far UVC presented the highest degree of DNA damage as evidenced by yH2AX staining. Globally, these cells displayed transcriptional changes in cell-cycle and senescence pathways. Thus, the introduction of 222-nm far UVC lamps for disinfection purposes should be carefully considered and designed with the inherent dangers involved.
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    Kujundzic, E.; Matalkah, F.; Howard, C. J.; Hernandez, M.; Miller, S. L. UV Air Cleaners and Upper-Room Air Ultraviolet Germicidal Irradiation for Controlling Airborne Bacteria and Fungal Spores. J. Occup. Environ. Hyg. 2006, 3 (10), 536– 546,  DOI: 10.1080/15459620600909799
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    9
    UV air cleaners and upper-room air ultraviolet germicidal irradiation for controlling airborne bacteria and fungal spores
    Kujundzic Elmira; Matalkah Fatimah; Howard Cody J; Hernandez Mark; Miller Shelly L
    Journal of occupational and environmental hygiene (2006), 3 (10), 536-46 ISSN:1545-9624.
    In-room air cleaners (ACs) and upper-room air ultraviolet germicidal irradiation (UVGI) are engineering control technologies that can help reduce the concentrations of airborne bacteria and fungal spores in the indoor environment. This study investigated six different types of ACs and quantified their ability to remove and/or inactivate airborne bacteria and fungal spores. Four of the air cleaners incorporated UV lamp(s) into their flow path. In addition, the efficacy of combining ACs with upper-room air UVGI was investigated. With the ventilation system providing zero or six air changes per hour, the air cleaners were tested separately or with the upper-room air UVGI system in operation in an 87-m3 test room. Active bacteria cells and fungal spores were aerosolized into the room such that their numbers and physiologic state were comparable both with and without air cleaning and upper-room air UVGI. In addition, the disinfection performance of a UV-C lamp internal to one of the ACs was evaluated by estimating the percentage of airborne bacteria cells and fungal spores captured on the air filter medium surface that were inactivated with UV exposure. Average airborne microbial clean air delivery rates (CADRm) varied between 26-981 m3 hr-1 depending on the AC, and between 1480-2370 m3 hr-1, when using air cleaners in combination with upper-room air UVGI. Culturing, direct microscopy, and optical particle counting revealed similar CADRm. The ACs performed similarly when challenged with three different microorganisms. Testing two of the ACs showed that no additional air cleaning was provided with the operation of an internal UV-C lamp; the internal UV-C lamps, however, inactivated 75% of fungal spores and 97% of bacteria cells captured in the air filter medium within 60 min.
  10. 10
    Wang, C. C.; Prather, K. A.; Sznitman, J.; Jimenez, J. L.; Lakdawala, S. S.; Tufekci, Z.; Marr, L. C. Airborne Transmission of Respiratory Viruses. Science 2021, 373 (6558), eabd9149  DOI: 10.1126/science.abd9149
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    Greenhalgh, T.; Jimenez, J. L.; Prather, K. A.; Tufekci, Z.; Fisman, D.; Schooley, R. Ten Scientific Reasons in Support of Airborne Transmission of SARS-CoV-2. Lancet 2021, 397, 1603,  DOI: 10.1016/S0140-6736(21)00869-2
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    11
    Ten scientific reasons in support of airborne transmission of SARS-CoV-2
    Greenhalgh, Trisha; Jimenez, Jose L.; Prather, Kimberly A.; Tufekci, Zeynep; Fisman, David; Schooley, Robert
    Lancet (2021), 397 (10285), 1603-1605CODEN: LANCAO; ISSN:0140-6736. (Elsevier Ltd.)
    Heneghan and colleagues' systematic review, funded by WHO, published in March, 2021, as a preprint, states: "The lack of recoverable viral culture samples of SARS-CoV-2 prevents firm conclusions to be drawn about airborne transmission" (F1000Research 2021; published online March 24. https://doi.org/10.12688/f1000research.52091.1 (preprint). This conclusion, and the wide circulation of the review's findings, is concerning because of the public health implications. In this report, ten streams of evidence that collectively support the hypothesis that SARS-CoV-2 is transmitted primarily by the airborne route is presented. The authors propose that it is a scientific error to use lack of direct evidence of SARS-CoV-2 in some air samples to cast doubt on airborne transmission while overlooking the quality and strength of the overall evidence base. There is consistent, strong evidence that SARS-CoV-2 spreads by airborne transmission. Although other routes can contribute, we believe that the airborne route is likely to be dominant. The public health community should act accordingly and without further delay.
  12. 12
    Klompas, M.; Milton, D. K.; Rhee, C.; Baker, M. A.; Leekha, S. Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs: A Narrative Review. Ann. Int. Med. 2021, 174 (12), 1710– 1718,  DOI: 10.7326/M21-2780
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    12
    Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs : A Narrative Review
    Klompas Michael; Rhee Chanu; Baker Meghan A; Milton Donald K; Leekha Surbhi
    Annals of internal medicine (2021), 174 (12), 1710-1718 ISSN:.
    Policies to prevent respiratory virus transmission in health care settings have traditionally divided organisms into Droplet versus Airborne categories. Droplet organisms (for example, influenza) are said to be transmitted via large respiratory secretions that rapidly fall to the ground within 1 to 2 meters and are adequately blocked by surgical masks. Airborne pathogens (for example, measles), by contrast, are transmitted by aerosols that are small enough and light enough to carry beyond 2 meters and to penetrate the gaps between masks and faces; health care workers are advised to wear N95 respirators and to place these patients in negative-pressure rooms. Respirators and negative-pressure rooms are also recommended when caring for patients with influenza or SARS-CoV-2 who are undergoing "aerosol-generating procedures," such as intubation. An increasing body of evidence, however, questions this framework. People routinely emit respiratory particles in a range of sizes, but most are aerosols, and most procedures do not generate meaningfully more aerosols than ordinary breathing, and far fewer than coughing, exercise, or labored breathing. Most transmission nonetheless occurs at close range because virus-laden aerosols are most concentrated at the source; they then diffuse and dilute with distance, making long-distance transmission rare in well-ventilated spaces. The primary risk factors for nosocomial transmission are community incidence rates, viral load, symptoms, proximity, duration of exposure, and poor ventilation. Failure to appreciate these factors may lead to underappreciation of some risks (for example, overestimation of the protection provided by medical masks, insufficient attention to ventilation) or misallocation of limited resources (for example, reserving N95 respirators and negative-pressure rooms only for aerosol-generating procedures or requiring negative-pressure rooms for all patients with SARS-CoV-2 infection regardless of stage of illness). Enhanced understanding of the factors governing respiratory pathogen transmission may inform the development of more effective policies to prevent nosocomial transmission of respiratory pathogens.
  13. 13
    Qian, H.; Miao, T.; Liu, L.; Zheng, X.; Luo, D.; Li, Y. Indoor Transmission of SARS-CoV-2. Indoor Air 2021, 31 (3), 639– 645,  DOI: 10.1111/ina.12766
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    Indoor transmission of SARS-CoV-2
    Qian, Hua; Miao, Te; Liu, Li; Zheng, Xiaohong; Luo, Danting; Li, Yuguo
    Indoor Air (2021), 31 (3), 639-645CODEN: INAIE5; ISSN:1600-0668. (Wiley-Blackwell)
    It is essential to understand where and how severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is transmitted. Case reports were extd. from the local Municipal Health Commissions of 320 prefectural municipalities in China (not including Hubei Province). We identified all outbreaks involving three or more cases and reviewed the major characteristics of the enclosed spaces in which the outbreaks were reported and their assocd. indoor environmental aspects. Three hundred and eighteen outbreaks with three or more cases were identified, comprising a total of 1245 confirmed cases in 120 prefectural cities. Among the identified outbreaks, 53.8% involved three cases, 26.4% involved four cases, and only 1.6% involved ten or more cases. Home-based outbreaks were the dominant category (254 of 318 outbreaks; 79.9%), followed by transport-based outbreaks (108; 34.0%), and many outbreaks occurred in more than one category of venue. All identified outbreaks of three or more cases occurred in indoor environments, which confirm that sharing indoor spaces with one or more infected persons is a major SARS-CoV-2 infection risk.
  14. 14
    Adam, D. C.; Wu, P.; Wong, J. Y.; Lau, E. H. Y.; Tsang, T. K.; Cauchemez, S.; Leung, G. M.; Cowling, B. J. Clustering and Superspreading Potential of SARS-CoV-2 Infections in Hong Kong. Nat. Med. 2020, 26 (11), 1714– 1719,  DOI: 10.1038/s41591-020-1092-0
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    14
    Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong
    Adam, Dillon C.; Wu, Peng; Wong, Jessica Y.; Lau, Eric H. Y.; Tsang, Tim K.; Cauchemez, Simon; Leung, Gabriel M.; Cowling, Benjamin J.
    Nature Medicine (New York, NY, United States) (2020), 26 (11), 1714-1719CODEN: NAMEFI; ISSN:1078-8956. (Nature Research)
    Abstr.: Superspreading events (SSEs) have characterized previous epidemics of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) infections1-6. For SARS-CoV-2, the degree to which SSEs are involved in transmission remains unclear, but there is growing evidence that SSEs might be a typical feature of COVID-197,8. Using contact tracing data from 1,038 SARS-CoV-2 cases confirmed between 23 Jan. and 28 Apr. 2020 in Hong Kong, we identified and characterized all local clusters of infection. We identified 4-7 SSEs across 51 clusters (n = 309 cases) and estd. that 19% (95% confidence interval, 15-24%) of cases seeded 80% of all local transmission. Transmission in social settings was assocd. with more secondary cases than households when controlling for age (P = 0.002). Decreasing the delay between symptom onset and case confirmation did not result in fewer secondary cases (P = 0.98), although the odds that an individual being quarantined as a contact interrupted transmission was 14.4 (95% CI, 1.9-107.2). Public health authorities should focus on rapidly tracing and quarantining contacts, along with implementing restrictions targeting social settings to reduce the risk of SSEs and suppress SARS-CoV-2 transmission.
  15. 15
    Peng, Z.; Rojas, A. L. P.; Kropff, E.; Bahnfleth, W.; Buonanno, G.; Dancer, S. J.; Kurnitski, J.; Li, Y.; Loomans, M. G. L. C.; Marr, L. C.; Morawska, L.; Nazaroff, W.; Noakes, C.; Querol, X.; Sekhar, C.; Tellier, R.; Greenhalgh, T.; Bourouiba, L.; Boerstra, A.; Tang, J. W.; Miller, S. L.; Jimenez, J. L. Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks. Environ. Sci. Technol. 2022, 56 (2), 1125– 1137,  DOI: 10.1021/acs.est.1c06531
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    15
    Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks
    Peng, Z.; Rojas, A. L. Pineda; Kropff, E.; Bahnfleth, W.; Buonanno, G.; Dancer, S. J.; Kurnitski, J.; Li, Y.; Loomans, M. G. L. C.; Marr, L. C.; Morawska, L.; Nazaroff, W.; Noakes, C.; Querol, X.; Sekhar, C.; Tellier, R.; Greenhalgh, T.; Bourouiba, L.; Boerstra, A.; Tang, J. W.; Miller, S. L.; Jimenez, J. L.
    Environmental Science & Technology (2022), 56 (2), 1125-1137CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
    Some infectious diseases, including COVID-19, can undergo airborne transmission. This may happen at close proximity, but as time indoors increases, infections can occur in shared room air despite distancing. We propose 2 indicators of infection risk for this situation, i.e., relative risk parameter (Hr) and risk parameter (H). They combine the key factors that control airborne disease transmission indoors: virus-contg. aerosol generation rate, breathing flow rate, masking and its quality, ventilation and aerosol-removal rates, no. of occupants, and duration of exposure. COVID-19 outbreaks show a clear trend that is consistent with airborne infection and enable recommendations to minimize transmission risk. Transmission in typical prepandemic indoor spaces is highly sensitive to mitigation efforts. Previous outbreaks of measles, influenza, and tuberculosis were also assessed. Measles outbreaks occur at much lower risk parameter values than COVID-19, while tuberculosis outbreaks are obsd. at higher risk parameter values. Because both diseases are accepted as airborne, the fact that COVID-19 is less contagious than measles does not rule out airborne transmission. It is important that future outbreak reports include information on masking, ventilation and aerosol-removal rates, no. of occupants, and duration of exposure, to investigate airborne transmission.
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    Li, Y.; Cheng, P.; Jia, W. Poor Ventilation Worsens Short-Range Airborne Transmission of Respiratory Infection. Indoor Air 2022, 32 (1), e12946  DOI: 10.1111/ina.12946
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    Poor ventilation worsens short-range airborne transmission of respiratory infection
    Li, Yuguo; Cheng, Pan; Jia, Wei
    Indoor Air (2022), 32 (1), e12946CODEN: INAIE5; ISSN:1600-0668. (Wiley-Blackwell)
    To explain the obsd. phenomenon that most SARS-CoV-2 transmission occurs indoors whereas its outdoor transmission is rare, a simple macroscopic aerosol balance model is developed to link short- and long-range airborne transmission. The model considers the involvement of exhaled droplets with initial diam. ≤50μm in the short-range airborne route, whereas only a fraction of these droplets with an initial diam. within 15μm or equivalently a final diam. within 5μm considered in the long-range airborne route. One surprising finding is that the room ventilation rate significantly affects the short-range airborne route, in contrast to traditional belief. When the ventilation rate in a room is insufficient, the airborne infection risks due to both short- and long-range transmission are high. A ventilation rate of 10 L/s per person provides a similar concn. vs distance decay profile to that in outdoor settings, which provides addnl. justification for the widely adopted ventilation std. of 10 L/s per person. The newly obtained data do not support the basic assumption in the existing ventilation std. ASHRAE 62.1 (2019) that the required people outdoor air rate is const. if the std. is used directly for respiratory infection control. Instead, it is necessary to increase the ventilation rate when the phys. distance between people is less than approx. 2 m.
  17. 17
    Jimenez, J. L.; Peng, Z.; Pagonis, D. Systematic Way to Understand and Classify the Shared-Room Airborne Transmission Risk of Indoor Spaces. Indoor Air 2022, 32 (5), e13025  DOI: 10.1111/ina.13025
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    Morawska, L.; Allen, J.; Bahnfleth, W.; Bluyssen, P. M.; Boerstra, A.; Buonanno, G.; Cao, J.; Dancer, S. J.; Floto, A.; Franchimon, F.; Greenhalgh, T.; Haworth, C.; Hogeling, J.; Isaxon, C.; Jimenez, J. L.; Kurnitski, J.; Li, Y.; Loomans, M.; Marks, G.; Marr, L. C.; Mazzarella, L.; Melikov, A. K.; Miller, S.; Milton, D. K.; Nazaroff, W.; Nielsen, P. V.; Noakes, C.; Peccia, J.; Prather, K.; Querol, X.; Sekhar, C.; Seppänen, O.; Tanabe, S.-I.; Tang, J. W.; Tellier, R.; Tham, K. W.; Wargocki, P.; Wierzbicka, A.; Yao, M. A Paradigm Shift to Combat Indoor Respiratory Infection. Science 2021, 372 (6543), 689– 691,  DOI: 10.1126/science.abg2025
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    18
    A paradigm shift to combat indoor respiratory infection
    Morawska, Lidia; Allen, Joseph; Bahnfleth, William; Bluyssen, Philomena M.; Boerstra, Atze; Buonanno, Giorgio; Cao, Junji; Dancer, Stephanie J.; Floto, Andres; Franchimon, Francesco; Greenhalgh, Trisha; Haworth, Charles; Hogeling, Jaap; Isaxon, Christina; Jimenez, Jose L.; Kurnitski, Jarek; Li, Yuguo; Loomans, Marcel; Marks, Guy; Marr, Linsey C.; Mazzarella, Livio; Melikov, Arsen Krikor; Miller, Shelly; Milton, Donald K.; Nazaroff, William; Nielsen, Peter V.; Noakes, Catherine; Peccia, Jordan; Prather, Kim; Querol, Xavier; Sekhar, Chandra; Seppanen, Olli; Tanabe, Shin-ichi; Tang, Julian W.; Tellier, Raymond; Tham, Kwok Wai; Wargocki, Pawel; Wierzbicka, Aneta; Yao, Maosheng
    Science (Washington, DC, United States) (2021), 372 (6543), 689-691CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    A review. There is great disparity in the way we think about and address different sources of environmental infection. Governments have for decades promulgated a large amt. of legislation and invested heavily in food safety, sanitation, and drinking water for public health purposes. By contrast, airborne pathogens and respiratory infections, whether seasonal influenza or COVID-19, are addressed fairly weakly, if at all, in terms of regulations, stds., and building design and operation, pertaining to the air we breathe. We suggest that the rapid growth in our understanding of the mechanisms behind respiratory infection transmission should drive a paradigm shift in how we view and address the transmission of respiratory infections to protect against unnecessary suffering and economic losses. It starts with a recognition that preventing respiratory infection, like reducing waterborne or foodborne disease, is a tractable problem.
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    ASHRAE. Ventilation for Acceptable Indoor Air Quality: ANSI/ASHRAE Standard 62.1-2019; ANSI/ASHRAE, 2019.
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    Peng, Z.; Jimenez, J. L. Radical Chemistry in Oxidation Flow Reactors for Atmospheric Chemistry Research. Chem. Soc. Rev. 2020, 49 (9), 2570– 2616,  DOI: 10.1039/C9CS00766K
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    Radical chemistry in oxidation flow reactors for atmospheric chemistry research
    Peng, Zhe; Jimenez, Jose L.
    Chemical Society Reviews (2020), 49 (9), 2570-2616CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Environmental chambers have been playing a vital role in atm. chem. research for seven decades. In last decade, oxidn. flow reactors (OFR) have emerged as a promising alternative to chambers to study complex multigenerational chem. OFR can generate higher-than-ambient concns. of oxidants via H2O, O2 and O3 photolysis by low-pressure-Hg-lamp emissions and reach hours to days of equiv. photochem. aging in just minutes of real time. The use of OFR for volatile-org.-compd. (VOC) oxidn. and secondary-org.-aerosol formation has grown very rapidly recently. However, the lack of detailed understanding of OFR photochem. left room for speculation that OFR chem. may be generally irrelevant to the troposphere, since its initial oxidant generation is similar to stratosphere. Recently, a series of studies have been conducted to address important open questions on OFR chem. and to guide exptl. design and interpretation. In this Review, we present a comprehensive picture connecting the chemistries of hydroxyl (OH) and hydroperoxy radicals, oxidized nitrogen species and org. peroxy radicals (RO2) in OFR. Potential lack of tropospheric relevance assocd. with these chemistries, as well as the phys. conditions resulting in it will also be reviewed. When atm. oxidn. is dominated by OH, OFR conditions can often be similar to ambient conditions, as OH dominates against undesired non-OH effects. One key reason for tropospherically-irrelevant/undesired VOC fate is that under some conditions, OH is drastically reduced while non-tropospheric/undesired VOC reactants are not. The most frequent problems are running expts. with too high precursor concns., too high UV and/or too low humidity. On other hand, another cause of deviation from ambient chem. in OFR is that some tropospherically-relevant non-OH chem. (e.g. VOC photolysis in UVA and UVB) is not sufficiently represented under some conditions. In addn., the fate of RO2 produced from VOC oxidn. can be kept relevant to the troposphere. However, in some cases RO2 lifetime can be too short for atmospherically-relevant RO2 chem., including its isomerization. OFR applications using only photolysis of injected O3 to generate OH are less preferable than those using both 185 and 254 nm photons (without O3 injection) for several reasons. When a relatively low equiv. photochem. age (<∼1 d) and high NO are needed, OH and NO generation by org.-nitrite photolysis in the UVA range is preferable. We also discuss how to achieve the atm. relevance for different purposes in OFR exptl. planning.
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    Atkinson, R.; Arey, J. Atmospheric Degradation of Volatile Organic Compounds. Chem. Rev. 2003, 103 (12), 4605– 4638,  DOI: 10.1021/cr0206420
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    Atmospheric Degradation of Volatile Organic Compounds
    Atkinson, Roger; Arey, Janet
    Chemical Reviews (Washington, DC, United States) (2003), 103 (12), 4605-4638CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review concerning the removal of biogenic and anthropogenic volatile org. compds. (VOC) emitted to the atm. through phys. (deposition) or chem. processes, or their atm. transformation is given. Topics discussed include: tropospheric VOC transformation processes (initial reactions and lifetimes, reaction mechanisms); atm. chem. of alkanes (kinetic data for initial OH- and NO3- reactions, reaction mechanism); atm. chem. of alkenes (rate consts. for initial reaction of alkenes with OH-, NO3-, and O3; mechanism of the OH- reaction; NO3- reaction; reaction with O3); arom. hydrocarbons (kinetics of OH- reactions, reactions of phenols, reactions of unsatd. 1,4-dicarbonyls and di-unsatd. 1,6-dicarbonyls); atm. reactions of oxygenated VOC (aldehydes, ketones, aliph. alcs., ethers, alkyl nitrates); and conclusions and future research needs (exptl. studies, theor. studies and crit. reviews and evaluations).
  22. 22
    Ziemann, P. J.; Atkinson, R. Kinetics, Products, and Mechanisms of Secondary Organic Aerosol Formation. Chem. Soc. Rev. 2012, 41 (19), 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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    Blitz, M. A.; Heard, D. E.; Pilling, M. J. Wavelength Dependent Photodissociation of CH3OOH: Quantum Yields for CH3O and OH, and Measurement of the OH+ CH3OOH Rate Coefficient. J. Photochem. Photobiol. A Chem. 2005, 176 (1–3), 107– 113,  DOI: 10.1016/j.jphotochem.2005.09.017
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    23
    Wavelength dependent photodissociation of CH3OOH
    Blitz, Mark A.; Heard, Dwayne E.; Pilling, Michael J.
    Journal of Photochemistry and Photobiology, A: Chemistry (2005), 176 (1-3), 107-113CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    Me hydroperoxide, CH3OOH, has been synthesized with >99.5% purity, confirmed using UV absorption spectroscopy and high-pressure liq. chromatog. (HPLC) followed by post-column derivatization. The UV absorption cross-section for CH3OOH was measured and for < 325 nm was in good agreement with the literature. Laser-flash photolysis combined with laser-induced fluorescence (LIF) spectroscopy has been used to measure both OH and CH3O photofragments following the photolysis of CH3OOH in the wavelength range 223-355 nm. Using the previously measured unity quantum yield for OH at 248 nm as a ref., the LIF signals immediately following photolysis were used to measure wavelength dependent quantum yields for OH and CH3O, taking into account changes in laser pulse energy and absorption cross-section. The quantum yields for both species were unity within exptl. error. The rate coeff. for the reaction of OH with CH3OOH (R1a) to generate CH3O2 + H2O products was measured at 295 K to be k (R1a) = (9.0 ± 0.2) × 10-12 cm3 mol.-1 s-1, considerably higher (by about a factor of two) than previous values measured by Vaghjiani and Ravishankara [G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 (1989) 1948-1959] and Niki et al. [H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 2190-2193].
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    Vaghjiani, G. L.; Ravishankara, A. R. Photodissociation of H2O2 and CH3OOH at 248 Nm and 298 K: Quantum Yields for OH, O(3P) and H(2S). J. Chem. Phys. 1990, 92 (2), 996– 1003,  DOI: 10.1063/1.458081
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    24
    Photodissociation of hydrogen peroxide and methyl hydroperoxide (CH3OOH) at 248 nm and 298 K: quantum yields for hydroxyl, atomic oxygen, and atomic hydrogen (O(3P), and H(2S))
    Vaghjiani, Ghanshyam L.; Ravishankara, A. R.
    Journal of Chemical Physics (1990), 92 (2), 996-1003CODEN: JCPSA6; ISSN:0021-9606.
    The quantum yields of OH, O, and H in H2O2 photolysis (248 nm, 298 K) were 2.09 ± 0.36, <0.002 and <0.0002 for OH, O and H, resp. For CH3OOH photolysis, the quantum yields were detd. as 1 ± 0.18, <0.007 and 0.038 ± 0.007 for OH, O and H, resp. In both H2O2 and CH3OOH photolysis, the obsd. O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH was consistent with the known continuous and unstructured absorption spectra of these mols. in this wavelength region, where the key process is the dissociative (~A1A ← ~X1A) transition to give OH (X2Π, ν'' = 0) fragment.
  25. 25
    Link, M. F.; Farmer, D. K.; Berg, T.; Flocke, F.; Ravishankara, A. R. Measuring Photodissociation Product Quantum Yields Using Chemical Ionization Mass Spectrometry: A Case Study with Ketones. J. Phys. Chem. A 2021, 125 (31), 6836– 6844,  DOI: 10.1021/acs.jpca.1c03140
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    25
    Measuring photodissociation product quantum yields using chemical ionization mass spectrometry: A case study with ketones
    Link, Michael F.; Farmer, Delphine K.; Berg, Tyson; Flocke, Frank; Ravishankara, A. R.
    Journal of Physical Chemistry A (2021), 125 (31), 6836-6844CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Measurements of photolysis quantum yields are challenging because of the difficulties in measuring the first-generation photodissocn. products, interference from other products or contaminants, sufficient photon fluxes and/or low absorption cross sections of the photolyte to make detectable amts. of products, and quantification of the photon flux. In the case of acetone (and other atmospherically relevant ketones) the uncertainty in the photolysis quantum yield creates uncertainty in the calcd. OH radical and acyl peroxy nitrate prodn. in the atm. We present a new method for detg. photodissocn. product quantum yields by measuring acyl peroxy radicals (RC(O)O2) produced in the photolysis of ketones in air using chem. ionization mass spectrometry (CIMS). We show good agreement of our CIMS method with previously published quantum yields of the acyl radical from photolysis of biacetyl and Me Et ketone (MEK) at 254 nm. Addnl., we highlight the capabilities of this CIMS method through the measurement of photolysis branching ratios for MEK. We suggest future applications of CIMS (in the lab. and field) to measure RC(O)O2 and assocd. photolysis processes.
  26. 26
    Rajakumar, B.; Gierczak, T.; Flad, J. E.; Ravishankara, A. R.; Burkholder, J. B. The CH3CO Quantum Yield in the 248 Nm Photolysis of Acetone, Methyl Ethyl Ketone, and Biacetyl. J. Photochem. Photobiol. A Chem. 2008, 199 (2–3), 336– 344,  DOI: 10.1016/j.jphotochem.2008.06.015
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    The CH3CO quantum yield in the 248 nm photolysis of acetone, methyl ethyl ketone, and biacetyl
    Rajakumar, B.; Gierczak, Tomasz; Flad, Jonathan E.; Ravishankara, A. R.; Burkholder, James B.
    Journal of Photochemistry and Photobiology, A: Chemistry (2008), 199 (2-3), 336-344CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    We report measurements of the CH3CO quantum yield, ΦCH3CO , following the 248 nm pulsed laser photolysis of acetone (CH3C(O)CH3), Me Et ketone (CH3C(O)CH2CH3), and biacetyl (CH3C(O)C(O)CH3). CH3CO quantum yields at 248 nm were measured at 296 K, relative to CH3CO ref. systems. CH3CO was detected using cavity ring-down spectroscopy at wavelengths between 490 and 660 nm. Measurements were performed between 60 and 670 Torr (He, N2 bath gases) and the obtained CH3CO quantum yields in the low-pressure limit were 0.535 ± 0.09, 0.41 ± 0.08, and 0.76 ± 0.11, for acetone, Me Et ketone, and biacetyl, resp. The quoted uncertainties are 2σ (95% confidence level) and include estd. systematic errors. An increase in ΦCH3CO with increasing bath gas pressure, which depended on the identity of the collision partner (He, N2), was obsd. The present results are compared with previous quantum yield detns.
  27. 27
    Collins, D. B.; Farmer, D. K. Unintended Consequences of Air Cleaning Chemistry. Environ. Sci. Technol. 2021, 55, 12172,  DOI: 10.1021/acs.est.1c02582
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    Unintended Consequences of Air Cleaning Chemistry
    Collins, Douglas B.; Farmer, Delphine K.
    Environmental Science & Technology (2021), 55 (18), 12172-12179CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
    A review. Amplified interest in maintaining clean indoor air assocd. with the airborne transmission risks of SARS-CoV-2 have led to an expansion in the market for com. available air cleaning systems. While the optimal way to mitigate indoor air pollutants or contaminants is to control (remove) the source, air cleaners are a tool for use when abs. source control is not possible. Interventions for indoor air quality management include phys. removal of pollutants through ventilation or collection on filters and sorbent materials, along with chem. reactive processes that transform pollutants or seek to deactivate biol. entities. This perspective intends to highlight the perhaps unintended consequences of various air cleaning approaches via indoor air chem. Introduction of new chem. agents or reactive processes can initiate complex chem. that results in the release of reactive intermediates and(or) byproducts into the indoor environment. Since air cleaning systems are often continuously running to maximize their effectiveness and most people spend a vast majority of their time indoors, human exposure to both primary and secondary products from air cleaners may represent significant exposure risk. We highlight the need for further study of chem. reactive air cleaning and disinfection methods before broader adoption.
  28. 28
    Logue, J. M.; McKone, T. E.; Sherman, M. H.; Singer, B. C. Hazard Assessment of Chemical Air Contaminants Measured in Residences. Indoor Air 2011, 21 (2), 92– 109,  DOI: 10.1111/j.1600-0668.2010.00683.x
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    Hazard assessment of chemical air contaminants measured in residences
    Logue, J. M.; McKone, T. E.; Sherman, M. H.; Singer, B. C.
    Indoor Air (2011), 21 (2), 92-109CODEN: INAIE5; ISSN:0905-6947. (Wiley-Blackwell)
    Identifying air pollutants that pose a potential hazard indoors can facilitate exposure mitigation. In this study, we compiled summary results from 77 published studies reporting measurements of chem. pollutants in residences in the United States and in countries with similar lifestyles. These data were used to calc. representative mid-range and upper-bound concns. relevant to chronic exposures for 267 pollutants and representative peak concns. relevant to acute exposures for five activity-assocd. pollutants. Representative concns. are compared to available chronic and acute health stds. for 97 pollutants. Fifteen pollutants appear to exceed chronic health stds. in a large fraction of homes. Nine other pollutants are identified as potential chronic health hazards in a substantial minority of homes, and an addnl. nine are identified as potential hazards in a very small percentage of homes. Nine pollutants are identified as priority hazards based on the robustness of measured concn. data and the fraction of residences that appear to be impacted: acetaldehyde; acrolein; benzene; 1,3-butadiene; 1,4-dichlorobenzene; formaldehyde; naphthalene; nitrogen dioxide; and PM2.5. Activity-based emissions are shown to pose potential acute health hazards for PM2.5, formaldehyde, CO, chloroform, and NO2.
  29. 29
    Mattila, J. M.; Arata, C.; Abeleira, A.; Zhou, Y.; Wang, C.; Katz, E. F.; Goldstein, A. H.; Abbatt, J. P. D.; DeCarlo, P. F.; Vance, M. E.; Farmer, D. K. Contrasting Chemical Complexity and the Reactive Organic Carbon Budget of Indoor and Outdoor Air. Environmental Science & Technology. 2022, 56, 109– 118,  DOI: 10.1021/acs.est.1c03915
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    Contrasting Chemical Complexity and the Reactive Organic Carbon Budget of Indoor and Outdoor Air
    Mattila, James M.; Arata, Caleb; Abeleira, Andrew; Zhou, Yong; Wang, Chen; Katz, Erin F.; Goldstein, Allen H.; Abbatt, Jonathan P. D.; DeCarlo, Peter F.; Vance, Marina E.; Farmer, Delphine K.
    Environmental Science & Technology (2022), 56 (1), 109-118CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
    Reactive org. carbon (ROC) comprises a substantial fraction of the total atm. carbon budget. Emissions of ROC fuel atm. oxidn. chem. to produce secondary pollutants including ozone, carbon dioxide, and particulate matter. Compared to the outdoor atm., the indoor org. carbon budget is comparatively understudied. We characterized indoor ROC in a test house during unoccupied, cooking, and cleaning scenarios using various online mass spectrometry and gas chromatog. measurements of gaseous and particulate orgs. Cooking greatly impacted indoor ROC concns. and bulk physicochem. properties (e.g., volatility and oxidn. state), while cleaning yielded relatively insubstantial changes. Addnl., cooking enhanced the reactivities of hydroxyl radicals and ozone toward indoor ROC. We obsd. consistently higher median ROC concns. indoors (≥223μg C m-3) compared to outdoors (54μg C m-3), demonstrating that buildings can be a net source of reactive carbon to the outdoor atm., following its removal by ventilation. We est. the unoccupied test house emitted 0.7 g C day-1 from ROC to outdoors. Indoor ROC emissions may thus play an important role in air quality and secondary pollutant formation outdoors, particularly in urban and suburban areas, and indoors during the use of oxidant-generating air purifiers.
  30. 30
    Price, D. J.; Day, D. A.; Pagonis, D.; Stark, H.; Algrim, L. B.; Handschy, A. V.; Liu, S.; Krechmer, J. E.; Miller, S. L.; Hunter, J. F.; de Gouw, J. A.; Ziemann, P. J.; Jimenez, J. L. Budgets of Organic Carbon Composition and Oxidation in Indoor Air. Environ. Sci. Technol. 2019, 53, 13053,  DOI: 10.1021/acs.est.9b04689
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    Budgets of Organic Carbon Composition and Oxidation in Indoor Air
    Price, Derek J.; Day, Douglas A.; Pagonis, Demetrios; Stark, Harald; Algrim, Lucas B.; Handschy, Anne V.; Liu, Shang; Krechmer, Jordan E.; Miller, Shelly L.; Hunter, James F.; de Gouw, Joost A.; Ziemann, Paul J.; Jimenez, Jose L.
    Environmental Science & Technology (2019), 53 (22), 13053-13063CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The chem. compn. of indoor air at the University of Colorado-Boulder art museum was measured with a suite of gas- and particle-phase instruments. More than 80% of the total obsd. org. C (TOOC) mass (100 μg/m3) consisted of reduced compds. (C oxidn. state [OSC] <-0.5) with high volatility (log10C* >7) and low C no. (nC <6). Museum TOOC was compared to other indoor and outdoor sites, which increased according to the following trend: remote < rural ≤ urban < indoor ≤ megacity. Museum TOOC was comparable to a university classroom and three times less than residential environments. Total reactive flux trends were: remote < indoor < rural < urban < megacity. High volatile org. compd. (VOC) concns. compensated low oxidant concns. indoors resulting in an appreciable reactive flux. Total OH-, O3, NO3-, and Cl atom reactivities at each site followed a trend similar to TOOC. High human occupancy events increased all oxidant reactivities in the museum 65-125%. O3, NO3-, OH-, and Cl reactivities were 13 h, 15 h, 23 days, and 189 days, resp., corresponding to >88% of indoor VOC-oxidant reactivity being consumed outdoors after ventilation.
  31. 31
    Ye, Q.; Krechmer, J. E.; Shutter, J. D.; Barber, V. P.; Li, Y.; Helstrom, E.; Franco, L. J.; Cox, J. L.; Hrdina, A. I. H.; Goss, M. B.; Tahsini, N.; Canagaratna, M.; Keutsch, F. N.; Kroll, J. H. Real-Time Laboratory Measurements of VOC Emissions, Removal Rates, and Byproduct Formation from Consumer-Grade Oxidation-Based Air Cleaners. Environmental Science & Technology Letters 2021, 8, 1020,  DOI: 10.1021/acs.estlett.1c00773
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    Real-Time Laboratory Measurements of VOC Emissions, Removal Rates, and Byproduct Formation from Consumer-Grade Oxidation-Based Air Cleaners
    Ye, Qing; Krechmer, Jordan E.; Shutter, Joshua D.; Barber, Victoria P.; Li, Yaowei; Helstrom, Erik; Franco, Lesly J.; Cox, Joshua L.; Hrdina, Amy I. H.; Goss, Matthew B.; Tahsini, Nadia; Canagaratna, Manjula; Keutsch, Frank N.; Kroll, Jesse H.
    Environmental Science & Technology Letters (2021), 8 (12), 1020-1025CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Levels of volatile org. compds. (VOCs) in the indoor environment can be decreased by the use of "air cleaners", devices that remove VOCs by sorption and/or oxidative degrdn. However, efficacies of these technologies for removing VOCs tend to be poorly constrained, as does the formation of oxidn. byproducts. Here, we examine the influence of several oxidn.-based air cleaners, specifically ones marketed as consumer-grade products, on the amts. and compn. of VOCs. Expts. were conducted in an environmental chamber, with a suite of real-time anal. instruments to measure direct emissions, VOC removal efficacies (by the addn. of either limonene and toluene), and byproduct formation. We find that the air cleaners themselves can be a source of org. gases, that removal efficacy can be exceedingly variable, and that VOC loss is primarily driven by phys. removal in some cleaners. When oxidative degrdn. of VOCs was obsd., it was accompanied by the formation of a range of oxidn. byproducts, including formaldehyde and other oxygenates. Some consumer-grade portable air cleaners can be ineffective in removing VOCs and that the air delivered may contain a range of org. compds., due to direct emission and/or byproduct formation.
  32. 32
    Joo, T.; Rivera-Rios, J. C.; Alvarado-Velez, D.; Westgate, S.; Ng, N. L. Formation of Oxidized Gases and Secondary Organic Aerosol from a Commercial Oxidant-Generating Electronic Air Cleaner. Environ. Sci. Technol. Lett. 2021, 8, 691,  DOI: 10.1021/acs.estlett.1c00416
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    Formation of Oxidized Gases and Secondary Organic Aerosol from a Commercial Oxidant-Generating Electronic Air Cleaner
    Joo, Taekyu; Rivera-Rios, Jean C.; Alvarado-Velez, Daniel; Westgate, Sabrina; Lee Ng, Nga
    Environmental Science & Technology Letters (2021), 8 (8), 691-698CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    The COVID-19 pandemic increased the demand for indoor air cleaners. While some com. electronic air cleaners can be effective in reducing primary pollutants and inactivating bioaerosol, studies on the formation of secondary products from oxidn. chem. during their use are limited. Here, we measured oxygenated volatile org. compds. (OVOCs) and the chem. compn. of particles generated from a hydroxyl radical generator in an office. During operation, enhancements in OVOCs, esp. low-mol.-wt. org. acids, were detected. Rapid increases in particle no. and mass concns. were obsd., corresponding to the formation of highly oxidized secondary org. aerosol (SOA) (O:C ~ 1.3), with an enhanced signal at m/z 44 (CO2+) in the org. mass spectra. These results suggest that org. acids generated during VOC oxidn. contributed to particle nucleation and SOA formation. Nitrate, sulfate, and chloride also increased during the oxidn. without a corresponding increase in ammonium, suggesting org. nitrate, org. sulfate, and org. chloride formation. As secondary species are reported to have detrimental health effects, further studies should not be limited to the inactivation of bioaerosol or redn. of particular VOCs, but should also evaluate potential OVOCs and SOA formation from electronic air cleaners in different indoor environments.
  33. 33
    Peng, Z.; Day, D. A.; Stark, H.; Li, R.; Lee-Taylor, J.; Palm, B. B.; Brune, W. H.; Jimenez, J. L. HOx Radical Chemistry in Oxidation Flow Reactors with Low-Pressure Mercury Lamps Systematically Examined by Modeling. Atmospheric Measurement Techniques 2015, 8 (11), 4863– 4890,  DOI: 10.5194/amt-8-4863-2015
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    HOx radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling
    Peng, Z.; Day, D. A.; Stark, H.; Li, R.; Lee-Taylor, J.; Palm, B. B.; Brune, W. H.; Jimenez, J. L.
    Atmospheric Measurement Techniques (2015), 8 (11), 4863-4890CODEN: AMTTC2; ISSN:1867-8548. (Copernicus Publications)
    Oxidn. flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are commonly used in atm. chem. and other fields. OFR254 requires the addn. of externally formed O3 since OH is formed from O3 photolysis, while OFR185 does not since O2 can be photolyzed to produce O3, and OH can also be formed from H2O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and lab. studies. We show that the radical chem. in OFRs can be characterized as a function of UV light intensity, H2O concn., and total external OH reactivity (OHRext, e.g., from volatile org. compds. (VOCs), NOx, and SO2). OH exposure is decreased by added external OH reactivity. OFR185 is esp. sensitive to this effect at low UV intensity due to low primary OH prodn. OFR254 can be more resilient against OH suppression at high injected O3 (e.g., 70 ppm), as a larger primary OH source from O3, as well as enhanced recycling of HO2 to OH, make external perturbations to the radical chem. less significant. However if the external OH reactivity in OFR254 is much larger than OH reactivity from injected O3, OH suppression can reach 2 orders of magnitude. For a typical input of 7 ppm O3 (OHRO3 = 10 s-1), 10-fold OH suppression is obsd. at OHRext ∼ 100 s-1, which is similar or lower than used in many lab. studies. The range of modeled OH suppression for literature expts. is consistent with the measured values except for those with isoprene. The finding on OH suppression may have important implications for the interpretation of past lab. studies, as applying OHexp measurements acquired under different conditions could lead to over a 1-order-of-magnitude error in the estd. OHexp. The uncertainties of key model outputs due to uncertainty in all rate consts. and absorption cross-sections in the model are within ±25% for OH exposure and within ±60% for other parameters. These uncertainties are small relative to the dynamic range of outputs. Uncertainty anal. shows that most of the uncertainty is contributed by photolysis rates of O3, O2, and H2O and reactions of OH and HO2 with themselves or with some abundant species, i.e., O3 and H2O2. OHexp calcd. from direct integration and estd. from SO2 decay in the model with laminar and measured residence time distributions (RTDs) are generally within a factor of 2 from the plug-flow OHexp. However, in the models with RTDs, OHexp estd. from SO2 is systematically lower than directly integrated OHexp in the case of significant SO2 consumption. We thus recommended using OHexp estd. from the decay of the species under study when possible, to obtain the most appropriate information on photochem. aging in the OFR. Using HOx - recycling vs. destructive external OH reactivity only leads to small changes in OHexp under most conditions. Changing the identity (rate const.) of external OH reactants can result in substantial changes in OHexp due to different redns. in OH suppression as the reactant is consumed. We also report two equations for estg. OH exposure in OFR254. We find that the equation estg. OHexp from measured O3 consumption performs better than an alternative equation that does not use it, and thus recommend measuring both input and output O3 concns. in OFR254 expts. This study contributes to establishing a firm and systematic understanding of the gas-phase HOx and Ox chem. in these reactors, and enables better expt. planning and interpretation as well as improved design of future reactors.
  34. 34
    Peng, Z.; Jimenez, J. L. Modeling of the Chemistry in Oxidation Flow Reactors with High Initial NO. Atmos. Chem. Phys. 2017, 17 (19), 11991– 12010,  DOI: 10.5194/acp-17-11991-2017
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    Modeling of the chemistry in oxidation flow reactors with high initial NO
    Peng, Zhe; Jimenez, Jose L.
    Atmospheric Chemistry and Physics (2017), 17 (19), 11991-12010CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Oxidn. flow reactors (OFRs) are increasingly employed in atm. chem. research because of their high efficiency of OH radical prodn. from low-pressure Hg lamp emissions at both 185 and 254 nm (OFR185) or 254 nm only (OFR254). OFRs have been thought to be limited to studying low-NO chem. (in which peroxy radicals (RO2) react preferentially with HO2) because NO is very rapidly oxidized by the high concns. of O2, HO2, and OH in OFRs. However, many groups are performing expts. by aging combustion exhaust with high NO levels or adding NO in the hopes of simulating high-NO chem. (in which RO2 + NO dominates). This work systematically explores the chem. in OFRs with high initial NO. Using box modeling, we investigate the interconversion of Ncontaining species and the uncertainties due to kinetic parameters. Simple initial injection of NO in OFR185 can result in more RO2 reacted with NO than with HO2 and minor non-tropospheric photolysis, but only under a very narrow set of conditions (high water mixing ratio, low UV intensity, low external OH reactivity (OHRext), and initial NO concn. (NOin) of tens to hundreds of ppb) that account for a very small fraction of the input parameter space. These conditions are generally far away from exptl. conditions of published OFR studies with high initial NO. In particular, studies of aerosol formation from vehicle emissions in OFRs often used OHRext and NOin several orders of magnitude higher. Due to extremely high OHRext and NOin, some studies may have resulted in substantial non-tropospheric photolysis, strong delay to RO2 chem. due to peroxynitrate formation, VOC reactions with NO2 dominating over those with OH, and faster reactions of OH-arom. adducts with NO2 than those with O2, all of which are irrelevant to ambient VOC photooxidn. chem. Some of the neg. effects are the worst for alkene and arom. precursors. To avoid undesired chem., vehicle emissions generally need to be dild. by a factor of >100 before being injected into an OFR. However, sufficiently dild. vehicle emissions generally do not lead to high-NO chem. in OFRs but are rather dominated by the low-NO RO2 + HO2 pathway. To ensure high-NO conditions without substantial atmospherically irrelevant chem. in a more controlled fashion, new techniques are needed.
  35. 35
    Stockwell, W. R.; Kirchner, F.; Kuhn, M.; Seefeld, S. A New Mechanism for Regional Atmospheric Chemistry Modeling. J. Geophys. Res. 1997, 102 (D22), 25847– 25879,  DOI: 10.1029/97JD00849
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    A new mechanism for regional atmospheric chemistry modeling
    Stockwell, William R.; Kirchner, Frank; Kuhn, Michael; Seefeld, Stephan
    Journal of Geophysical Research, [Atmospheres] (1997), 102 (D22), 25847-25879CODEN: JGRDE3 ISSN:. (American Geophysical Union)
    A new gas-phase chem. mechanism for the modeling of regional atm. chem., the "Regional Atm. Chem. Mechanism" (RACM) is presented. The mechanism is intended to be valid for remote to polluted conditions and from the Earth's surface through the upper troposphere. The RACM mechanism is based upon the earlier Regional Acid Deposition Model, version 2 (RADM2) mechanism [Stockwell et al., 1990] and the more detailed Euro-RADM mechanism [Stockwell and Kley, 1994]. The RACM mechanism includes rate consts. and product yields from the most recent lab. measurements, and it has been tested against environmental chamber data. A new condensed reaction mechanism is included for biogenic compds.: isoprene, α-pinene, and d-limonene. The branching ratios for alkane decay were reevaluated, and in the revised mechanism the aldehyde to ketone ratios were significantly reduced. The relatively large amts. of nitrates resulting from the reactions of unbranched alkenes with NO3 are now included, and the prodn. of HO from the ozonolysis of alkenes has a much greater yield. The arom. chem. has been revised through the use of new lab. data. The yield of cresol prodn. from aroms. was reduced, while the reactions of HO, NO3, and O3 with unsatd. dicarbonyl species and unsatd. peroxynitrate are now included in the RACM mechanism. The peroxyacetyl nitrate chem. and the org. peroxy radical-peroxy radical reactions were revised, and org. peroxy radical + NO3 reactions were added.
  36. 36
    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 (4), 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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    McDonald, B. C.; de Gouw, J. A.; Gilman, J. B.; Jathar, S. H.; Akherati, A.; Cappa, C. D.; Jimenez, J. L.; Lee-Taylor, J.; Hayes, P. L.; McKeen, S. A.; Cui, Y. Y.; Kim, S.-W.; Gentner, D. R.; Isaacman-VanWertz, G.; Goldstein, A. H.; Harley, R. A.; Frost, G. J.; Roberts, J. M.; Ryerson, T. B.; Trainer, M. Volatile Chemical Products Emerging as Largest Petrochemical Source of Urban Organic Emissions. Science 2018, 359 (6377), 760– 764,  DOI: 10.1126/science.aaq0524
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    Volatile chemical products emerging as largest petrochemical source of urban organic emissions
    McDonald, Brian C.; de Gouw, Joost A.; Gilman, Jessica B.; Jathar, Shantanu H.; Akherati, Ali; Cappa, Christopher D.; Jimenez, Jose L.; Lee-Taylor, Julia; Hayes, Patrick L.; McKeen, Stuart A.; Cui, Yu Yan; Kim, Si-Wan; Gentner, Drew R.; Isaacman-VanWertz, Gabriel; Goldstein, Allen H.; Harley, Robert A.; Frost, Gregory J.; Roberts, James M.; Ryerson, Thomas B.; Trainer, Michael
    Science (Washington, DC, United States) (2018), 359 (6377), 760-764CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A gap in emission inventories of urban volatile org. compd. (VOC) sources, which contribute to regional ozone and aerosol burdens, has increased as transportation emissions in the United States and Europe have declined rapidly. A detailed mass balance demonstrates that the use of volatile chem. products (VCPs)-including pesticides, coatings, printing inks, adhesives, cleaning agents, and personal care products-now constitutes half of fossil fuel VOC emissions in industrialized cities. The high fraction of VCP emissions is consistent with obsd. urban outdoor and indoor air measurements. We show that human exposure to carbonaceous aerosols of fossil origin is transitioning away from transportation-related sources and toward VCPs. Existing U.S. regulations on VCPs emphasize mitigating ozone and air toxics, but they currently exempt many chems. that lead to secondary org. aerosols.
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    Upper room germicidal ultraviolet fixtures. AeroMed Technologies. https://aeromed.com/product/upper-room-guv-fixtures/ (accessed 2022-02-21).
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    Xu, P.; Kujundzic, E.; Peccia, J.; Schafer, M. P.; Moss, G.; Hernandez, M.; Miller, S. L. Impact of Environmental Factors on Efficacy of Upper-Room Air Ultraviolet Germicidal Irradiation for Inactivating Airborne Mycobacteria. Environ. Sci. Technol. 2005, 39 (24), 9656– 9664,  DOI: 10.1021/es0504892
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    Impact of Environmental Factors on Efficacy of Upper-Room Air Ultraviolet Germicidal Irradiation for Inactivating Airborne Mycobacteria
    Xu, Peng; Kujundzic, Elmira; Peccia, Jordan; Schafer, Millie P.; Moss, Gene; Hernandez, Mark; Miller, Shelly L.
    Environmental Science and Technology (2005), 39 (24), 9656-9664CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    This study evaluated the efficacy of an upper-room air UV germicidal irradn. (UVGI) system for inactivating airborne bacteria, which irradiates the upper part of a room while minimizing radiation exposure to persons in the lower part of the room. A full-scale test room (87 m3), fitted with a UVGI system consisting of 9 louvered wall and ceiling fixtures (504 W with all lamps operating) was operated at 24° and 34°, between 25 and 90% relative humidity (RH), and at three ventilation rates. Mycobacterium parafortuitum cells were aerosolized into the room such that their nos. and physiol. state were comparable both with and without the UVGI system operating. Airborne bacteria were collected in duplicate using liq. impingers and quantified with direct epifluorescent microscopy and std. culturing assay. The performance of the UVGI system was degraded significantly when the RH was increased from 50% to 75-90%, the horizontal UV fluence rate distribution was skewed to one side compared to being evenly dispersed, and the room air temp. was stratified from hot at the ceiling to cold at the floor. The inactivation rate increased linearly with effective UV fluence rate up to 5 μW/cm2; an increase in the fluence rate above this level did not yield a proportional increase in inactivation rate.
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    Xu, P.; Peccia, J.; Fabian, P.; Martyny, J. W.; Fennelly, K. P.; Hernandez, M.; Miller, S. L. Efficacy of Ultraviolet Germicidal Irradiation of Upper-Room Air in Inactivating Airborne Bacterial Spores and Mycobacteria in Full-Scale Studies. Atmos. Environ. 2003, 37 (3), 405– 419,  DOI: 10.1016/S1352-2310(02)00825-7
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    Efficacy of ultraviolet germicidal irradiation of upper-room air in inactivating airborne bacterial spores and mycobacteria in full-scale studies
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    Atmospheric Environment (2003), 37 (3), 405-419CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
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  3. Sara Bjerre Sørensen, Frederik Rask Dalby, Søren Kristian Olsen, Kasper Kristensen. Influence of Germicidal UV (222 nm) Lamps on Ozone, Ultrafine Particles, and Volatile Organic Compounds in Indoor Office Spaces. Environmental Science & Technology 2024, 58 (45) , 20073-20080. https://doi.org/10.1021/acs.est.4c03903
  4. Olivia J. Jenks, Zhe Peng, Melinda K. Schueneman, Madison Rutherford, Anne V. Handschy, Douglas A. Day, Jose L. Jimenez, Joost A. de Gouw. Effects of 222 nm Germicidal Ultraviolet Light on Aerosol and VOC Formation from Limonene. ACS ES&T Air 2024, 1 (7) , 725-733. https://doi.org/10.1021/acsestair.4c00065
  5. Seongjun Park, Youngbo Won, Donghyun Rim. Formation and Transport of Secondary Contaminants Associated with Germicidal Ultraviolet Light Systems in an Occupied Classroom. Environmental Science & Technology 2024, 58 (27) , 12051-12061. https://doi.org/10.1021/acs.est.4c00575
  6. Michael F. Link, Rileigh Robertson, Megan S. Claflin, Dustin Poppendieck. Quantification of Byproduct Formation from Portable Air Cleaners Using a Proposed Standard Test Method. Environmental Science & Technology 2024, 58 (18) , 7916-7923. https://doi.org/10.1021/acs.est.3c09331
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  16. Matthew B. Goss, Jesse H. Kroll. Organic aerosol formation from 222 nm germicidal light: ozone-initiated vs. non-ozone pathways. Environmental Science: Processes & Impacts 2025, 8 https://doi.org/10.1039/D4EM00384E
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  19. David J. Brenner. The public‐health significance of far‐UVC‐induced indoor ozone and its associated secondary chemistry. Photochemistry and Photobiology 2024, 100 (5) , 1186-1190. https://doi.org/10.1111/php.13892
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  24. Linsey C. Marr, Jonathan M. Samet. Reducing Transmission of Airborne Respiratory Pathogens: A New Beginning as the COVID-19 Emergency Ends. Environmental Health Perspectives 2024, 132 (5) https://doi.org/10.1289/EHP13878
  25. Devabhaktuni Srikrishna. Pentagon Found Daily, Metagenomic Detection of Novel Bioaerosol Threats to Be Cost-Prohibitive: Can Virtualization and AI Make It Cost-Effective?. Health Security 2024, 22 (2) , 108-129. https://doi.org/10.1089/hs.2023.0048
  26. P. Jacob Bueno de Mesquita, Rosemary K. Sokas, Mary B. Rice, Edward A. Nardell. Far-UVC: Technology Update with an Untapped Potential to Mitigate Airborne Infections. Annals of the American Thoracic Society 2023, 20 (12) , 1700-1702. https://doi.org/10.1513/AnnalsATS.202305-460VP
  27. Kangqi Guo, Yue Pan, Hoi Fung Ronaldo Chan, Kin-Fai Ho, Chun Chen. Far-UVC disinfection of airborne and surface virus in indoor environments: Laboratory experiments and numerical simulations. Building and Environment 2023, 245 , 110900. https://doi.org/10.1016/j.buildenv.2023.110900
  28. Adrian F. Tuck. Actinic Radiation, Viruses, Bacteria, the Open Air Factor (OAF) and Indoor Sterilization with UV-C Radiation. Processes 2023, 11 (10) , 2882. https://doi.org/10.3390/pr11102882
  29. Zhancong Liang, Tim Yiu Cheung, Wing Lam Chan, Chee Kent Lim, Alvin. C. K. Lai, Patrick. K. H. Lee, Chak K. Chan. Negligible increase in indoor endotoxin activity by 222 nm far-UVC illumination on bioaerosols. Environmental Science: Atmospheres 2023, 3 (8) , 1212-1220. https://doi.org/10.1039/D3EA00059A
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Cite this: Environ. Sci. Technol. Lett. 2023, 10, 1, 6–13
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  • Abstract

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    Figure 1

    Figure 1. Schematics of a germicidal ultraviolet air disinfection system at (a) 254 nm and at (b) 222 nm in a room; (c) absorption cross sections of several important gas-phase species relevant to this study (a discontinuity in the spectrum of methacrolein is due to lack of data).

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    Figure 2

    Figure 2. Final quantity/concentration of the main (types of) species of interest in this study under different GUV and ventilation conditions. In the GUV254 cases, the volume-weighted average concentrations for the whole room are shown. The stable chemical species concentrations are similar between the irradiated and unirradiated zones, while the radical and SARS-CoV-2 concentrations in the unirradiated zone can be significantly lower and higher, respectively (Table S3). Note that some panels use a log scale for concentrations while other panels use a linear scale. SOA is assumed to have a molar weight of 200 g mol–1.

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  • References

    This article references 54 other publications.

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      Air Disinfection for Airborne Infection Control with a Focus on COVID-19: Why Germicidal UV is Essential
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      A review. Aerosol transmission is now widely accepted as the principal way that COVID-19 is spread, as has the importance of ventilation-natural and mech. But in other than healthcare facilities, mech. ventilation is designed for comfort, not airborne infection control, and cannot achieve the 6 to 12 room air changes per h recommended for airborne infection control. More efficient air filters have been recommended in ventilation ducts despite a lack of convincing evidence that SARS-CoV-2 virus spreads through ventilation systems. Most transmission appears to occur in rooms where both an infectious source COVID-19 case and other susceptible occupants share the same air. Only two established room-based technologies are available to supplement mech. ventilation: portable room air cleaners and upper room germicidal UV air disinfection. Portable room air cleaners can be effective, but performance is limited by their clean air delivery rate relative to room vol. SARS-CoV-2 is highly susceptible to GUV, an 80-yr-old technol. that has been shown to safely, quietly, effectively and economically produce the equiv. of 10 to 20 or more air changes per h under real life conditions. For these reasons, upper room GUV is the essential engineering intervention for reducing COVID-19 spread.
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      Accidental exposure to UV radiation produced by germicidal lamp: case report and risk assessment
      Zaffina, Salvatore; Camisa, Vincenzo; Lembo, Marco; Vinci, Maria Rosaria; Tucci, Mario Graziano; Borra, Massimo; Napolitano, Antonio; Cannata, Vittorio
      Photochemistry and Photobiology (2012), 88 (4), 1001-1004CODEN: PHCBAP; ISSN:0031-8655. (Wiley-Blackwell)
      UV radiation is known to cause both benefits and harmful effects on humans. The adverse effects mainly involve two target organs, skin and eye, and can be further divided into short- and long-term effects. The present case report describes an accidental exposure of two health-care workers to UV radiation produced by a germicidal lamp in a hospital pharmacy. The germicidal lamp presented a spectrum with an intense UV-C component as well as a modest UV-B contribution. Overexposure to UV-C radiation was over 100 times as large as the ICNIRP exposure limits. A few hours after the exposure, the two subjects reported symptoms of acute UV injury and both of them continued having significant clin. signs for over 2 years. In this study, we describe acute and potentially irreversible effects caused by high UV exposure. In addn., we present the results of risk assessment by occupational exposure to germicidal lamps.
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      Ma, B.; Gundy, P. M.; Gerba, C. P.; Sobsey, M. D.; Linden, K. G. UV Inactivation of SARS-CoV-2 across the UVC Spectrum: KrCl* Excimer, Mercury-Vapor, and Light-Emitting-Diode (LED) Sources. Appl. Environ. Microbiol. 2021, 87 (22), e0153221  DOI: 10.1128/AEM.01532-21
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      Buonanno, M.; Welch, D.; Shuryak, I.; Brenner, D. J. Far-UVC Light (222 Nm) Efficiently and Safely Inactivates Airborne Human Coronaviruses. Sci. Rep. 2020, 10 (1), 10285,  DOI: 10.1038/s41598-020-67211-2
      7
      Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses
      Buonanno, Manuela; Welch, David; Shuryak, Igor; Brenner, David J.
      Scientific Reports (2020), 10 (1), 10285CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Abstr.: A direct approach to limit airborne viral transmissions is to inactivate them within a short time of their prodn. Germicidal UV light, typically at 254 nm, is effective in this context but, used directly, can be a health hazard to skin and eyes. By contrast, far-UVC light (207-222 nm) efficiently kills pathogens potentially without harm to exposed human tissues. We previously demonstrated that 222-nm far-UVC light efficiently kills airborne influenza virus and we extend those studies to explore far-UVC efficacy against airborne human coronaviruses alpha HCoV-229E and beta HCoV-OC43. Low doses of 1.7 and 1.2 mJ/cm2 inactivated 99.9% of aerosolized coronavirus 229E and OC43, resp. As all human coronaviruses have similar genomic sizes, far-UVC light would be expected to show similar inactivation efficiency against other human coronaviruses including SARS-CoV-2. Based on the beta-HCoV-OC43 results, continuous far-UVC exposure in occupied public locations at the current regulatory exposure limit (∼3 mJ/cm2/h) would result in ∼90% viral inactivation in ∼8 min, 95% in ∼11 min, 99% in ∼16 min and 99.9% inactivation in ∼25 min. Thus while staying within current regulatory dose limits, low-dose-rate far-UVC exposure can potentially safely provide a major redn. in the ambient level of airborne coronaviruses in occupied public locations.
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      Ong, Q.; Wee, W.; Dela Cruz, J.; Teo, J. W. R.; Han, W. 222-Nanometer Far-UVC Exposure Results in DNA Damage and Transcriptional Changes to Mammalian Cells. Int. J. Mol. Sci. 2022, 23 (16), 9112,  DOI: 10.3390/ijms23169112
      8
      222-Nanometer Far-UVC Exposure Results in DNA Damage and Transcriptional Changes to Mammalian Cells
      Ong, Qunxiang; Wee, Winson; Dela Cruz, Joshua; Teo, Jin Wah Ronnie; Han, Weiping
      International Journal of Molecular Sciences (2022), 23 (16), 9112CODEN: IJMCFK; ISSN:1422-0067. (MDPI AG)
      UV germicidal tools have recently gained attention as a disinfection strategy against the COVID-19 pandemic, but the safety profile arising from their exposure has been controversial and impeded larger-scale implementation. We compare the emerging 222-nm far UVC and 277-nm UVC LED disinfection modules with the traditional UVC mercury lamp emitting at 254 nm to understand their effects on human retinal cell line ARPE-19 and HEK-A keratinocytes. Cells illuminated with 222-nm far UVC survived, while those treated with 254-nm and 277-nm wavelengths underwent apoptosis via the JNK/ATF2 pathway. However, cells exposed to 222-nm far UVC presented the highest degree of DNA damage as evidenced by yH2AX staining. Globally, these cells displayed transcriptional changes in cell-cycle and senescence pathways. Thus, the introduction of 222-nm far UVC lamps for disinfection purposes should be carefully considered and designed with the inherent dangers involved.
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      Kujundzic, E.; Matalkah, F.; Howard, C. J.; Hernandez, M.; Miller, S. L. UV Air Cleaners and Upper-Room Air Ultraviolet Germicidal Irradiation for Controlling Airborne Bacteria and Fungal Spores. J. Occup. Environ. Hyg. 2006, 3 (10), 536– 546,  DOI: 10.1080/15459620600909799
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      UV air cleaners and upper-room air ultraviolet germicidal irradiation for controlling airborne bacteria and fungal spores
      Kujundzic Elmira; Matalkah Fatimah; Howard Cody J; Hernandez Mark; Miller Shelly L
      Journal of occupational and environmental hygiene (2006), 3 (10), 536-46 ISSN:1545-9624.
      In-room air cleaners (ACs) and upper-room air ultraviolet germicidal irradiation (UVGI) are engineering control technologies that can help reduce the concentrations of airborne bacteria and fungal spores in the indoor environment. This study investigated six different types of ACs and quantified their ability to remove and/or inactivate airborne bacteria and fungal spores. Four of the air cleaners incorporated UV lamp(s) into their flow path. In addition, the efficacy of combining ACs with upper-room air UVGI was investigated. With the ventilation system providing zero or six air changes per hour, the air cleaners were tested separately or with the upper-room air UVGI system in operation in an 87-m3 test room. Active bacteria cells and fungal spores were aerosolized into the room such that their numbers and physiologic state were comparable both with and without air cleaning and upper-room air UVGI. In addition, the disinfection performance of a UV-C lamp internal to one of the ACs was evaluated by estimating the percentage of airborne bacteria cells and fungal spores captured on the air filter medium surface that were inactivated with UV exposure. Average airborne microbial clean air delivery rates (CADRm) varied between 26-981 m3 hr-1 depending on the AC, and between 1480-2370 m3 hr-1, when using air cleaners in combination with upper-room air UVGI. Culturing, direct microscopy, and optical particle counting revealed similar CADRm. The ACs performed similarly when challenged with three different microorganisms. Testing two of the ACs showed that no additional air cleaning was provided with the operation of an internal UV-C lamp; the internal UV-C lamps, however, inactivated 75% of fungal spores and 97% of bacteria cells captured in the air filter medium within 60 min.
    10. 10
      Wang, C. C.; Prather, K. A.; Sznitman, J.; Jimenez, J. L.; Lakdawala, S. S.; Tufekci, Z.; Marr, L. C. Airborne Transmission of Respiratory Viruses. Science 2021, 373 (6558), eabd9149  DOI: 10.1126/science.abd9149
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      Greenhalgh, T.; Jimenez, J. L.; Prather, K. A.; Tufekci, Z.; Fisman, D.; Schooley, R. Ten Scientific Reasons in Support of Airborne Transmission of SARS-CoV-2. Lancet 2021, 397, 1603,  DOI: 10.1016/S0140-6736(21)00869-2
      11
      Ten scientific reasons in support of airborne transmission of SARS-CoV-2
      Greenhalgh, Trisha; Jimenez, Jose L.; Prather, Kimberly A.; Tufekci, Zeynep; Fisman, David; Schooley, Robert
      Lancet (2021), 397 (10285), 1603-1605CODEN: LANCAO; ISSN:0140-6736. (Elsevier Ltd.)
      Heneghan and colleagues' systematic review, funded by WHO, published in March, 2021, as a preprint, states: "The lack of recoverable viral culture samples of SARS-CoV-2 prevents firm conclusions to be drawn about airborne transmission" (F1000Research 2021; published online March 24. https://doi.org/10.12688/f1000research.52091.1 (preprint). This conclusion, and the wide circulation of the review's findings, is concerning because of the public health implications. In this report, ten streams of evidence that collectively support the hypothesis that SARS-CoV-2 is transmitted primarily by the airborne route is presented. The authors propose that it is a scientific error to use lack of direct evidence of SARS-CoV-2 in some air samples to cast doubt on airborne transmission while overlooking the quality and strength of the overall evidence base. There is consistent, strong evidence that SARS-CoV-2 spreads by airborne transmission. Although other routes can contribute, we believe that the airborne route is likely to be dominant. The public health community should act accordingly and without further delay.
    12. 12
      Klompas, M.; Milton, D. K.; Rhee, C.; Baker, M. A.; Leekha, S. Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs: A Narrative Review. Ann. Int. Med. 2021, 174 (12), 1710– 1718,  DOI: 10.7326/M21-2780
      12
      Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs : A Narrative Review
      Klompas Michael; Rhee Chanu; Baker Meghan A; Milton Donald K; Leekha Surbhi
      Annals of internal medicine (2021), 174 (12), 1710-1718 ISSN:.
      Policies to prevent respiratory virus transmission in health care settings have traditionally divided organisms into Droplet versus Airborne categories. Droplet organisms (for example, influenza) are said to be transmitted via large respiratory secretions that rapidly fall to the ground within 1 to 2 meters and are adequately blocked by surgical masks. Airborne pathogens (for example, measles), by contrast, are transmitted by aerosols that are small enough and light enough to carry beyond 2 meters and to penetrate the gaps between masks and faces; health care workers are advised to wear N95 respirators and to place these patients in negative-pressure rooms. Respirators and negative-pressure rooms are also recommended when caring for patients with influenza or SARS-CoV-2 who are undergoing "aerosol-generating procedures," such as intubation. An increasing body of evidence, however, questions this framework. People routinely emit respiratory particles in a range of sizes, but most are aerosols, and most procedures do not generate meaningfully more aerosols than ordinary breathing, and far fewer than coughing, exercise, or labored breathing. Most transmission nonetheless occurs at close range because virus-laden aerosols are most concentrated at the source; they then diffuse and dilute with distance, making long-distance transmission rare in well-ventilated spaces. The primary risk factors for nosocomial transmission are community incidence rates, viral load, symptoms, proximity, duration of exposure, and poor ventilation. Failure to appreciate these factors may lead to underappreciation of some risks (for example, overestimation of the protection provided by medical masks, insufficient attention to ventilation) or misallocation of limited resources (for example, reserving N95 respirators and negative-pressure rooms only for aerosol-generating procedures or requiring negative-pressure rooms for all patients with SARS-CoV-2 infection regardless of stage of illness). Enhanced understanding of the factors governing respiratory pathogen transmission may inform the development of more effective policies to prevent nosocomial transmission of respiratory pathogens.
    13. 13
      Qian, H.; Miao, T.; Liu, L.; Zheng, X.; Luo, D.; Li, Y. Indoor Transmission of SARS-CoV-2. Indoor Air 2021, 31 (3), 639– 645,  DOI: 10.1111/ina.12766
      13
      Indoor transmission of SARS-CoV-2
      Qian, Hua; Miao, Te; Liu, Li; Zheng, Xiaohong; Luo, Danting; Li, Yuguo
      Indoor Air (2021), 31 (3), 639-645CODEN: INAIE5; ISSN:1600-0668. (Wiley-Blackwell)
      It is essential to understand where and how severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is transmitted. Case reports were extd. from the local Municipal Health Commissions of 320 prefectural municipalities in China (not including Hubei Province). We identified all outbreaks involving three or more cases and reviewed the major characteristics of the enclosed spaces in which the outbreaks were reported and their assocd. indoor environmental aspects. Three hundred and eighteen outbreaks with three or more cases were identified, comprising a total of 1245 confirmed cases in 120 prefectural cities. Among the identified outbreaks, 53.8% involved three cases, 26.4% involved four cases, and only 1.6% involved ten or more cases. Home-based outbreaks were the dominant category (254 of 318 outbreaks; 79.9%), followed by transport-based outbreaks (108; 34.0%), and many outbreaks occurred in more than one category of venue. All identified outbreaks of three or more cases occurred in indoor environments, which confirm that sharing indoor spaces with one or more infected persons is a major SARS-CoV-2 infection risk.
    14. 14
      Adam, D. C.; Wu, P.; Wong, J. Y.; Lau, E. H. Y.; Tsang, T. K.; Cauchemez, S.; Leung, G. M.; Cowling, B. J. Clustering and Superspreading Potential of SARS-CoV-2 Infections in Hong Kong. Nat. Med. 2020, 26 (11), 1714– 1719,  DOI: 10.1038/s41591-020-1092-0
      14
      Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong
      Adam, Dillon C.; Wu, Peng; Wong, Jessica Y.; Lau, Eric H. Y.; Tsang, Tim K.; Cauchemez, Simon; Leung, Gabriel M.; Cowling, Benjamin J.
      Nature Medicine (New York, NY, United States) (2020), 26 (11), 1714-1719CODEN: NAMEFI; ISSN:1078-8956. (Nature Research)
      Abstr.: Superspreading events (SSEs) have characterized previous epidemics of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) infections1-6. For SARS-CoV-2, the degree to which SSEs are involved in transmission remains unclear, but there is growing evidence that SSEs might be a typical feature of COVID-197,8. Using contact tracing data from 1,038 SARS-CoV-2 cases confirmed between 23 Jan. and 28 Apr. 2020 in Hong Kong, we identified and characterized all local clusters of infection. We identified 4-7 SSEs across 51 clusters (n = 309 cases) and estd. that 19% (95% confidence interval, 15-24%) of cases seeded 80% of all local transmission. Transmission in social settings was assocd. with more secondary cases than households when controlling for age (P = 0.002). Decreasing the delay between symptom onset and case confirmation did not result in fewer secondary cases (P = 0.98), although the odds that an individual being quarantined as a contact interrupted transmission was 14.4 (95% CI, 1.9-107.2). Public health authorities should focus on rapidly tracing and quarantining contacts, along with implementing restrictions targeting social settings to reduce the risk of SSEs and suppress SARS-CoV-2 transmission.
    15. 15
      Peng, Z.; Rojas, A. L. P.; Kropff, E.; Bahnfleth, W.; Buonanno, G.; Dancer, S. J.; Kurnitski, J.; Li, Y.; Loomans, M. G. L. C.; Marr, L. C.; Morawska, L.; Nazaroff, W.; Noakes, C.; Querol, X.; Sekhar, C.; Tellier, R.; Greenhalgh, T.; Bourouiba, L.; Boerstra, A.; Tang, J. W.; Miller, S. L.; Jimenez, J. L. Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks. Environ. Sci. Technol. 2022, 56 (2), 1125– 1137,  DOI: 10.1021/acs.est.1c06531
      15
      Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks
      Peng, Z.; Rojas, A. L. Pineda; Kropff, E.; Bahnfleth, W.; Buonanno, G.; Dancer, S. J.; Kurnitski, J.; Li, Y.; Loomans, M. G. L. C.; Marr, L. C.; Morawska, L.; Nazaroff, W.; Noakes, C.; Querol, X.; Sekhar, C.; Tellier, R.; Greenhalgh, T.; Bourouiba, L.; Boerstra, A.; Tang, J. W.; Miller, S. L.; Jimenez, J. L.
      Environmental Science & Technology (2022), 56 (2), 1125-1137CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Some infectious diseases, including COVID-19, can undergo airborne transmission. This may happen at close proximity, but as time indoors increases, infections can occur in shared room air despite distancing. We propose 2 indicators of infection risk for this situation, i.e., relative risk parameter (Hr) and risk parameter (H). They combine the key factors that control airborne disease transmission indoors: virus-contg. aerosol generation rate, breathing flow rate, masking and its quality, ventilation and aerosol-removal rates, no. of occupants, and duration of exposure. COVID-19 outbreaks show a clear trend that is consistent with airborne infection and enable recommendations to minimize transmission risk. Transmission in typical prepandemic indoor spaces is highly sensitive to mitigation efforts. Previous outbreaks of measles, influenza, and tuberculosis were also assessed. Measles outbreaks occur at much lower risk parameter values than COVID-19, while tuberculosis outbreaks are obsd. at higher risk parameter values. Because both diseases are accepted as airborne, the fact that COVID-19 is less contagious than measles does not rule out airborne transmission. It is important that future outbreak reports include information on masking, ventilation and aerosol-removal rates, no. of occupants, and duration of exposure, to investigate airborne transmission.
    16. 16
      Li, Y.; Cheng, P.; Jia, W. Poor Ventilation Worsens Short-Range Airborne Transmission of Respiratory Infection. Indoor Air 2022, 32 (1), e12946  DOI: 10.1111/ina.12946
      16
      Poor ventilation worsens short-range airborne transmission of respiratory infection
      Li, Yuguo; Cheng, Pan; Jia, Wei
      Indoor Air (2022), 32 (1), e12946CODEN: INAIE5; ISSN:1600-0668. (Wiley-Blackwell)
      To explain the obsd. phenomenon that most SARS-CoV-2 transmission occurs indoors whereas its outdoor transmission is rare, a simple macroscopic aerosol balance model is developed to link short- and long-range airborne transmission. The model considers the involvement of exhaled droplets with initial diam. ≤50μm in the short-range airborne route, whereas only a fraction of these droplets with an initial diam. within 15μm or equivalently a final diam. within 5μm considered in the long-range airborne route. One surprising finding is that the room ventilation rate significantly affects the short-range airborne route, in contrast to traditional belief. When the ventilation rate in a room is insufficient, the airborne infection risks due to both short- and long-range transmission are high. A ventilation rate of 10 L/s per person provides a similar concn. vs distance decay profile to that in outdoor settings, which provides addnl. justification for the widely adopted ventilation std. of 10 L/s per person. The newly obtained data do not support the basic assumption in the existing ventilation std. ASHRAE 62.1 (2019) that the required people outdoor air rate is const. if the std. is used directly for respiratory infection control. Instead, it is necessary to increase the ventilation rate when the phys. distance between people is less than approx. 2 m.
    17. 17
      Jimenez, J. L.; Peng, Z.; Pagonis, D. Systematic Way to Understand and Classify the Shared-Room Airborne Transmission Risk of Indoor Spaces. Indoor Air 2022, 32 (5), e13025  DOI: 10.1111/ina.13025
      There is no corresponding record for this reference.
    18. 18
      Morawska, L.; Allen, J.; Bahnfleth, W.; Bluyssen, P. M.; Boerstra, A.; Buonanno, G.; Cao, J.; Dancer, S. J.; Floto, A.; Franchimon, F.; Greenhalgh, T.; Haworth, C.; Hogeling, J.; Isaxon, C.; Jimenez, J. L.; Kurnitski, J.; Li, Y.; Loomans, M.; Marks, G.; Marr, L. C.; Mazzarella, L.; Melikov, A. K.; Miller, S.; Milton, D. K.; Nazaroff, W.; Nielsen, P. V.; Noakes, C.; Peccia, J.; Prather, K.; Querol, X.; Sekhar, C.; Seppänen, O.; Tanabe, S.-I.; Tang, J. W.; Tellier, R.; Tham, K. W.; Wargocki, P.; Wierzbicka, A.; Yao, M. A Paradigm Shift to Combat Indoor Respiratory Infection. Science 2021, 372 (6543), 689– 691,  DOI: 10.1126/science.abg2025
      18
      A paradigm shift to combat indoor respiratory infection
      Morawska, Lidia; Allen, Joseph; Bahnfleth, William; Bluyssen, Philomena M.; Boerstra, Atze; Buonanno, Giorgio; Cao, Junji; Dancer, Stephanie J.; Floto, Andres; Franchimon, Francesco; Greenhalgh, Trisha; Haworth, Charles; Hogeling, Jaap; Isaxon, Christina; Jimenez, Jose L.; Kurnitski, Jarek; Li, Yuguo; Loomans, Marcel; Marks, Guy; Marr, Linsey C.; Mazzarella, Livio; Melikov, Arsen Krikor; Miller, Shelly; Milton, Donald K.; Nazaroff, William; Nielsen, Peter V.; Noakes, Catherine; Peccia, Jordan; Prather, Kim; Querol, Xavier; Sekhar, Chandra; Seppanen, Olli; Tanabe, Shin-ichi; Tang, Julian W.; Tellier, Raymond; Tham, Kwok Wai; Wargocki, Pawel; Wierzbicka, Aneta; Yao, Maosheng
      Science (Washington, DC, United States) (2021), 372 (6543), 689-691CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      A review. There is great disparity in the way we think about and address different sources of environmental infection. Governments have for decades promulgated a large amt. of legislation and invested heavily in food safety, sanitation, and drinking water for public health purposes. By contrast, airborne pathogens and respiratory infections, whether seasonal influenza or COVID-19, are addressed fairly weakly, if at all, in terms of regulations, stds., and building design and operation, pertaining to the air we breathe. We suggest that the rapid growth in our understanding of the mechanisms behind respiratory infection transmission should drive a paradigm shift in how we view and address the transmission of respiratory infections to protect against unnecessary suffering and economic losses. It starts with a recognition that preventing respiratory infection, like reducing waterborne or foodborne disease, is a tractable problem.
    19. 19
      ASHRAE. Ventilation for Acceptable Indoor Air Quality: ANSI/ASHRAE Standard 62.1-2019; ANSI/ASHRAE, 2019.
      There is no corresponding record for this reference.
    20. 20
      Peng, Z.; Jimenez, J. L. Radical Chemistry in Oxidation Flow Reactors for Atmospheric Chemistry Research. Chem. Soc. Rev. 2020, 49 (9), 2570– 2616,  DOI: 10.1039/C9CS00766K
      20
      Radical chemistry in oxidation flow reactors for atmospheric chemistry research
      Peng, Zhe; Jimenez, Jose L.
      Chemical Society Reviews (2020), 49 (9), 2570-2616CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Environmental chambers have been playing a vital role in atm. chem. research for seven decades. In last decade, oxidn. flow reactors (OFR) have emerged as a promising alternative to chambers to study complex multigenerational chem. OFR can generate higher-than-ambient concns. of oxidants via H2O, O2 and O3 photolysis by low-pressure-Hg-lamp emissions and reach hours to days of equiv. photochem. aging in just minutes of real time. The use of OFR for volatile-org.-compd. (VOC) oxidn. and secondary-org.-aerosol formation has grown very rapidly recently. However, the lack of detailed understanding of OFR photochem. left room for speculation that OFR chem. may be generally irrelevant to the troposphere, since its initial oxidant generation is similar to stratosphere. Recently, a series of studies have been conducted to address important open questions on OFR chem. and to guide exptl. design and interpretation. In this Review, we present a comprehensive picture connecting the chemistries of hydroxyl (OH) and hydroperoxy radicals, oxidized nitrogen species and org. peroxy radicals (RO2) in OFR. Potential lack of tropospheric relevance assocd. with these chemistries, as well as the phys. conditions resulting in it will also be reviewed. When atm. oxidn. is dominated by OH, OFR conditions can often be similar to ambient conditions, as OH dominates against undesired non-OH effects. One key reason for tropospherically-irrelevant/undesired VOC fate is that under some conditions, OH is drastically reduced while non-tropospheric/undesired VOC reactants are not. The most frequent problems are running expts. with too high precursor concns., too high UV and/or too low humidity. On other hand, another cause of deviation from ambient chem. in OFR is that some tropospherically-relevant non-OH chem. (e.g. VOC photolysis in UVA and UVB) is not sufficiently represented under some conditions. In addn., the fate of RO2 produced from VOC oxidn. can be kept relevant to the troposphere. However, in some cases RO2 lifetime can be too short for atmospherically-relevant RO2 chem., including its isomerization. OFR applications using only photolysis of injected O3 to generate OH are less preferable than those using both 185 and 254 nm photons (without O3 injection) for several reasons. When a relatively low equiv. photochem. age (<∼1 d) and high NO are needed, OH and NO generation by org.-nitrite photolysis in the UVA range is preferable. We also discuss how to achieve the atm. relevance for different purposes in OFR exptl. planning.
    21. 21
      Atkinson, R.; Arey, J. Atmospheric Degradation of Volatile Organic Compounds. Chem. Rev. 2003, 103 (12), 4605– 4638,  DOI: 10.1021/cr0206420
      21
      Atmospheric Degradation of Volatile Organic Compounds
      Atkinson, Roger; Arey, Janet
      Chemical Reviews (Washington, DC, United States) (2003), 103 (12), 4605-4638CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review concerning the removal of biogenic and anthropogenic volatile org. compds. (VOC) emitted to the atm. through phys. (deposition) or chem. processes, or their atm. transformation is given. Topics discussed include: tropospheric VOC transformation processes (initial reactions and lifetimes, reaction mechanisms); atm. chem. of alkanes (kinetic data for initial OH- and NO3- reactions, reaction mechanism); atm. chem. of alkenes (rate consts. for initial reaction of alkenes with OH-, NO3-, and O3; mechanism of the OH- reaction; NO3- reaction; reaction with O3); arom. hydrocarbons (kinetics of OH- reactions, reactions of phenols, reactions of unsatd. 1,4-dicarbonyls and di-unsatd. 1,6-dicarbonyls); atm. reactions of oxygenated VOC (aldehydes, ketones, aliph. alcs., ethers, alkyl nitrates); and conclusions and future research needs (exptl. studies, theor. studies and crit. reviews and evaluations).
    22. 22
      Ziemann, P. J.; Atkinson, R. Kinetics, Products, and Mechanisms of Secondary Organic Aerosol Formation. Chem. Soc. Rev. 2012, 41 (19), 6582– 6605,  DOI: 10.1039/c2cs35122f
      22
      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.
    23. 23
      Blitz, M. A.; Heard, D. E.; Pilling, M. J. Wavelength Dependent Photodissociation of CH3OOH: Quantum Yields for CH3O and OH, and Measurement of the OH+ CH3OOH Rate Coefficient. J. Photochem. Photobiol. A Chem. 2005, 176 (1–3), 107– 113,  DOI: 10.1016/j.jphotochem.2005.09.017
      23
      Wavelength dependent photodissociation of CH3OOH
      Blitz, Mark A.; Heard, Dwayne E.; Pilling, Michael J.
      Journal of Photochemistry and Photobiology, A: Chemistry (2005), 176 (1-3), 107-113CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      Me hydroperoxide, CH3OOH, has been synthesized with >99.5% purity, confirmed using UV absorption spectroscopy and high-pressure liq. chromatog. (HPLC) followed by post-column derivatization. The UV absorption cross-section for CH3OOH was measured and for < 325 nm was in good agreement with the literature. Laser-flash photolysis combined with laser-induced fluorescence (LIF) spectroscopy has been used to measure both OH and CH3O photofragments following the photolysis of CH3OOH in the wavelength range 223-355 nm. Using the previously measured unity quantum yield for OH at 248 nm as a ref., the LIF signals immediately following photolysis were used to measure wavelength dependent quantum yields for OH and CH3O, taking into account changes in laser pulse energy and absorption cross-section. The quantum yields for both species were unity within exptl. error. The rate coeff. for the reaction of OH with CH3OOH (R1a) to generate CH3O2 + H2O products was measured at 295 K to be k (R1a) = (9.0 ± 0.2) × 10-12 cm3 mol.-1 s-1, considerably higher (by about a factor of two) than previous values measured by Vaghjiani and Ravishankara [G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 (1989) 1948-1959] and Niki et al. [H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 2190-2193].
    24. 24
      Vaghjiani, G. L.; Ravishankara, A. R. Photodissociation of H2O2 and CH3OOH at 248 Nm and 298 K: Quantum Yields for OH, O(3P) and H(2S). J. Chem. Phys. 1990, 92 (2), 996– 1003,  DOI: 10.1063/1.458081
      24
      Photodissociation of hydrogen peroxide and methyl hydroperoxide (CH3OOH) at 248 nm and 298 K: quantum yields for hydroxyl, atomic oxygen, and atomic hydrogen (O(3P), and H(2S))
      Vaghjiani, Ghanshyam L.; Ravishankara, A. R.
      Journal of Chemical Physics (1990), 92 (2), 996-1003CODEN: JCPSA6; ISSN:0021-9606.
      The quantum yields of OH, O, and H in H2O2 photolysis (248 nm, 298 K) were 2.09 ± 0.36, <0.002 and <0.0002 for OH, O and H, resp. For CH3OOH photolysis, the quantum yields were detd. as 1 ± 0.18, <0.007 and 0.038 ± 0.007 for OH, O and H, resp. In both H2O2 and CH3OOH photolysis, the obsd. O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH was consistent with the known continuous and unstructured absorption spectra of these mols. in this wavelength region, where the key process is the dissociative (~A1A ← ~X1A) transition to give OH (X2Π, ν'' = 0) fragment.
    25. 25
      Link, M. F.; Farmer, D. K.; Berg, T.; Flocke, F.; Ravishankara, A. R. Measuring Photodissociation Product Quantum Yields Using Chemical Ionization Mass Spectrometry: A Case Study with Ketones. J. Phys. Chem. A 2021, 125 (31), 6836– 6844,  DOI: 10.1021/acs.jpca.1c03140
      25
      Measuring photodissociation product quantum yields using chemical ionization mass spectrometry: A case study with ketones
      Link, Michael F.; Farmer, Delphine K.; Berg, Tyson; Flocke, Frank; Ravishankara, A. R.
      Journal of Physical Chemistry A (2021), 125 (31), 6836-6844CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Measurements of photolysis quantum yields are challenging because of the difficulties in measuring the first-generation photodissocn. products, interference from other products or contaminants, sufficient photon fluxes and/or low absorption cross sections of the photolyte to make detectable amts. of products, and quantification of the photon flux. In the case of acetone (and other atmospherically relevant ketones) the uncertainty in the photolysis quantum yield creates uncertainty in the calcd. OH radical and acyl peroxy nitrate prodn. in the atm. We present a new method for detg. photodissocn. product quantum yields by measuring acyl peroxy radicals (RC(O)O2) produced in the photolysis of ketones in air using chem. ionization mass spectrometry (CIMS). We show good agreement of our CIMS method with previously published quantum yields of the acyl radical from photolysis of biacetyl and Me Et ketone (MEK) at 254 nm. Addnl., we highlight the capabilities of this CIMS method through the measurement of photolysis branching ratios for MEK. We suggest future applications of CIMS (in the lab. and field) to measure RC(O)O2 and assocd. photolysis processes.
    26. 26
      Rajakumar, B.; Gierczak, T.; Flad, J. E.; Ravishankara, A. R.; Burkholder, J. B. The CH3CO Quantum Yield in the 248 Nm Photolysis of Acetone, Methyl Ethyl Ketone, and Biacetyl. J. Photochem. Photobiol. A Chem. 2008, 199 (2–3), 336– 344,  DOI: 10.1016/j.jphotochem.2008.06.015
      26
      The CH3CO quantum yield in the 248 nm photolysis of acetone, methyl ethyl ketone, and biacetyl
      Rajakumar, B.; Gierczak, Tomasz; Flad, Jonathan E.; Ravishankara, A. R.; Burkholder, James B.
      Journal of Photochemistry and Photobiology, A: Chemistry (2008), 199 (2-3), 336-344CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      We report measurements of the CH3CO quantum yield, ΦCH3CO , following the 248 nm pulsed laser photolysis of acetone (CH3C(O)CH3), Me Et ketone (CH3C(O)CH2CH3), and biacetyl (CH3C(O)C(O)CH3). CH3CO quantum yields at 248 nm were measured at 296 K, relative to CH3CO ref. systems. CH3CO was detected using cavity ring-down spectroscopy at wavelengths between 490 and 660 nm. Measurements were performed between 60 and 670 Torr (He, N2 bath gases) and the obtained CH3CO quantum yields in the low-pressure limit were 0.535 ± 0.09, 0.41 ± 0.08, and 0.76 ± 0.11, for acetone, Me Et ketone, and biacetyl, resp. The quoted uncertainties are 2σ (95% confidence level) and include estd. systematic errors. An increase in ΦCH3CO with increasing bath gas pressure, which depended on the identity of the collision partner (He, N2), was obsd. The present results are compared with previous quantum yield detns.
    27. 27
      Collins, D. B.; Farmer, D. K. Unintended Consequences of Air Cleaning Chemistry. Environ. Sci. Technol. 2021, 55, 12172,  DOI: 10.1021/acs.est.1c02582
      27
      Unintended Consequences of Air Cleaning Chemistry
      Collins, Douglas B.; Farmer, Delphine K.
      Environmental Science & Technology (2021), 55 (18), 12172-12179CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      A review. Amplified interest in maintaining clean indoor air assocd. with the airborne transmission risks of SARS-CoV-2 have led to an expansion in the market for com. available air cleaning systems. While the optimal way to mitigate indoor air pollutants or contaminants is to control (remove) the source, air cleaners are a tool for use when abs. source control is not possible. Interventions for indoor air quality management include phys. removal of pollutants through ventilation or collection on filters and sorbent materials, along with chem. reactive processes that transform pollutants or seek to deactivate biol. entities. This perspective intends to highlight the perhaps unintended consequences of various air cleaning approaches via indoor air chem. Introduction of new chem. agents or reactive processes can initiate complex chem. that results in the release of reactive intermediates and(or) byproducts into the indoor environment. Since air cleaning systems are often continuously running to maximize their effectiveness and most people spend a vast majority of their time indoors, human exposure to both primary and secondary products from air cleaners may represent significant exposure risk. We highlight the need for further study of chem. reactive air cleaning and disinfection methods before broader adoption.
    28. 28
      Logue, J. M.; McKone, T. E.; Sherman, M. H.; Singer, B. C. Hazard Assessment of Chemical Air Contaminants Measured in Residences. Indoor Air 2011, 21 (2), 92– 109,  DOI: 10.1111/j.1600-0668.2010.00683.x
      28
      Hazard assessment of chemical air contaminants measured in residences
      Logue, J. M.; McKone, T. E.; Sherman, M. H.; Singer, B. C.
      Indoor Air (2011), 21 (2), 92-109CODEN: INAIE5; ISSN:0905-6947. (Wiley-Blackwell)
      Identifying air pollutants that pose a potential hazard indoors can facilitate exposure mitigation. In this study, we compiled summary results from 77 published studies reporting measurements of chem. pollutants in residences in the United States and in countries with similar lifestyles. These data were used to calc. representative mid-range and upper-bound concns. relevant to chronic exposures for 267 pollutants and representative peak concns. relevant to acute exposures for five activity-assocd. pollutants. Representative concns. are compared to available chronic and acute health stds. for 97 pollutants. Fifteen pollutants appear to exceed chronic health stds. in a large fraction of homes. Nine other pollutants are identified as potential chronic health hazards in a substantial minority of homes, and an addnl. nine are identified as potential hazards in a very small percentage of homes. Nine pollutants are identified as priority hazards based on the robustness of measured concn. data and the fraction of residences that appear to be impacted: acetaldehyde; acrolein; benzene; 1,3-butadiene; 1,4-dichlorobenzene; formaldehyde; naphthalene; nitrogen dioxide; and PM2.5. Activity-based emissions are shown to pose potential acute health hazards for PM2.5, formaldehyde, CO, chloroform, and NO2.
    29. 29
      Mattila, J. M.; Arata, C.; Abeleira, A.; Zhou, Y.; Wang, C.; Katz, E. F.; Goldstein, A. H.; Abbatt, J. P. D.; DeCarlo, P. F.; Vance, M. E.; Farmer, D. K. Contrasting Chemical Complexity and the Reactive Organic Carbon Budget of Indoor and Outdoor Air. Environmental Science & Technology. 2022, 56, 109– 118,  DOI: 10.1021/acs.est.1c03915
      29
      Contrasting Chemical Complexity and the Reactive Organic Carbon Budget of Indoor and Outdoor Air
      Mattila, James M.; Arata, Caleb; Abeleira, Andrew; Zhou, Yong; Wang, Chen; Katz, Erin F.; Goldstein, Allen H.; Abbatt, Jonathan P. D.; DeCarlo, Peter F.; Vance, Marina E.; Farmer, Delphine K.
      Environmental Science & Technology (2022), 56 (1), 109-118CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Reactive org. carbon (ROC) comprises a substantial fraction of the total atm. carbon budget. Emissions of ROC fuel atm. oxidn. chem. to produce secondary pollutants including ozone, carbon dioxide, and particulate matter. Compared to the outdoor atm., the indoor org. carbon budget is comparatively understudied. We characterized indoor ROC in a test house during unoccupied, cooking, and cleaning scenarios using various online mass spectrometry and gas chromatog. measurements of gaseous and particulate orgs. Cooking greatly impacted indoor ROC concns. and bulk physicochem. properties (e.g., volatility and oxidn. state), while cleaning yielded relatively insubstantial changes. Addnl., cooking enhanced the reactivities of hydroxyl radicals and ozone toward indoor ROC. We obsd. consistently higher median ROC concns. indoors (≥223μg C m-3) compared to outdoors (54μg C m-3), demonstrating that buildings can be a net source of reactive carbon to the outdoor atm., following its removal by ventilation. We est. the unoccupied test house emitted 0.7 g C day-1 from ROC to outdoors. Indoor ROC emissions may thus play an important role in air quality and secondary pollutant formation outdoors, particularly in urban and suburban areas, and indoors during the use of oxidant-generating air purifiers.
    30. 30
      Price, D. J.; Day, D. A.; Pagonis, D.; Stark, H.; Algrim, L. B.; Handschy, A. V.; Liu, S.; Krechmer, J. E.; Miller, S. L.; Hunter, J. F.; de Gouw, J. A.; Ziemann, P. J.; Jimenez, J. L. Budgets of Organic Carbon Composition and Oxidation in Indoor Air. Environ. Sci. Technol. 2019, 53, 13053,  DOI: 10.1021/acs.est.9b04689
      30
      Budgets of Organic Carbon Composition and Oxidation in Indoor Air
      Price, Derek J.; Day, Douglas A.; Pagonis, Demetrios; Stark, Harald; Algrim, Lucas B.; Handschy, Anne V.; Liu, Shang; Krechmer, Jordan E.; Miller, Shelly L.; Hunter, James F.; de Gouw, Joost A.; Ziemann, Paul J.; Jimenez, Jose L.
      Environmental Science & Technology (2019), 53 (22), 13053-13063CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The chem. compn. of indoor air at the University of Colorado-Boulder art museum was measured with a suite of gas- and particle-phase instruments. More than 80% of the total obsd. org. C (TOOC) mass (100 μg/m3) consisted of reduced compds. (C oxidn. state [OSC] <-0.5) with high volatility (log10C* >7) and low C no. (nC <6). Museum TOOC was compared to other indoor and outdoor sites, which increased according to the following trend: remote < rural ≤ urban < indoor ≤ megacity. Museum TOOC was comparable to a university classroom and three times less than residential environments. Total reactive flux trends were: remote < indoor < rural < urban < megacity. High volatile org. compd. (VOC) concns. compensated low oxidant concns. indoors resulting in an appreciable reactive flux. Total OH-, O3, NO3-, and Cl atom reactivities at each site followed a trend similar to TOOC. High human occupancy events increased all oxidant reactivities in the museum 65-125%. O3, NO3-, OH-, and Cl reactivities were 13 h, 15 h, 23 days, and 189 days, resp., corresponding to >88% of indoor VOC-oxidant reactivity being consumed outdoors after ventilation.
    31. 31
      Ye, Q.; Krechmer, J. E.; Shutter, J. D.; Barber, V. P.; Li, Y.; Helstrom, E.; Franco, L. J.; Cox, J. L.; Hrdina, A. I. H.; Goss, M. B.; Tahsini, N.; Canagaratna, M.; Keutsch, F. N.; Kroll, J. H. Real-Time Laboratory Measurements of VOC Emissions, Removal Rates, and Byproduct Formation from Consumer-Grade Oxidation-Based Air Cleaners. Environmental Science & Technology Letters 2021, 8, 1020,  DOI: 10.1021/acs.estlett.1c00773
      31
      Real-Time Laboratory Measurements of VOC Emissions, Removal Rates, and Byproduct Formation from Consumer-Grade Oxidation-Based Air Cleaners
      Ye, Qing; Krechmer, Jordan E.; Shutter, Joshua D.; Barber, Victoria P.; Li, Yaowei; Helstrom, Erik; Franco, Lesly J.; Cox, Joshua L.; Hrdina, Amy I. H.; Goss, Matthew B.; Tahsini, Nadia; Canagaratna, Manjula; Keutsch, Frank N.; Kroll, Jesse H.
      Environmental Science & Technology Letters (2021), 8 (12), 1020-1025CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      Levels of volatile org. compds. (VOCs) in the indoor environment can be decreased by the use of "air cleaners", devices that remove VOCs by sorption and/or oxidative degrdn. However, efficacies of these technologies for removing VOCs tend to be poorly constrained, as does the formation of oxidn. byproducts. Here, we examine the influence of several oxidn.-based air cleaners, specifically ones marketed as consumer-grade products, on the amts. and compn. of VOCs. Expts. were conducted in an environmental chamber, with a suite of real-time anal. instruments to measure direct emissions, VOC removal efficacies (by the addn. of either limonene and toluene), and byproduct formation. We find that the air cleaners themselves can be a source of org. gases, that removal efficacy can be exceedingly variable, and that VOC loss is primarily driven by phys. removal in some cleaners. When oxidative degrdn. of VOCs was obsd., it was accompanied by the formation of a range of oxidn. byproducts, including formaldehyde and other oxygenates. Some consumer-grade portable air cleaners can be ineffective in removing VOCs and that the air delivered may contain a range of org. compds., due to direct emission and/or byproduct formation.
    32. 32
      Joo, T.; Rivera-Rios, J. C.; Alvarado-Velez, D.; Westgate, S.; Ng, N. L. Formation of Oxidized Gases and Secondary Organic Aerosol from a Commercial Oxidant-Generating Electronic Air Cleaner. Environ. Sci. Technol. Lett. 2021, 8, 691,  DOI: 10.1021/acs.estlett.1c00416
      32
      Formation of Oxidized Gases and Secondary Organic Aerosol from a Commercial Oxidant-Generating Electronic Air Cleaner
      Joo, Taekyu; Rivera-Rios, Jean C.; Alvarado-Velez, Daniel; Westgate, Sabrina; Lee Ng, Nga
      Environmental Science & Technology Letters (2021), 8 (8), 691-698CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      The COVID-19 pandemic increased the demand for indoor air cleaners. While some com. electronic air cleaners can be effective in reducing primary pollutants and inactivating bioaerosol, studies on the formation of secondary products from oxidn. chem. during their use are limited. Here, we measured oxygenated volatile org. compds. (OVOCs) and the chem. compn. of particles generated from a hydroxyl radical generator in an office. During operation, enhancements in OVOCs, esp. low-mol.-wt. org. acids, were detected. Rapid increases in particle no. and mass concns. were obsd., corresponding to the formation of highly oxidized secondary org. aerosol (SOA) (O:C ~ 1.3), with an enhanced signal at m/z 44 (CO2+) in the org. mass spectra. These results suggest that org. acids generated during VOC oxidn. contributed to particle nucleation and SOA formation. Nitrate, sulfate, and chloride also increased during the oxidn. without a corresponding increase in ammonium, suggesting org. nitrate, org. sulfate, and org. chloride formation. As secondary species are reported to have detrimental health effects, further studies should not be limited to the inactivation of bioaerosol or redn. of particular VOCs, but should also evaluate potential OVOCs and SOA formation from electronic air cleaners in different indoor environments.
    33. 33
      Peng, Z.; Day, D. A.; Stark, H.; Li, R.; Lee-Taylor, J.; Palm, B. B.; Brune, W. H.; Jimenez, J. L. HOx Radical Chemistry in Oxidation Flow Reactors with Low-Pressure Mercury Lamps Systematically Examined by Modeling. Atmospheric Measurement Techniques 2015, 8 (11), 4863– 4890,  DOI: 10.5194/amt-8-4863-2015
      33
      HOx radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling
      Peng, Z.; Day, D. A.; Stark, H.; Li, R.; Lee-Taylor, J.; Palm, B. B.; Brune, W. H.; Jimenez, J. L.
      Atmospheric Measurement Techniques (2015), 8 (11), 4863-4890CODEN: AMTTC2; ISSN:1867-8548. (Copernicus Publications)
      Oxidn. flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are commonly used in atm. chem. and other fields. OFR254 requires the addn. of externally formed O3 since OH is formed from O3 photolysis, while OFR185 does not since O2 can be photolyzed to produce O3, and OH can also be formed from H2O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and lab. studies. We show that the radical chem. in OFRs can be characterized as a function of UV light intensity, H2O concn., and total external OH reactivity (OHRext, e.g., from volatile org. compds. (VOCs), NOx, and SO2). OH exposure is decreased by added external OH reactivity. OFR185 is esp. sensitive to this effect at low UV intensity due to low primary OH prodn. OFR254 can be more resilient against OH suppression at high injected O3 (e.g., 70 ppm), as a larger primary OH source from O3, as well as enhanced recycling of HO2 to OH, make external perturbations to the radical chem. less significant. However if the external OH reactivity in OFR254 is much larger than OH reactivity from injected O3, OH suppression can reach 2 orders of magnitude. For a typical input of 7 ppm O3 (OHRO3 = 10 s-1), 10-fold OH suppression is obsd. at OHRext ∼ 100 s-1, which is similar or lower than used in many lab. studies. The range of modeled OH suppression for literature expts. is consistent with the measured values except for those with isoprene. The finding on OH suppression may have important implications for the interpretation of past lab. studies, as applying OHexp measurements acquired under different conditions could lead to over a 1-order-of-magnitude error in the estd. OHexp. The uncertainties of key model outputs due to uncertainty in all rate consts. and absorption cross-sections in the model are within ±25% for OH exposure and within ±60% for other parameters. These uncertainties are small relative to the dynamic range of outputs. Uncertainty anal. shows that most of the uncertainty is contributed by photolysis rates of O3, O2, and H2O and reactions of OH and HO2 with themselves or with some abundant species, i.e., O3 and H2O2. OHexp calcd. from direct integration and estd. from SO2 decay in the model with laminar and measured residence time distributions (RTDs) are generally within a factor of 2 from the plug-flow OHexp. However, in the models with RTDs, OHexp estd. from SO2 is systematically lower than directly integrated OHexp in the case of significant SO2 consumption. We thus recommended using OHexp estd. from the decay of the species under study when possible, to obtain the most appropriate information on photochem. aging in the OFR. Using HOx - recycling vs. destructive external OH reactivity only leads to small changes in OHexp under most conditions. Changing the identity (rate const.) of external OH reactants can result in substantial changes in OHexp due to different redns. in OH suppression as the reactant is consumed. We also report two equations for estg. OH exposure in OFR254. We find that the equation estg. OHexp from measured O3 consumption performs better than an alternative equation that does not use it, and thus recommend measuring both input and output O3 concns. in OFR254 expts. This study contributes to establishing a firm and systematic understanding of the gas-phase HOx and Ox chem. in these reactors, and enables better expt. planning and interpretation as well as improved design of future reactors.
    34. 34
      Peng, Z.; Jimenez, J. L. Modeling of the Chemistry in Oxidation Flow Reactors with High Initial NO. Atmos. Chem. Phys. 2017, 17 (19), 11991– 12010,  DOI: 10.5194/acp-17-11991-2017
      34
      Modeling of the chemistry in oxidation flow reactors with high initial NO
      Peng, Zhe; Jimenez, Jose L.
      Atmospheric Chemistry and Physics (2017), 17 (19), 11991-12010CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Oxidn. flow reactors (OFRs) are increasingly employed in atm. chem. research because of their high efficiency of OH radical prodn. from low-pressure Hg lamp emissions at both 185 and 254 nm (OFR185) or 254 nm only (OFR254). OFRs have been thought to be limited to studying low-NO chem. (in which peroxy radicals (RO2) react preferentially with HO2) because NO is very rapidly oxidized by the high concns. of O2, HO2, and OH in OFRs. However, many groups are performing expts. by aging combustion exhaust with high NO levels or adding NO in the hopes of simulating high-NO chem. (in which RO2 + NO dominates). This work systematically explores the chem. in OFRs with high initial NO. Using box modeling, we investigate the interconversion of Ncontaining species and the uncertainties due to kinetic parameters. Simple initial injection of NO in OFR185 can result in more RO2 reacted with NO than with HO2 and minor non-tropospheric photolysis, but only under a very narrow set of conditions (high water mixing ratio, low UV intensity, low external OH reactivity (OHRext), and initial NO concn. (NOin) of tens to hundreds of ppb) that account for a very small fraction of the input parameter space. These conditions are generally far away from exptl. conditions of published OFR studies with high initial NO. In particular, studies of aerosol formation from vehicle emissions in OFRs often used OHRext and NOin several orders of magnitude higher. Due to extremely high OHRext and NOin, some studies may have resulted in substantial non-tropospheric photolysis, strong delay to RO2 chem. due to peroxynitrate formation, VOC reactions with NO2 dominating over those with OH, and faster reactions of OH-arom. adducts with NO2 than those with O2, all of which are irrelevant to ambient VOC photooxidn. chem. Some of the neg. effects are the worst for alkene and arom. precursors. To avoid undesired chem., vehicle emissions generally need to be dild. by a factor of >100 before being injected into an OFR. However, sufficiently dild. vehicle emissions generally do not lead to high-NO chem. in OFRs but are rather dominated by the low-NO RO2 + HO2 pathway. To ensure high-NO conditions without substantial atmospherically irrelevant chem. in a more controlled fashion, new techniques are needed.
    35. 35
      Stockwell, W. R.; Kirchner, F.; Kuhn, M.; Seefeld, S. A New Mechanism for Regional Atmospheric Chemistry Modeling. J. Geophys. Res. 1997, 102 (D22), 25847– 25879,  DOI: 10.1029/97JD00849
      35
      A new mechanism for regional atmospheric chemistry modeling
      Stockwell, William R.; Kirchner, Frank; Kuhn, Michael; Seefeld, Stephan
      Journal of Geophysical Research, [Atmospheres] (1997), 102 (D22), 25847-25879CODEN: JGRDE3 ISSN:. (American Geophysical Union)
      A new gas-phase chem. mechanism for the modeling of regional atm. chem., the "Regional Atm. Chem. Mechanism" (RACM) is presented. The mechanism is intended to be valid for remote to polluted conditions and from the Earth's surface through the upper troposphere. The RACM mechanism is based upon the earlier Regional Acid Deposition Model, version 2 (RADM2) mechanism [Stockwell et al., 1990] and the more detailed Euro-RADM mechanism [Stockwell and Kley, 1994]. The RACM mechanism includes rate consts. and product yields from the most recent lab. measurements, and it has been tested against environmental chamber data. A new condensed reaction mechanism is included for biogenic compds.: isoprene, α-pinene, and d-limonene. The branching ratios for alkane decay were reevaluated, and in the revised mechanism the aldehyde to ketone ratios were significantly reduced. The relatively large amts. of nitrates resulting from the reactions of unbranched alkenes with NO3 are now included, and the prodn. of HO from the ozonolysis of alkenes has a much greater yield. The arom. chem. has been revised through the use of new lab. data. The yield of cresol prodn. from aroms. was reduced, while the reactions of HO, NO3, and O3 with unsatd. dicarbonyl species and unsatd. peroxynitrate are now included in the RACM mechanism. The peroxyacetyl nitrate chem. and the org. peroxy radical-peroxy radical reactions were revised, and org. peroxy radical + NO3 reactions were added.
    36. 36
      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 (4), 806– 811,  DOI: 10.1021/acs.jchemed.9b00033
      36
      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.
    37. 37
      McDonald, B. C.; de Gouw, J. A.; Gilman, J. B.; Jathar, S. H.; Akherati, A.; Cappa, C. D.; Jimenez, J. L.; Lee-Taylor, J.; Hayes, P. L.; McKeen, S. A.; Cui, Y. Y.; Kim, S.-W.; Gentner, D. R.; Isaacman-VanWertz, G.; Goldstein, A. H.; Harley, R. A.; Frost, G. J.; Roberts, J. M.; Ryerson, T. B.; Trainer, M. Volatile Chemical Products Emerging as Largest Petrochemical Source of Urban Organic Emissions. Science 2018, 359 (6377), 760– 764,  DOI: 10.1126/science.aaq0524
      37
      Volatile chemical products emerging as largest petrochemical source of urban organic emissions
      McDonald, Brian C.; de Gouw, Joost A.; Gilman, Jessica B.; Jathar, Shantanu H.; Akherati, Ali; Cappa, Christopher D.; Jimenez, Jose L.; Lee-Taylor, Julia; Hayes, Patrick L.; McKeen, Stuart A.; Cui, Yu Yan; Kim, Si-Wan; Gentner, Drew R.; Isaacman-VanWertz, Gabriel; Goldstein, Allen H.; Harley, Robert A.; Frost, Gregory J.; Roberts, James M.; Ryerson, Thomas B.; Trainer, Michael
      Science (Washington, DC, United States) (2018), 359 (6377), 760-764CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A gap in emission inventories of urban volatile org. compd. (VOC) sources, which contribute to regional ozone and aerosol burdens, has increased as transportation emissions in the United States and Europe have declined rapidly. A detailed mass balance demonstrates that the use of volatile chem. products (VCPs)-including pesticides, coatings, printing inks, adhesives, cleaning agents, and personal care products-now constitutes half of fossil fuel VOC emissions in industrialized cities. The high fraction of VCP emissions is consistent with obsd. urban outdoor and indoor air measurements. We show that human exposure to carbonaceous aerosols of fossil origin is transitioning away from transportation-related sources and toward VCPs. Existing U.S. regulations on VCPs emphasize mitigating ozone and air toxics, but they currently exempt many chems. that lead to secondary org. aerosols.
    38. 38
      Upper room germicidal ultraviolet fixtures. AeroMed Technologies. https://aeromed.com/product/upper-room-guv-fixtures/ (accessed 2022-02-21).
      There is no corresponding record for this reference.
    39. 39
      Xu, P.; Kujundzic, E.; Peccia, J.; Schafer, M. P.; Moss, G.; Hernandez, M.; Miller, S. L. Impact of Environmental Factors on Efficacy of Upper-Room Air Ultraviolet Germicidal Irradiation for Inactivating Airborne Mycobacteria. Environ. Sci. Technol. 2005, 39 (24), 9656– 9664,  DOI: 10.1021/es0504892
      39
      Impact of Environmental Factors on Efficacy of Upper-Room Air Ultraviolet Germicidal Irradiation for Inactivating Airborne Mycobacteria
      Xu, Peng; Kujundzic, Elmira; Peccia, Jordan; Schafer, Millie P.; Moss, Gene; Hernandez, Mark; Miller, Shelly L.
      Environmental Science and Technology (2005), 39 (24), 9656-9664CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      This study evaluated the efficacy of an upper-room air UV germicidal irradn. (UVGI) system for inactivating airborne bacteria, which irradiates the upper part of a room while minimizing radiation exposure to persons in the lower part of the room. A full-scale test room (87 m3), fitted with a UVGI system consisting of 9 louvered wall and ceiling fixtures (504 W with all lamps operating) was operated at 24° and 34°, between 25 and 90% relative humidity (RH), and at three ventilation rates. Mycobacterium parafortuitum cells were aerosolized into the room such that their nos. and physiol. state were comparable both with and without the UVGI system operating. Airborne bacteria were collected in duplicate using liq. impingers and quantified with direct epifluorescent microscopy and std. culturing assay. The performance of the UVGI system was degraded significantly when the RH was increased from 50% to 75-90%, the horizontal UV fluence rate distribution was skewed to one side compared to being evenly dispersed, and the room air temp. was stratified from hot at the ceiling to cold at the floor. The inactivation rate increased linearly with effective UV fluence rate up to 5 μW/cm2; an increase in the fluence rate above this level did not yield a proportional increase in inactivation rate.
    40. 40
      Xu, P.; Peccia, J.; Fabian, P.; Martyny, J. W.; Fennelly, K. P.; Hernandez, M.; Miller, S. L. Efficacy of Ultraviolet Germicidal Irradiation of Upper-Room Air in Inactivating Airborne Bacterial Spores and Mycobacteria in Full-Scale Studies. Atmos. Environ. 2003, 37 (3), 405– 419,  DOI: 10.1016/S1352-2310(02)00825-7
      40
      Efficacy of ultraviolet germicidal irradiation of upper-room air in inactivating airborne bacterial spores and mycobacteria in full-scale studies
      Xu, Peng; Peccia, Jordan; Fabian, Patricia; Martyny, John W.; Fennelly, Kevin P.; Hernandez, Mark; Miller, Shelly L.
      Atmospheric Environment (2003), 37 (3), 405-419CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
      The efficacy of UV germicidal irradn. (UVGI) for inactivating airborne bacterial spores and vegetative mycobacteria cells was evaluated under full-scale conditions. Airborne bacteria inactivation expts. were conducted in a test room (87 m3), fitted with a modern UVGI system (216 W all lamps operating, av. upper zone UV irradiance 42±19 μW cm-2) and maintained at 25°C and 50% relative humidity, at two ventilation rates (0 and 6 air changes per h). Bacillus subtilis (spores), Mycobacterium parafortuitum, and Mycobacterium bovis BCG cells were aerosolized continuously into the room such that their nos. and physiol. state were comparable both with and without the UVGI and ventilation system operating. Air samples were collected using glass impingers (9 breathing-zone locations) and multi-stage impactors, and collected bacteria were quantified using direct microscopy and std. culturing assays. UVGI reduced the room-av. concn. of culturable airborne bacteria between 46% and 80% for B. subtilis spores, between 83% and 98% for M. parafortuitum, and 96-97% for M. bovis BCG cells, depending on the ventilation rate. An addnl. set of expts., in which M. parafortuitum was aerosolized into the test room and then allowed to decay under varying UVGI and ventilation rates, yielded an inactivation rate of 16±1.2 h-1 for the UVGI system, all lamps operating. The Z value (inactivation rate normalized to UVGI irradiance) was estd. to be 1.2±0.15×10-3 cm2 μW-1 s-1 for aerosolized M. parafortuitum at 50% relative humidity.
    41. 41
      Daisey, J. M.; Angell, W. J.; Apte, M. G. Indoor Air Quality, Ventilation and Health Symptoms in Schools: An Analysis of Existing Information. Indoor Air 2003, 13 (1), 53– 64,  DOI: 10.1034/j.1600-0668.2003.00153.x
      41
      Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information
      Daisey, J. M.; Angell, W. J.; Apte, M. G.
      Indoor Air (2003), 13 (1), 53-64CODEN: INAIE5; ISSN:0905-6947. (Blackwell Munksgaard)
      A review concerning indoor air quality (IAQ), ventilation, and building-related health problems in schools and identification of commonly reported building-related health symptoms involving schools until 1999 is given. Topics discussed include: methods; ventilation rate and CO2 concn. measurements in schools; indoor pollutants (total volatile org. compds. [TVOC], formaldehyde, bioaerosol contaminants, total airborne bacteria, dust mite and animal allergens, fungi); relationships among health symptoms, ventilation, and other indoor environmental factors in schools; and relationships among health symptoms and measured exposure to environmental pollutants in schools.
    42. 42
      Buonanno, G.; Morawska, L.; Stabile, L. Quantitative Assessment of the Risk of Airborne Transmission of SARS-CoV-2 Infection: Prospective and Retrospective Applications. Environ. Int. 2020, 145, 106112,  DOI: 10.1016/j.envint.2020.106112
      42
      Quantitative assessment of the risk of airborne transmission of SARS-CoV-2 infection: Prospective and retrospective applications
      Buonanno, G.; Morawska, L.; Stabile, L.
      Environment International (2020), 145 (), 106112CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
      Airborne transmission is a recognized pathway of contagion; however, it is rarely quant. evaluated. The numerous outbreaks that have occurred during the SARS-CoV-2 pandemic are putting a demand on researchers to develop approaches capable of both predicting contagion in closed environments (predictive assessment) and analyzing previous infections (retrospective assessment). This study presents a novel approach for quant. assessment of the individual infection risk of susceptible subjects exposed in indoor microenvironments in the presence of an asymptomatic infected SARS-CoV-2 subject. The application of a Monte Carlo method allowed the risk for an exposed healthy subject to be evaluated or, starting from an acceptable risk, the max. exposure time. We applied the proposed approach to four distinct scenarios for a prospective assessment, highlighting that, in order to guarantee an acceptable risk of 10-3 for exposed subjects in naturally ventilated indoor environments, the exposure time could be well below one hour. Such max. exposure time clearly depends on the viral load emission of the infected subject and on the exposure conditions; thus, longer exposure times were estd. for mech. ventilated indoor environments and lower viral load emissions. The proposed approach was used for retrospective assessment of documented outbreaks in a restaurant in Guangzhou (China) and at a choir rehearsal in Mount Vernon (USA), showing that, in both cases, the high attack rate values can be justified only assuming the airborne transmission as the main route of contagion. Moreover, we show that such outbreaks are not caused by the rare presence of a superspreader, but can be likely explained by the co-existence of conditions, including emission and exposure parameters, leading to a highly probable event, which can be defined as a "superspreading event".
    43. 43
      Riley, E. C.; Murphy, G.; Riley, R. L. Airborne Spread of Measles in a Suburban Elementary School. Am. J. Epidemiol. 1978, 107 (5), 421– 432,  DOI: 10.1093/oxfordjournals.aje.a112560
      43
      Airborne spread of measles in a suburban elementary school
      Riley E C; Murphy G; Riley R L
      American journal of epidemiology (1978), 107 (5), 421-32 ISSN:0002-9262.
      A measles epidemic in a modern suburban elementary school in upstate New York in spring, 1974, is analyzed in terms of a model which provides a basis for apportioning the chance of infection from classmates sharing the same home room, from airborne organisms recirculated by the ventilating system, and from exposure in school buses. The epidemic was notable because of its explosive nature and its occurrence in a school where 97% of the children had been vaccinated. Many had been vaccinated at less than one year of age. The index case was a girl in second grade who produced 28 secondary cases in 14 different classrooms. Organisms recirculated by the ventilating system were strongly implicated. After two subsequent generations, 60 children had been infected, and the epidemic subsided. From estimates of major physical and biologic factors, it was possible to calculate that the index case produced approximately 93 units of airborne infection (quanta) per minute. The epidemic pattern suggested that the secondaries were less infectious by an order of magnitude. The exceptional infectiousness of the index case, inadequate immunization of many of the children, and the high percentage of air recirculated throughout the school, are believed to account for the extent and sharpness of the outbreak.
    44. 44
      EPA. Chapter 6─Inhalation Rates. In Exposure Factors Handbook; U.S. Environmental Protection Agency, 2011.
      There is no corresponding record for this reference.
    45. 45
      Peng, Z.; Jimenez, J. L. Exhaled CO 2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities. Environmental Science & Technology Letters 2021, 8 (5), 392– 397,  DOI: 10.1021/acs.estlett.1c00183
      45
      Exhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities
      Peng, Zhe; Jimenez, Jose L.
      Environmental Science & Technology Letters (2021), 8 (5), 392-397CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      CO2 is co-exhaled with aerosols contg. SARS-CoV-2 by COVID-19-infected people and can be used as a proxy of SARS-CoV-2 concns. indoors. Indoor CO2 measurements by low-cost sensors hold promise for mass monitoring of indoor aerosol transmission risk for COVID-19 and other respiratory diseases. We derive anal. expressions of CO2-based risk proxies and apply them to various typical indoor environments. The relative infection risk in a given environment scales with excess CO2 level, and thus, keeping CO2 as low as feasible in a space allows optimization of the protection provided by ventilation. The CO2 level corresponding to a given abs. infection risk varies by >2 orders of magnitude for different environments and activities. Although large uncertainties, mainly from virus exhalation rates, are still assocd. with infection risk ests., our study provides more specific and practical recommendations for low-cost CO2-based indoor infection risk monitoring.
    46. 46
      Ren, X.; Olson, J. R.; Crawford, J. H.; Brune, W. H.; Mao, J.; Long, R. B.; Chen, Z.; Chen, G.; Avery, M. A.; Sachse, G. W.; Barrick, J. D.; Diskin, G. S.; Huey, L. G.; Fried, A.; Cohen, R. C.; Heikes, B.; Wennberg, P. O.; Singh, H. B.; Blake, D. R.; Shetter, R. E. HOx chemistry during INTEX-A 2004: Observation, Model Calculation, and Comparison with Previous Studies. J. Geophys. Res. 2008, 113 (D5). D05310 DOI: 10.1029/2007JD009166 .
      46
      HOx chemistry during INTEX-A 2004: observation, model calculation, and comparison with previous studies
      Ren, Xinrong; Olson, Jennifer R.; Crawford, James H.; Brune, William H.; Mao, Jingqiu; Long, Robert B.; Chen, Zhong; Chen, Gao; Avery, Melody A.; Sachse, Glen W.; Barrick, John D.; Diskin, Glenn S.; Huey, L. Greg; Fried, Alan; Cohen, Ronald C.; Heikes, Brian; Wennberg, Paul O.; Singh, Hanwant B.; Blake, Donald R.; Shetter, Richard E.
      Journal of Geophysical Research, [Atmospheres] (2008), 113 (D5), D05310/1-D05310/13CODEN: JGRDE3 ISSN:. (American Geophysical Union)
      OH and HO2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chem. Transport Expt.-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atm. oxidn. chem. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, obsd. OH was generally 0.95 of modeled OH; below 8 km, obsd. HO2 was generally 1.20 of modeled HO2. This obsd.-to-modeled comparison is similar to that for TRACE-P, another mid-latitude study for which the median obsd.-to-modeled ratio was 1.08 for OH and 1.34 for HO2, and to that for PEM-TB, a tropical study for which the median obsd.-to-modeled ratio was 1.17 for OH and 0.97 for HO2. HO2 behavior above 8 km was markedly different. The obsd.-to-modeled HO2 ratio increased from ∼1.2 at 8 km to ∼3 at 11 km with the obsd.-to-modeled ratio correlating with NO. Above 8 km, the obsd.-to-modeled HO2 and obsd. NO were both considerably greater than observations from previous campaigns. In addn., the obsd.-to-modeled HO2/OH, which is sensitive to cycling reactions between OH and HO2, increased from ∼ 1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HOx source and addnl. reactants that cycle HOx from OH to HO2. In the continental planetary boundary layer, the obsd.-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HOx sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examn. of possible unknown OH sinks and HOx sources.
    47. 47
      Peng, Z.; Lee-Taylor, J.; Orlando, J. J.; Tyndall, G. S.; Jimenez, J. L. Organic Peroxy Radical Chemistry in Oxidation Flow Reactors and Environmental Chambers and Their Atmospheric Relevance. Atmos. Chem. Phys. 2019, 19 (2), 813– 834,  DOI: 10.5194/acp-19-813-2019
      47
      Organic peroxy radical chemistry in oxidation flow reactors and environmental chambers and their atmospheric relevance
      Peng, Zhe; Lee-Taylor, Julia; Orlando, John J.; Tyndall, Geoffrey S.; Jimenez, Jose L.
      Atmospheric Chemistry and Physics (2019), 19 (2), 813-834CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Oxidn. flow reactors (OFRs) are a promising complement to environmental chambers for investigating atm. oxidn. processes and secondary aerosol formation. However, questions have been raised about how representative the chem. within OFRs is of that in the troposphere. We investigate the fates of org. peroxy radicals (RO2), which play a central role in atm. org. chem., in OFRs and environmental chambers by chem. kinetic modeling and compare to a variety of ambient conditions to help define a range of atmospherically relevant OFR operating conditions. For most types of RO2, their bimol. fates in OFRs are mainly RO2 CHO2 and RO2CNO, similar to chambers and atm. studies. For substituted primary RO2 and acyl RO2, RO2CRO2 can make a significant contribution to the fate of RO2 in OFRs, chambers and the atm., but RO2 CRO2 in OFRs is in general somewhat less important than in the atm. At high NO, RO2CNO dominates RO2 fate in OFRs, as in the atm. At a high UV lamp setting in OFRs, RO2 COH can be a major RO2 fate and RO2 isomerization can be negligible for common multifunctional RO2, both of which deviate from common atm. conditions. In the OFR254 operation mode (for which OH is generated only from the photolysis of added O3), we cannot identify any conditions that can simultaneously avoid significant org. photolysis at 254 nm and lead to RO2 lifetimes long enough (∼ 10 s) to allow atmospherically relevant RO2 isomerization. In the OFR185 mode (for which OH is generated from reactions initiated by 185 nm photons), high relative humidity, low UV intensity and low precursor concns. are recommended for the atmospherically relevant gas-phase chem. of both stable species and RO2. These conditions ensure minor or negligible RO2COH and a relative importance of RO2 isomerization in RO2 fate in OFRs within ∼ ×2 of that in the atm. Under these conditions, the photochem. age within OFR185 systems can reach a few equiv. days at most, encompassing the typical ages for max. secondary org. aerosol (SOA) prodn. A small increase in OFR temp. may allow the relative importance of RO2 isomerization to approach the ambient values. To study the heterogeneous oxidn. of SOA formed under atmospherically relevant OFR conditions, a different UV source with higher intensity is needed after the SOA formation stage, which can be done with another reactor in series. Finally, we recommend evaluating the atm. relevance of RO2 chem. by always reporting measured and/or estd. OH, HO2, NO, NO2 and OH reactivity (or at least precursor compn. and concn.) in all chamber and flow reactor expts. An easy-to-use RO2 fate estimator program is included with this paper to facilitate the investigation of this topic in future studies.
    48. 48
      Shrivastava, M.; Cappa, C. D.; Fan, J.; Goldstein, A. H.; Guenther, A. B.; Jimenez, J. L.; Kuang, C.; Laskin, A.; Martin, S. T.; Ng, N. L.; Petaja, T.; Pierce, J. R.; Rasch, P. J.; Roldin, P.; Seinfeld, J. H.; Shilling, J.; Smith, J. N.; Thornton, J. A.; Volkamer, R.; Wang, J.; Worsnop, D. R.; Zaveri, R. A.; Zelenyuk, A.; Zhang, Q. Recent Advances in Understanding Secondary Organic Aerosol: Implications for Global Climate Forcing. Rev. Geophys. 2017, 55 (2), 509– 559,  DOI: 10.1002/2016RG000540
      There is no corresponding record for this reference.
    49. 49
      U.S. Department of Health & Human Services. Agency for Toxic Substances and Disease Registry Minimal Risk Levels, 2018. https://www.atsdr.cdc.gov/about/index.html (accessed 2022-01-21).
      There is no corresponding record for this reference.
    50. 50
      Whalen, J. J. Environmental Control for Tuberculosis; Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings Guide; 2009-105; NIOSH, 2009.
      There is no corresponding record for this reference.
    51. 51
      Riley, R. L.; Permutt, S. Room Air Disinfection by Ultraviolet Irradiation of Upper Air. Air Mixing and Germicidal Effectiveness. Arch. Environ. Health 1971, 22 (2), 208– 219,  DOI: 10.1080/00039896.1971.10665834
      51
      Room air disinfection by ultraviolet irradiation of upper air. Air mixing and germicidal effectiveness
      Riley R L; Permutt S
      Archives of environmental health (1971), 22 (2), 208-19 ISSN:0003-9896.
      There is no expanded citation for this reference.
    52. 52
      Morawska, L.; Tang, J. W.; Bahnfleth, W.; Bluyssen, P. M.; Boerstra, A.; Buonanno, G.; Cao, J.; Dancer, S.; Floto, A.; Franchimon, F.; Haworth, C.; Hogeling, J.; Isaxon, C.; Jimenez, J. L.; Kurnitski, J.; Li, Y.; Loomans, M.; Marks, G.; Marr, L. C.; Mazzarella, L.; Melikov, A. K.; Miller, S.; Milton, D. K.; Nazaroff, W.; Nielsen, P. V.; Noakes, C.; Peccia, J.; Querol, X.; Sekhar, C.; Seppänen, O.; Tanabe, S.-I.; Tellier, R.; Tham, K. W.; Wargocki, P.; Wierzbicka, A.; Yao, M. How Can Airborne Transmission of COVID-19 Indoors Be Minimised?. Environ. Int. 2020, 142, 105832,  DOI: 10.1016/j.envint.2020.105832
      52
      How can airborne transmission of COVID-19 indoors be minimized?
      Morawska, Lidia; Tang, Julian W.; Bahnfleth, William; Bluyssen, Philomena M.; Boerstra, Atze; Buonanno, Giorgio; Cao, Junji; Dancer, Stephanie; Floto, Andres; Franchimon, Francesco; Haworth, Charles; Hogeling, Jaap; Isaxon, Christina; Jimenez, Jose L.; Kurnitski, Jarek; Li, Yuguo; Loomans, Marcel; Marks, Guy; Marr, Linsey C.; Mazzarella, Livio; Melikov, Arsen Krikor; Miller, Shelly; Milton, Donald K.; Nazaroff, William; Nielsen, Peter V.; Noakes, Catherine; Peccia, Jordan; Querol, Xavier; Sekhar, Chandra; Seppanen, Olli; Tanabe, Shin-ichi; Tellier, Raymond; Tham, Kwok Wai; Wargocki, Pawel; Wierzbicka, Aneta; Yao, Maosheng
      Environment International (2020), 142 (), 105832CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
      During the rapid rise in COVID-19 illnesses and deaths globally, and notwithstanding recommended precautions, questions are voiced about routes of transmission for this pandemic disease. Inhaling small airborne droplets is probable as a third route of infection, in addn. to more widely recognized transmission via larger respiratory droplets and direct contact with infected people or contaminated surfaces. While uncertainties remain regarding the relative contributions of the different transmission pathways, we argue that existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Appropriate building engineering controls include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding. Often, such measures can be easily implemented and without much cost, but if only they are recognized as significant in contributing to infection control goals. We believe that the use of engineering controls in public buildings, including hospitals, shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms or public transport, in parallel with effective application of other controls (including isolation and quarantine, social distancing and hand hygiene), would be an addnl. important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public.
    53. 53
      Lee, P.; Davidson, J. Evaluation of Activated Carbon Filters for Removal of Ozone at the PPB Level. Am. Ind. Hyg. Assoc. J. 1999, 60 (5), 589– 600,  DOI: 10.1080/00028899908984478
      53
      Evaluation of activated carbon filters for removal of ozone at the ppb level
      Lee, Poshin; Davidson, Jane
      American Industrial Hygiene Association Journal (1999), 60 (5), 589-600CODEN: AIHAAP; ISSN:0002-8894. (American Industrial Hygiene Association)
      Performance of filters for the removal of ozone at ambient concn. is characterized. The removal efficiency and pressure drop of 10 com. filters, including 8 made of granule or powd. activated carbon, 1 activated carbon fiber filter, and 1 packed bed made of an ozone catalyst, were measured for an influent ozone concn. of 120 ppb at 50% relative humidity and 2.54 m/s face velocity. Activated carbon filters can be very effective at ozone removal, although not indefinitely because chem. reactions of ozone and carbon change the carbon. Initial efficiencies of the 1.27-cm thick flat samples varied from 4.6 to 98.3%. Anal. of the structure and compn. of the filters with SEM and x-ray photoemission spectrometry showed that chem. reactions permanently changed the compn. of the carbon and decreased the surface area. Consequently, removal efficiency decreased with use. Moreover, it was not feasible to regenerate the filters by simply removing them from ozone-laden air. Changes in relative humidity, from 20 to 80%, had no measurable effect on the performance of a granular activated carbon filter. However, because the rate of adsorption of water is faster and the pores are smaller in activated carbon fiber, the efficiency of the fiber filter decreased when the relative humidity was raised from 20 to 50%. A quality factor, equal to the ratio of a threshold breakthrough time and pressure drop, is used to compare filters. In general, those with higher carbon surface area per unit vol. had higher efficiencies and greater pressure drops. Future work should address the removal of ozone in the presence of other gases.
    54. 54
      Mochida, I.; Korai, Y.; Shirahama, M.; Kawano, S.; Hada, T.; Seo, Y.; Yoshikawa, M.; Yasutake, A. Removal of SOx and NOx over Activated Carbon Fibers. Carbon N. Y. 2000, 38 (2), 227– 239,  DOI: 10.1016/S0008-6223(99)00179-7
      There is no corresponding record for this reference.

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    • Details about the model setup (reaction scheme, species lumping, indoor emission, outdoor air composition, initial conditions, effective virus-removal rates of GUV), RO2 fates in the GUV254 cases, concentrations in the irradiated and unirradiated zones in the GUV254 cases, and sensitivity cases for highly polluted conditions (PDF)


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