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Historical Changes in Seasonal Aerosol Acidity in the Po Valley (Italy) as Inferred from Fog Water and Aerosol Measurements
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Anthropogenic Impacts on the Atmosphere

Historical Changes in Seasonal Aerosol Acidity in the Po Valley (Italy) as Inferred from Fog Water and Aerosol Measurements
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  • Marco Paglione*
    Marco Paglione
    Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    *Email: [email protected]. Phone: +390516399560.
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    Orcidhttp://orcid.org/0000-0002-4423-2570
  • Stefano Decesari
    Stefano Decesari
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
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    Orcidhttp://orcid.org/0000-0001-6486-3786
  • Matteo Rinaldi
    Matteo Rinaldi
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
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  • Leone Tarozzi
    Leone Tarozzi
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    Italian National Research Council—Institute of Marine Sciences (CNR-ISMAR), Bologna 40129, Italy
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  • Francesco Manarini
    Francesco Manarini
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
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  • Stefania Gilardoni
    Stefania Gilardoni
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    Italian National Research Council—Institute of Polar Sciences (CNR-ISP), Bologna 40129, Italy
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    Orcidhttp://orcid.org/0000-0002-7312-5571
  • Maria Cristina Facchini
    Maria Cristina Facchini
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
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  • Sandro Fuzzi
    Sandro Fuzzi
    Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
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  • Dimitri Bacco
    Dimitri Bacco
    Regional Agency for Prevention, Environment and Energy (ARPAE) of Emilia-Romagna, Bologna 40122, Italy
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  • Arianna Trentini
    Arianna Trentini
    Regional Agency for Prevention, Environment and Energy (ARPAE) of Emilia-Romagna, Bologna 40122, Italy
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  • Spyros N. Pandis
    Spyros N. Pandis
    Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece
    Department of Chemical Engineering, University of Patras, Patras 26504, Greece
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  • Athanasios Nenes*
    Athanasios Nenes
    Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece
    School of Architecture, Civil & Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0003-3873-9970
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2021, 55, 11, 7307–7315
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https://doi.org/10.1021/acs.est.1c00651
Published May 17, 2021

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY-NC-ND 4.0 .

Abstract

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Acidity profoundly affects almost every aspect that shapes the composition of ambient particles and their environmental impact. Thermodynamic analysis of gas-particle composition datasets offers robust estimates of acidity, but they are not available for long periods of time. Fog composition datasets, however, are available for many decades; we develop a thermodynamic analysis to estimate the ammonia in equilibrium with fog water and to infer the pre-fog aerosol pH starting from fog chemical composition and pH. The acidity values from the new method agree with the results of thermodynamic analysis of the available gas-particle composition data. Applying the new method to historical (25 years) fog water composition at the rural station of San Pietro Capofiume (SPC) in the Po Valley (Italy) suggests that the aerosol has been mildly acidic, with its pH decreasing by 0.5–1.5 pH units over the last decades. The observed pH of the fog water also increased 1 unit over the same period. Analysis of the simulated aerosol pH reveals that the aerosol acidity trend is driven by a decrease in aerosol precursor concentrations, and changes in temperature and relative humidity. Currently, NOx controls would be most effective for PM2.5 reduction in the Po valley both during summer and winter. In the future, however, seasonal transitions to the NH3-sensitive region may occur, meaning that the NH3 reduction policy may become increasingly necessary.

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1. Introduction

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Aerosol acidity is a key driver of many important atmospheric processes, (1,2) affecting partitioning of semivolatile species, SOA formation, and solubilization of trace metals, with important implications for air quality, the health of human and ecosystems, and climate. Despite its importance, the direct measurement of the pH of atmospheric aerosol is highly challenging, with only a few techniques available for its determination. (3−5) Indirect proxies (e.g., aerosol ionic molar ratios) have been used extensively in the past to represent particle acidity, although their link with pH is largely qualitative. (2,6,7) The most reliable estimates of aerosol acidity to date are obtained by thermodynamic analysis of ambient observations. (6−8) In this approach, aerosol thermodynamic models (such as ISORROPIA-II (9)) are applied to determine the concentrations of species in the gas and aerosol phases that control the water uptake of the aerosol. If the observed gas-particle partitioning of species sensitive to aerosol pH (e.g., NH3(g)/NH4+, HNO3(g)/NO3–) and liquid water content (W) are sufficiently captured by the thermodynamic model, the pH calculated from the thermodynamic model would be a good estimate of the true value. (5)
Using thermodynamic analysis, datasets of aerosol acidity are appearing with increasing frequency (see the review by Pye et al. (2)); however, their number and temporal span are limited compared to a large number of PM composition datasets available because the required concurrent measurements of the gas-phase concentrations of NH3 and/or HNO3 are often unavailable. In most cases, even if it may not be the case for dust or sea salt particles, NH3 concentration is enough to derive aerosol acidity, as it acts as a buffer that controls aerosol pH. (1,10) In the absence of NH3 data, one can still obtain robust pH estimates if NO3–/HNO3 observations are available. (11) If neither HNO3 nor NH3 observations are available, the pH estimates can still be obtained, albeit with a constrained (but non-negligible) bias. (8,12)
To date, thermodynamic analysis has been applied exclusively to aerosol–gas-phase composition data. This approach is the most straightforward, as the observations provide the direct input required by the thermodynamic models to calculate semivolatile partitioning and the aerosol pH. Other nonconventional datasets, however, may provide aerosol pH with an appropriately formulated thermodynamic analysis. Fog water data offer such potential, as fogs—which act like natural “mist chamber” samplers—tend to scavenge significant fractions of aerosol mass and soluble vapors such as NH3, HNO3, and HCl. Gilardoni et al. (13) showed that aerosol down to about 100 nm was scavenged by fog droplets in the Po Valley—so most of the mass that controls aerosol pH was contained in the fog water. The fog water pH and liquid water content can also be directly measured, allowing for estimations of gas-phase aerosol precursors (the main ones being NH3, HNO3, and HCl (14)). Combining this fog water and gas-phase information allows for a thermodynamic analysis that can be used to constrain the pre-fog aerosol acidity. A combined aerosol–fog water thermodynamic analysis is also important to understand the role of aerosols in fogs as media for the production of secondary organic aerosol, especially during intense haze events. (15)
In this study, we evaluate the potential of using the fog water ionic composition data to estimate the pH of the preexisting aerosol. The method is evaluated using the data collected in the Po Valley of Italy. Concurrent measurements of aerosol and gas-phase composition before fog events are also used to estimate the aerosol pH and are compared against the fog-water-derived values. The fog-water-based method is then applied to the full dataset, which covers a period of over 25 years. (12) The resulting aerosol pH trend is then compared against the fog water pH trend, and its links to the emissions of aerosol precursors (i.e., SO2, NOx, and NH3) and meteorological parameters (T and RH) are explored.

2. Experimental Methods

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2.1. Fog Water Collection and Analysis

Fog water is collected systematically at the field station of San Pietro Capofiume (SPC) since 1989 (from November to March). A series of intensive fog campaigns also took place in SPC during the 1980s. (17−20) Fog water is sampled using an automated active string collector, extensively described by Fuzzi et al. (21) The collector consists of a system in which fog droplets, carried by the air stream created by a fan located at the rear part of a short wind tunnel, are forced to impact on a series of strings. The impacted droplets then coalesce with each other and drain off the stainless-steel strings into a funnel to eventually be collected in a sampling bottle. The airflow through the tunnel is approximately 17 m3 min–1 with a 50% collection efficiency of individual string at approximately 3 mm droplet radius.
A Particulate Volume Monitor PVM-100 is used to measure the liquid water content of the fog (LWCf) with a time resolution of 1 min. This monitor is used to activate the string collector when LWCf exceeds 0.08 g m–3. This threshold corresponds to visibility of approximately 200 m (22) and the fog is considered dense (the meteorological definition of fog is visibility less than 1 km).
Direct pH measurements of the liquid fog samples are carried out using a Crison micropH 2002 pH meter: this measurement is done in the laboratory of ISAC-CNR (at Bologna, about 40 km from SPC) as soon as the sample arrives from the field; usually, a short trip by car (30–40 min of transport). If for any reason the time is longer, the fog sample is refrigerated in the fridge or directly outside (stored in a corked sampling bottle, on the terrace of the laboratory) for at least 60 min to rebalance the temperature as much as possible with that of the ambient air. Fog water samples are filtered (47 mm quartz-fiber filters) within a few hours after collection to remove any suspended particles. The samples are then stored frozen until the chemical analysis. Liquid samples are analyzed for inorganic ions (NH4+, Na+, K+, Ca2+, Mg2+, Cl–, NO3–, SO42–) and low molecular weight organic acids (acetate, formate, methanesulfonate, and oxalate) by ion chromatography. (23) The results of this chemical characterization and the temporal trends in the observed chemical and physical parameters were presented by Giulianelli et al. (16) As reported in Giulianelli et al., (16) analytical problems affected the chromatographic data from 1989/1990 to 1992/1993 and these samples were excluded from our analysis. A total of 577 fog water samples were analyzed from November 1993 to December 2018, with an annual median number of samples of 23 (4–37). The smallest number of samples corresponds to the central years of the time series (2004–2006), for which the statistics should be considered less robust.

2.2. PM2.5 and Ammonia Measurements

Daily PM2.5 and gaseous ammonia (NH3) measurements are carried out by the Regional Agency for Prevention, Environment, and Energy (ARPAE) of Emilia-Romagna, Italy. The PM2.5 filter sampling started in the framework of the Supersito project (www.arpae.it/supersito) from November 2011 and continues to date following the protocol described by Ricciardelli et al. (24) PM2.5 daily samples are collected at SPC on quartz fiber filters (PALL Tissu Quartz 2500 QAO-UP 2500 47 mm filters) for the analysis of the major inorganic ions (NH4+, Na+, K+, Ca2+, Mg2+, Cl–, NO3–, SO42–). After the collection with a Single Channel Monitor (SWAM FAI Rome, Italy) operating at the standard flow rate of 38.3 L min–1 (EN12341), the samples are extracted in 10 mL of MilliQ water, sonicated for 15 min, filtered through 0.45 mm cellulose acetate filters, and then injected into an Ion Chromatography system (DIONEX, California). Gaseous ammonia (NH3) hourly measurements are available at SPC starting from August 2017, thanks to a chemiluminescence ammonia analyzer (model 201E, Teledyne-API, San Diego, CA) of the air quality network of ARPAE Emilia-Romagna (following a standard certified QA/QC procedure ISO9001:2015, www.arpae.it, Section S1).

2.3. Meteorological Data

Daily maximum, minimum, and mean values for T and RH are available at SPC since 1993. Since 2007, hourly data are available by the Hydro-Meteo-Climate Service of ARPAE-ER (simc.arpae.it/dext3r/). T and RH measurements are validated against the data measured at SPC continuously by a VAISALA meteorological station.

3. Modeling Methods

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3.1. Thermodynamic Analysis of Fog Water Composition to Obtain Pre-Fog Aerosol pH

The ISORROPIA-II (version 2.3) aerosol thermodynamic model (6) (http://isorropia.epfl.ch) is used to calculate the composition and phase state of a K+–Ca2+–Mg2+–NH4+–Na+–SO42––NO3––Cl––water inorganic aerosol in thermodynamic equilibrium with gas-phase precursors. ISORROPIA-II has been extensively used to predict the liquid water content and pH for inorganic aerosol. (1,8,12,25−27) Similar to previous studies, the definition of pH used here is
(1)
where γH+ is the hydronium ion activity coefficient (here, assumed unity), Haq+ (mol L–1) is the hydronium ion concentration in the aqueous phase of the aerosol, Hair+ (μg m–3) is the hydronium ion concentration per volume of air, and Wi and Wo (μg m–3) are particle water concentrations associated with the aerosol-inorganic and -organic species, respectively. The pH used here is consistent with the pHF definition of Pye et al. (2) In this work, we neglect the contribution of water from the organic species and their overall effect on the water activity and pH. Guo et al., (8) Song et al., (7) Vasilakos et al., (28) and Battaglia et al. (29) evaluated this assumption and found the organic species to have a secondary effect on the aerosol pH, being somewhere between 0.15 and 0.3 pH units. So, since the organic aerosol hygroscopicity is not measured in our study and given that neglecting Wo appears to have only a minor impact on the pH characterization, we determine pH only considering Wi.
We used ISORROPIA-II in the partitioning (“forward”) mode for metastable aerosol. In this configuration, the model is used to calculate the equilibrium partitioning given the total concentration (gas plus particles) of various species together with RH and T as input. Considering that the fog scavenging efficiency of inorganic species is generally higher than 70% and shows little variability, (13,30) here we apply the partitioning mode using the ionic composition of fog water (as a proxy of the particle composition and amount) and the corresponding gaseous ammonia concentrations, as estimated in Section 3.2.
The total (gas and aerosol) concentration of species i in the pre-fog atmosphere (Ci,air, μg m–3) is obtained by multiplying their concentration in fog water (Ci,fog, μg mL–1) with the liquid water content of the fog (LWCair, mL m–3)
(2)
Given the relatively high pH of the fog water, some ammonia is present in the gas phase; its concentration is estimated based on its measured fog water concentration and pH (Section 3.2). This is added to the concentration calculated using eq 2. A similar inference of gas-phase HNO3 and HCl is deemed unnecessary in this case, given the high value of pH and LWCf of fogs in SPC. (26)
We test in Section 4.1 the reliability of our approach on a limited subset of data for which parallel measurements of PM2.5 and NH3(g) are available, comparing NH3(g)/NH4+(p) predictions with the measured ammonia gas-particle partitioning. We also evaluate the impact of neglecting gaseous ammonia from the calculation, applying the model even with just aerosol-phase data as already done in previous studies. (6)

3.2. Estimating Gaseous Ammonia in Fogs

Although fogs scavenge most of the aerosol and a large fraction of the water-soluble gases of interest for pH determination (namely, NH3 and HNO3), a portion remains in the gas phase in quasiequilibrium with the fog water. If the atmospheric concentration of gaseous ammonia in fog (NH3(fog)) is known, it can be included in the thermodynamic analysis. To determine NH3(fog), we first assume that NH3 is in equilibrium with the fog water NH4+, and then correct to account for any apparent departure from this equilibrium because we use the pH of the bulk fog water. (31) Following Guo et al., (26) the fraction of available NH3 that partitions to the fog water at equilibrium, εNH4, is given by
(3)
where T is the measured temperature in K, LWCf is the measured liquid water content of the air in g m–3, R (=0.082 atm L mol–1 K–1) is the ideal-gas constant, and HNH3* is the effective Henry’s law constant calculated at the measured fog water pH
(4)
where HNH3 (=62 M atm–1) is the Henry law constant for NH3 at 298 K, Ka1 = 1.7 × 10–5 M, and [H+] is the hydronium ion concentration in the fog water.
Given that the fog water pH and concentration of ammonium and liquid water are known, eqs 3 and 4 can be combined to give NH3(fog)eq
(5)
where NH3(fog)eq is the equilibrium concentration of ammonia in the fog, and NH4(fog)+ is the ammonium concentration of the fog water.
Ricci et al., (32) investigating the uptake of soluble gases by fog droplets, showed that deviations from bulk equilibrium can occur and thus should be accounted for, at least approximately. For this, we define an equilibrium deviation ratio, Dr = NH3(fog)/NH3(fog)eq, as the ratio between the estimated (NH3(fog)) and bulk equilibrium gas-phase ammonia concentration. If Dr is constrained from observations, it can then be used together with eq 5 to provide the estimated concentration of ammonia in the fog
(6)
Given that our analysis refers to the bulk composition of fog water (in which we cannot keep track of the size and pH of the single droplets), actually, the deviation ratio (Dr) can change depending on the duration and characteristics of the fog event (as already hypothesized and empirically measured by Pandis and Seinfeld (31) and by Ricci et al. (32)), and especially, depending on the pH of the fog water solution. Pandis and Seinfeld (31) proved that for all fogs the difference in pH among droplets of different sizes (larger droplets tend to be more alkaline as they form on alkaline dust particles), even if the system is in equilibrium, leads to supersaturation of the bulk aqueous phase compared to the gas phase. To account for this effect too, using the available measured data (referring to the last 2 years analyzed, 2017–2018), we estimate (following the steps described in the Supporting Information, Section S1) the possible range of Dr values due to different fog events (Figure S1) and we explicitly determine the relationship between Dr and the measured fog-pH for the 25-year dataset (Figure S2).
We tested consequently different Dr values in the range suggested by our analysis as well as the uncorrected estimation (i.e., Dr = 1) corresponding to the equilibrium (Supporting Information, Section S1). In particular, we compare the performance of the different Dr estimations in terms of the agreement between (a) simulated and observed ammonia/ammonium concentrations at SPC and (b) estimated NH3(fog) concentrations and NH3(g) measurements available also in other places of the Po Valley, like Lombardy (where NH3(g) is measured since 2003).
The results of this comparison suggest that Dr expressed as a function of fog-pH is more appropriate, as it corresponds and correlates well with the available parallel PM2.5 and NH3(g) measurements at SPC (Figure S3) and leads to the estimated NH3(fog) concentrations, which are comparable with other Po Valley background sites (Figure S4).

4. Results and Discussion

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4.1. Model Evaluation

The ability of the thermodynamic analysis of fog water composition to estimate aerosol pH is tested first for a subset of data collected from August 2017 to October 2018 focusing on ammonia partitioning. For this period, measurements of both fog and PM2.5 compositions as well as NH3(g) concentrations are available. ISORROPIA-II was used to simulate the partitioning in the system using the fog composition and the estimated NH4(fog)+ + NH3(fog) as inputs for the total ammonia. These predictions are then compared against the measurements of PM2.5 NH4+ and NH3(g) concentrations.
The predictions of ISORROPIA-II for the ammonia/ammonium concentrations obtained from the fog water composition and the estimated NH3 inside the fog are quite consistent with those using the measured NH3(g) and PM2.5 aerosol composition (Figure 1) as inputs. The differences in concentrations are, on average, −1 ± 3% for the gas-phase ammonia and 8 ± 3% for the particulate ammonium (Figure S2). The two approaches also result in consistent pH values with an average difference of 0.06 ± 0.27 pH units (Figure 2).

Figure 1

Figure 1. Evaluation of the ISORROPIA-II ammonia/ammonium concentrations: simulated concentrations of ammonia (a) and ammonium (b) are plotted against the corresponding observed/estimated values. Red markers refer to the measured NH3(g) and PM2.5 aerosol composition; green markers represent the partitioning obtained from the fog water composition and the estimated NH3 inside the fog.

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Figure 2

Figure 2. pH values calculated using ISORROPIA-II from the PM2.5 composition (AerPM2.5 pH) and from the fog water composition (AerFOG pH). (a) Time series of AerPM2.5 (dashed lines) and AerFOG pH (markers) for the evaluation dataset. (b) The pH values of AerPM2.5 (y-axis) and AerFOG (x-axis) for parallel samples. The dashed lines indicate ΔpH = 1, 0, and −1. In both panels, black line and markers represent the calculation made without gaseous ammonia (NH3(g)) as input, while red line and markers represent the results using particle/fog NH4(p)+ + gaseous ammonia measured (NH3(g)) or estimated by fog composition (NH3(fog)), respectively.

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Moreover, a direct comparison of the fog water and PM2.5 composition for all of the parallel available samples (63 samples spanning from 2011 up to 2018) is also reported in Table S1 and Figure S6, and shows a general agreement. This confirms that the fog water thermodynamic analysis provides a consistent alternative for predicting the pH in the ambient aerosol with reasonable accuracy.
If we do not include the observed gas-phase NH3, the predicted pH decreases by 1.01 ± 0.12 units using PM2.5 composition (Figure 2). This difference (hereinafter, defined as ΔpH AerPM) is consistent with the findings of previous studies (6,8,33) that neglect considering the gas-phase NH3 when inferring aerosol pH. The difference of the fog-water-based pH predictions (AerFOG pH) with and without the estimated gas-phase ammonia (ΔpH AerFOG) is slightly higher at 1.09 ± 0.34, respectively (Figure 2). The calculation of ammonia concentration from the fog water composition is unambiguous and leads to considerably improved pH estimates.

4.2. Aerosol pH Trend in the Last 25 Years

The aerosol pH was estimated using the proposed approach based on the 577 fog water samples collected at SPC over the last 25 years. To investigate typical conditions, the daily mean values of RH and T (calculated as the average of the 24 h of each corresponding day) were used for the ISORROPIA-II calculations, considering them as the best approximation of the average conditions that particles were exposed to before the onset of fog.
The trend of the estimated aerosol pH over the last 25 years is shown in Figure 3. We used annual averages to help to ensure that errors are independent and identically distributed, as suggested by Hess et al. (34) The variability of the aerosol pH during each year is considerable. In 1997, for instance, the calculated pH ranged from 1.38 to 5.51. The variability among years is also remarkable with the highest annual average of 5.18 in 1993 and the lowest of 3.66 in 2010. Given this high variability, the general trend of the aerosol pH over time is identified by applying an interquartile-range rule to remove the extreme values from the annual averages (see the Supporting Information, Section S3), and we obtain a net pH decreasing trend for the whole period 1993–2018 at a 99% confidence interval (Figures 3, S7, and S8). The calculated means from applying the interquartile-range rule show that during 1993–2002, the average aerosol pH was 4.62 (with the highest annual average of 5.18 in 1993) while from 2003–2012 it was 4.53 (with the highest value of 5.10 in 2004) dropping to an average value of 4.27 over the 6 years between 2013 and 2018 (with a maximum value of 4.31 in 2016).

Figure 3

Figure 3. Fog and aerosol pH trends as measured directly in fog water or calculated using ISORROPIA-II. Black circles are the annual average of the pH measured; red squares are the annual average values of the aerosol pH calculated using ISORROPIA-II starting from the fog ionic composition with the gaseous estimated NH3(fog) as input (pH AerFOG + NH3(fog)). Green triangles are the annual average values of the aerosol pH calculated using ISORROPIA-II starting from PM2.5 ionic composition with the measured gaseous NH3 as input (pH AerPM2.5 + NH3(g)). Error bars represent the standard deviations of the measured/calculated pH values.

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Given the importance of the gas-phase ammonia concentration inside the fog layer, NH3(fog), in the determination of the aerosol pH trend over time, an additional evaluation of the pH variability due to different possible estimations of the gaseous ammonia concentrations is shown in the Supporting Information (Sections S4 and S5, Figures S9–S11). All tests confirm the decreasing trend of the aerosol pH based on the fog composition (AerFOG pH) within a limited range of variability depending on the chosen gaseous ammonia estimation. Aerosol acidity has increased by approximately 0.5–1 pH units over the examined 25-year period.
The fog water pH is also shown in Figure 3, which shows an increasing trend at a 99% confidence interval (using least-squares regression). Giulianelli et al. (16) had already analyzed this increasing trend of the fog pH at SPC and related it to the decreasing concentrations of the major acidic species NO3– and SO42– (Figure S12). SO2 and NOx emissions have been reduced in Italy but also throughout Europe—without a concurrent decrease in the fog water content during episodes. This behavior is consistent with other European and American datasets (see Pye et al. (2) and references therein).
These different trends, although counterintuitive at first glance, are consistent with the thermodynamics of the fog and aerosol systems. The water concentration in the fog is not driven by the amount of aerosol but rather only by the meteorological conditions, especially the cooling rate when fog is being formed. In contrast, the aerosol liquid water scales with the amount of aerosol. As a result, the aerosol pH tends to change much less with the aerosol amount (because liquid water changes accordingly). (1,10) At the same time, there is the potential for a small decrease in the pH with reduced aerosol because relatively larger amounts of ammonium need to volatilize to establish equilibrium between the gas and particulate phases. (1) As this ammonium leaves the particles, the main cation that neutralizes the acids is depleted. This means that the aerosol pH tends to become more acidic even if the strong acidic components (sulfate and nitrate formed from SO2 and NOx) are reduced. Another factor that can reduce aerosol pH is the change of meteorological parameters (RH and T) observed at SPC over the 25-year period (Figure S13). The role of the different drivers of the observed trends will be discussed in a subsequent section.

4.3. Effect of RH and T Variability on pH Predictions

In our analysis, as already mentioned, we considered daily mean values of relative humidity and temperature as the best approximation of the conditions that particles were exposed to before the onset of fog. To understand how predicted aerosol pH varies throughout each day depending on the RH and T, we repeat the simulation using the RH and T continuously measured during the fog-events and averaged over the fog-sampling time periods, which possibly represents the atmospheric conditions closest to the fog-scavenging.
The resulting pH also shows a decreasing annual average trend (Figure S14), with a reduction of ∼1.5 pH units over the 25-year period. The difference between the two trends is indicative of the effect of the diurnal variability in meteorology on the predicted aerosol pH. Taking these differences into account, we can state that the aerosol pH has decreased by 0.5–1.5 units over the last 25 years.
The different slope of the two pH trends in Figure S14 indicates the important role of RH in determining the aerosol pH by influencing the aerosol liquid water content. To further investigate this effect, we used a constant RH value equal to 99, 95, 90, 85, 80, 75, and 70% and repeated the pH calculation. The aerosol pH values calculated using RH = 99% are systematically higher than all the others (Figure S15). For lower values of the RH, the trend becomes weaker, highlighting the importance of RH in pH determination at least in a region such as Po Valley, characterized by high RH.

4.4. Drivers of Aerosol pH Reduction

To understand the drivers of the observed aerosol pH reduction, we apply a multiple linear regression analysis on the simulated aerosol pH, following the approach of Rosenfeld et al. (35) A first regression is applied to the aerosol pH with all of the independent variables (ionic composition and meteorological parameters) used by ISORROPIA-II to calculate the total R2. The regression is then repeated, sequentially omitting one variable at a time to retrieve the contribution of each individual variable to the total variance explained (or R2). The strongest contribution is by the total ammonium (NH4TOT, i.e., gas plus particle phase), representing, alone, 35% of the total variance explained, followed by sulfate (SO42–) and RH contributing 17 and 16% each, respectively, and calcium (Ca2+) contributing 11% (Table S2). Grouping the variables by category, we find that the major ions (NH4TOT, NO3–, SO42–, and Cl–) are responsible for 61% of the variability, the meteorological parameters (RH and T) for another 23%, and the nonvolatile cations (K+, Na+, Ca2+, Mg2+) for the remaining 16%.
These findings confirm the importance of the aerosol chemical composition changes over the years, but point out the important contribution of decreasing RH and increasing T already observed at SPC (Figure S13) in determining the pH trend. This influence should be accounted for when considering future climate change and its interaction with air quality.

4.5. Relationship between Wintertime/Fog-Related pH and Other Seasons

Our analysis shows a decreasing trend in aerosol pH based on the fog water chemical composition. However, the fog composition is representative only of the period between November and March when fog events actually occur, and in particular only of the days in which fog occurs. Nevertheless, based on the pH sensitivity to RH (Figure S15) and on the difference between the average RH during foggy days and during all winter (Table S3), we can expect a change in pH estimation of about 0.2–0.3 units due to the variability of RH in wintertime (which never drops below 80% on a seasonal average basis). Regarding seasons without fog events, the 7 years (2011–2018) of available aerosol data (24) allow a robust characterization of the SPC aerosol pH seasonality over the recent past. The strong seasonality of aerosol pH (calculated using the monthly average of the available gaseous ammonia measurements for constraining the model, as described in the Supporting Information, Section S9, Figure S16) is recognizable, with the highest average values of 4.19 during winter (in good agreement with the pH calculated from fog composition in the same period 2011–2018) and the lowest of 2.61 during summer (Figure S17). The variation of more than 1.5 pH units between winter and summer can be the result of the seasonal changes in meteorological conditions (mean wintertime temperature and RH are 276 K and 85%, while the summertime ones are 297 K and 62%, respectively) but also of the changing emissions like biomass burning in the wintertime, agricultural activities and photochemical processes during summer. (36) These seasonal ranges are consistent with the seasonality reported in other locations (e.g., Pye et al., (2) and references therein).

5. Implications

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Information on aerosol pH trends over time is limited in Europe as well as around the world, owing to the scarcity of relevant data. Long-term monitoring programs for cloud/fog composition and acidity are also limited, but there are locations around the world where such measurements have been made routinely, or at least periodically, over periods of a decade or more. Our analysis suggests a novel way to calculate the pre-fog aerosol pH using fog compositional data in a thermodynamically consistent way, which can be useful to evaluate long-term trend of particle acidity also in other regions of the world for which data are available (e.g., Central Valley in CA; Whiteface mountain, NY; etc. (2)).
The Po Valley dataset used in this work shows an increasing trend of the fog water pH. (16) We show for the first time that this increasing pH of cloud/fog water may not be indicative of the trend in aerosol acidity. In the case of the Po Valley, the fog and the aerosol have opposite trends during the last 25 years. We demonstrate that this aerosol pH reduction trend is thermodynamically robust and it is driven by the contemporary decrease of the corresponding air pollutants due to the environmental policies and by the changing meteorological parameters (RH and T), possibly a result of regional climate change. The Po Valley is dominated by ammonia emissions and its aerosol is characterized by pH and liquid water content levels similar to those in E. Asia during haze episodes. (26)
The Po Valley is a well-known air quality hotspot characterized by particulate matter (PM) levels well above the limit set by the European Air Quality Directive and by the World Health Organization. These high concentrations are dominated during the cold season (almost half of the year) by ammonium nitrate (which during winter represents 26–43% of total PM2.5 mass (20)). Given their semivolatility and their acid–base activity, nitrate, and ammonium are the aerosol species most sensitive to pH. Nenes et al. (37) developed a new conceptual framework explicitly considering pH, aerosol liquid water content, and temperature as the main parameters controlling secondary inorganic PM sensitivity. Figure 4 places the Po Valley aerosol pH (both based on fog and aerosol data) in this framework. Based on this, the SPC area from November to March (fog season) is in the regime in which aerosol nitrate formation is highly efficient. This is true for the aerosol pH calculated both by fog water (i.e., referring to foggy days) and by PM2.5 composition (i.e., including both foggy and non-foggy wintertime days of 2017–2018).

Figure 4

Figure 4. Distribution of aerosol pH versus aerosol liquid water content from both fog composition (blue points) and PM2.5 composition (brown points) inside the chemical domains of aerosol sensitivity to NH3 and NOx emissions as introduced by Nenes et al. (37) The blue line defines the characteristic pH when the aerosol is sensitive to changes in the available nitrate and the red line in ammonia. These are calculated for the average temperature of the winter periods (280 K). PM2.5 data are for 2017–2018 wintertime samples for which both NH4(p)+ and NH3(g) measurements are available. Bigger markers indicate the average values of the corresponding multiyear period reported below each of them: white symbols refer to calculations based on the fog water data and the orange circles refer to the aerosol and gas-phase ammonia data.

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Considering the definitions of Nenes et al. (37) and the average values of temperature and aerosol liquid water content corresponding to the PM2.5 samples collected at SPC during the cold season (280 K and 28.8 μg m–3, respectively), we calculate the threshold for the change of acidity regime to be at a pH of ∼0.5. This pH value is well below the lowest observed pH values (3.76 as an annual average for 2010). However, if the decreasing trend of aerosol pH continues in the future and the pH drops to the threshold level, aerosol nitrate will remain almost exclusively in the gas phase as HNO3, regardless of the amount present. If and when this change happens depends on the rate of future reduction in emissions of NH3 and NOx (precursors of aerosol ammonium and nitrate) and on the changes of T and RH. Increasing T and decreasing RH will tend to shift the threshold to higher pH values as the effect of the liquid water content decreases. For the time being, the wintertime composition has been gradually moving along the years from the top right (HNO3/NH3-sensitive regimes) toward the top left side of the graph (HNO3-sensitive regime) (Figure 4), mostly because the aerosol liquid water content, in particular, has decreased significantly due to changes in PM concentrations and also to decreasing RH (Figure S13). pH has seen less of a drop because of these changes.
Currently, the PM sensitivity regime remains in the HNO3-sensitive area throughout the year, with a tendency however to be close to the NH3-sensitive regime during summer (Figure 5). This seasonality of the PM sensitivity in the Po Valley is important for effective pollution control policies. This means that, currently, NOx controls would be most effective for PM2.5 reduction in the Po Valley both during summer and winter. In the future, however, seasonal transitions to the NH3-sensitive region may occur, meaning that the NH3 reduction policy may become increasingly necessary. Future work will focus on the implications of the emission and meteorological trends for the deposition of reactive nitrogen. (38)

Figure 5

Figure 5. Seasonality of the distribution of aerosol pH versus liquid water content inside the chemical domains of aerosol sensitivity introduced by Nenes et al. (37) Characteristic pH for defining when the aerosol is sensitive to changes in the available nitrate (blue lines) and ammonia (red lines) are calculated for the annual average temperature (=287 K, solid lines) ± its standard deviation (=295 K, dotted lines, and =278 K, dashed lines, respectively).

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Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.1c00651.

  • Deeper descriptions of the modeling methods (e.g., estimation of gaseous ammonia from fog water composition; NH3 estimation effect on the aerosol pH variability and RH sensitivity test); statistical analysis of the pH trend; details on the trends of pollutants and meteorological parameters; details on the results of the multiple linear regression analysis to study the drivers of aerosol pH reduction (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • Marco Paglione - Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece;  Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy;  Orcidhttp://orcid.org/0000-0002-4423-2570;  Email: [email protected]
    • Athanasios Nenes - Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece;  School of Architecture, Civil & Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland;  Orcidhttp://orcid.org/0000-0003-3873-9970;  Email: [email protected]
  • Authors
    • Stefano Decesari - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy;  Orcidhttp://orcid.org/0000-0001-6486-3786
    • Matteo Rinaldi - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    • Leone Tarozzi - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy;  Italian National Research Council—Institute of Marine Sciences (CNR-ISMAR), Bologna 40129, Italy
    • Francesco Manarini - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    • Stefania Gilardoni - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy;  Italian National Research Council—Institute of Polar Sciences (CNR-ISP), Bologna 40129, Italy;  Orcidhttp://orcid.org/0000-0002-7312-5571
    • Maria Cristina Facchini - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    • Sandro Fuzzi - Italian National Research Council—Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna 40129, Italy
    • Dimitri Bacco - Regional Agency for Prevention, Environment and Energy (ARPAE) of Emilia-Romagna, Bologna 40122, Italy
    • Arianna Trentini - Regional Agency for Prevention, Environment and Energy (ARPAE) of Emilia-Romagna, Bologna 40122, Italy
    • Spyros N. Pandis - Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras 26504, Greece;  Department of Chemical Engineering, University of Patras, Patras 26504, Greece
  • Author Contributions

    M.P. and A.N. developed the acidity inference method and designed the research; S.D., L.T., S.F., and M.C.F. managed the fog-sampling campaigns; M.P., S.D., S.G., M.R., and F.M. contributed to the chemical analyses; D.B. and A.T. performed aerosol sampling and ammonia measurements; M.P., A.N., S.N.P., and S.D. wrote the paper; S.N.P., M.R., S.F., M.C.F., S.G., and S.D. contributed to the scientific discussion and paper correction. All authors have given approval to the final version of the manuscript.

  • Funding

    This research was supported by the European Commission (project PyroTRACH, grant no. 726165; project BACCHUS, grant no. 603445; and project FORCES, grant no. 821205) and by the Regione Emilia-Romagna (project SUPERSITO, grant nos. DRG 428/10 and DGR 1971/2013).

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors are grateful to all the ARPAE Emilia-Romagna co-workers for their support and collaboration in aerosol sample analysis. ISAC-CNR is particularly grateful to Giordano Orsi, Valeriana Mancinelli, Lara Giulianelli, Francescopiero Calzolari, and all the other colleagues who collaborated in the preparation of the fog campaigns, the functioning of the instruments, and the fog and aerosol sampling in the field over 25 years.

References

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This article references 38 other publications.

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    8
    Fine-particle water and pH in the southeastern United States
    Guo, H.; Xu, L.; Bougiatioti, A.; Cerully, K. M.; Capps, S. L.; Hite, J. R., Jr.; Carlton, A. G.; Lee, S.-H.; Bergin, M. H.; Ng, N. L.; Nenes, A.; Weber, R. J.
    Atmospheric Chemistry and Physics (2015), 15 (9), 5211-5228CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Particle water and pH are predicted using meteorol. observations (relative humidity (RH), temp. (T)), gas/particle compn., and thermodn. modeling (ISORROPIA-II). A comprehensive uncertainty anal. is included, and the model is validated. We investigate mass concns. of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiol. (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aq. concn. and depends on both the presence of ions and amt. of particle liq. water. Levels of particle water, in turn, are detd. through water uptake by both the ionic species and org. compds. Thermodn. calcns. based on measured ion concns. can predict both pH and liq. water but may be biased since contributions of org. species to liq. water are not considered. In this study, contributions of both the inorg. and org. fractions to aerosol liq. water were considered, and predictions were in good agreement with measured liq. water based on differences in ambient and dry light scattering coeffs. (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg m-3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concns. (slope = 1.07, intercept = -0.12 μg m-3, R2 = 0.76). Based on this study, org. species on av. contributed 35% to the total water, with a substantially higher contribution (50 %) at night. However, not including contributions of org. water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of org. water could be sufficient for the purposes of aq. secondary org. aerosol (SOA) chem. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liq. water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas-aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissocn. state of individual ions or particle liq. water levels and do not correlate with particle pH.
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  9. 9
    Fountoukis, C.; Nenes, A. ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+–Ca2+–Mg2+–NH4+–Na+–SO42––NO3––Cl––H2O aerosols. Atmos. Chem. Phys. 2007, 7, 4639– 4659,  DOI: 10.5194/acp-7-4639-2007
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    9
    ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosols
    Fountoukis, C.; Nenes, A.
    Atmospheric Chemistry and Physics (2007), 7 (17), 4639-4659CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
    This study presents ISORROPIA II, a thermodn. equil. model for the K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosol system. A comprehensive evaluation of its performance is conducted against water uptake measurements for lab. aerosol and predictions of the SCAPE2 thermodn. module over a wide range of atmospherically relevant conditions. The two models agree well, to within 13% for aerosol water content and total PM mass, 16% for aerosol nitrate and 6% for aerosol chloride and ammonium. Largest discrepancies were found under conditions of low RH, primarily from differences in the treatment of water uptake and solid state compn. In terms of computational speed, ISORROPIA II was more than an order of magnitude faster than SCAPE2, with robust and rapid convergence under all conditions. The addn. of crustal species does not slow down the thermodn. calcns. (compared to the older ISORROPIA code) because of optimizations in the activity coeff. calcn. algorithm. Based on its computational rigor and performance, ISORROPIA II appears to be a highly attractive alternative for use in large scale air quality and atm. transport models.
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  10. 10
    Song, S.; Nenes, A.; Gao, M.; Zhang, Y.; Liu, P. F.; Shao, J. Y.; Ye, D. C.; Xu, W. Q.; Lei, L.; Sun, Y. L.; Liu, B. X.; Wang, S. X.; McElroy, M. B. Thermodynamic modeling suggests declines in water uptake and acidity of inorganic aerosols in Beijing winter haze events during 2014/2015–2018/2019. Environ. Sci. Technol. Lett. 2019, 6, 752– 760,  DOI: 10.1021/acs.estlett.9b00621
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    10
    Thermodynamic Modeling Suggests Declines in Water Uptake and Acidity of Inorganic Aerosols in Beijing Winter Haze Events during 2014/2015-2018/2019
    Song, Shaojie; Nenes, Athanasios; Gao, Meng; Zhang, Yuzhong; Liu, Pengfei; Shao, Jingyuan; Ye, Dechao; Xu, Weiqi; Lei, Lu; Sun, Yele; Liu, Baoxian; Wang, Shuxiao; McElroy, Michael B.
    Environmental Science & Technology Letters (2019), 6 (12), 752-760CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    In recent years, aggressive air pollution mitigation in northern China resulted in considerable changes in gas and aerosol chem. compn.; however, it is unclear whether aerosol water content and acidity responded to these changes. These two parameters have been shown to affect heterogeneous prodn. of winter haze aerosols. The authors performed thermodn. equil. modeling using chem. and meteorol. data in urban Beijing for four recent winter seasons and quantified changes of inorg. aerosols mass growth factor and pH. They focused on high relative humidity (>60%) conditions when sub-micron particles have been shown to be in a liq. state. From 2014/2015 to 2018/2019, modeled mass growth factor decreased ∼9-17% due to changes in aerosol compn. (more NO3-, less SO42- and Cl-); modeled pH increased ∼0.3-0.4 std. units, mainly due to increasing NH3. A buffer equation, derived from semi-volatile NH3 partitioning, helped understand the pH sensitivity to meteorol. and chem. variables. Results provide implications to evaluate potential chem. feedback in secondary aerosol prodn. and the effectiveness of NH3 control as a measure to alleviate winter haze.
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  11. 11
    Ibikunle, I.; Beyersdorf, A.; Campuzano-Jost, P.; Corr, C.; Crounse, J. D.; Dibb, J.; Diskin, G.; Huey, G.; Jimenez, J.-L.; Kim, M. J.; Nault, B. A.; Scheuer, E.; Teng, A.; Wennberg, P. O.; Anderson, B.; Crawford, J.; Weber, R.; Nenes, A. Fine particle pH and sensitivity to NH3 and HNO3 over summertime South Korea during KORUS-AQ. Atmos. Chem. Phys. Discuss. 2020, 1– 42,  DOI: 10.5194/acp-2020-501
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  12. 12
    Bougiatioti, A.; Nikolaou, P.; Stavroulas, I.; Kouvarakis, G.; Weber, R.; Nenes, A.; Kanakidou, M.; Mihalopoulos, N. Particle water and pH in the eastern Mediterranean: source variability and implications for nutrient availability. Atmos. Chem. Phys. 2016, 16, 4579– 4591,  DOI: 10.5194/acp-16-4579-2016
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    12
    Particle water and pH in the eastern mediterranean: source variability and implications for nutrient availability
    Bougiatioti, Aikaterini; Nikolaou, Panayiota; Stavroulas, Iasonas; Kouvarakis, Giorgos; Weber, Rodney; Nenes, Athanasios; Kanakidou, Maria; Mihalopoulos, Nikolaos
    Atmospheric Chemistry and Physics (2016), 16 (7), 4579-4591CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Particle water (liq. water content, LWC) and aerosol pH are important parameters of the aerosol phase, affecting heterogeneous chem. and bioavailability of nutrients that profoundly impact cloud formation, atm. compn., and atm. fluxes of nutrients to ecosystems. Few measurements of in situ LWC and pH, however, exist in the published literature. Using concurrent measurements of aerosol chem. compn., cloud condensation nuclei activity, and tandem light scattering coeffs., the particle water mass concns. assocd. with the aerosol inorg. (Winorg) and org. (Worg) components are detd. for measurements conducted at the Finokalia atm. observation station in the eastern Mediterranean between June and Nov. 2012. These data are interpreted using the ISORROPIA-II thermodn. model to predict the pH of aerosols originating from the various sources that influence air quality in the region. On av., closure between predicted aerosol water and that detd. by comparison of ambient with dry light scattering coeffs. was achieved to within 8% (slope=0.92, R2 = 0.8, n = 5201 points). Based on the scattering measurements, a parameterization is also derived, capable of reproducing the hygroscopic growth factor (f(RH)) within 15% of the measured values. The highest aerosol water concns. are obsd. during night time, when relative humidity is highest and the collapse of the boundary layer increases the aerosol concn. A significant diurnal variability is found for Worg with morning and afternoon av. mass concns. being 10-15 times lower than night time concns., thus rendering Winorg the main form of particle water during daytime. The av. value of total aerosol water was 2.19±1.75 μgm-3, contributing on av. up to 33% of the total submicron mass concn. Av. aerosol water assocd. with orgs., Worg, was equal to 0.56±0.37 μgm-3; thus, orgs. contributed about 27.5% to the total aerosol water, mostly during early morning, late evening, and night time hours. The aerosol was found to be highly acidic with calcd. aerosol pH varying from 0.5 to 2.8 throughout the study period. Biomass burning aerosol presented the highest values of pH in the submicron fraction and the lowest values in total water mass concn. The low pH values obsd. in the submicron mode and independently of air mass origin could increase nutrient availability and esp. P soly., which is the nutrient limiting sea water productivity of the eastern Mediterranean.
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  13. 13
    Gilardoni, S.; Massoli, P.; Giulianelli, L.; Rinaldi, M.; Paglione, M.; Pollini, F.; Lanconelli, C.; Poluzzi, V.; Carbone, S.; Hillamo, R.; Russell, L. M.; Facchini, M. C.; Fuzzi, S. Fog scavenging of organic and inorganic aerosol in the Po Valley. Atmos. Chem. Phys. 2014, 14, 6967– 6981,  DOI: 10.5194/acp-14-6967-2014
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    13
    Fog scavenging of organic and inorganic aerosol in the Po Valley
    Gilardoni, S.; Massoli, P.; Giulianelli, L.; Rinaldi, M.; Paglione, M.; Pollini, F.; Lanconelli, C.; Poluzzi, V.; Carbone, S.; Hillamo, R.; Russell, L. M.; Facchini, M. C.; Fuzzi, S.
    Atmospheric Chemistry and Physics (2014), 14 (13), 6967-6981, 15 pp.CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    The interaction of aerosol with atm. water affects the processing and wet removal of atm. particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol-water interaction at high relative humidity during fog events in the Po Valley within the framework of the Agenzia Regionale per la Prevenzione e l'Ambiente (ARPA) - Emilia Romagna supersite project. For the first time in this area, the changes in particle chem. compn. caused by fog are discussed along with changes in particle microphysics. During the expt., 14 fog events were obsd. The av. mass scavenging efficiency was 70% for nitrate, 68% for ammonium, 61% for sulfate, 50% for orgs., and 39% for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm Dva (vacuum aerodynamic diam.) and enriched in carbonaceous aerosol, mainly black carbon and water-insol. org. aerosol. For each fog event, the size-segregated scavenging efficiency of nitrate and org. aerosol (OA) was calcd. by comparing chem. species size distribution before and after fog formation. For both nitrate and OA, the size-segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was obsd., indicating that 44-51% of the variability of nitrate scavenging in smaller particles (below 300 nm Dva) was explained by changes in particle chem. compn. The size-segregated scavenging curves of OA followed those of nitrate, suggesting that org. scavenging was controlled by mixing with water-sol. species. In particular, functional group compn. and OA elemental anal. indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of org. functional group compn. during the expt. did not allow us to discriminate the effect of different org. functionalities on OA scavenging.
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  14. 14
    Young, A. H.; Keene, W. C.; Pszenny, A. A. P.; Sander, R.; Thornton, J. A.; Riedel, T. P.; Maben, J. R. Phase partitioning of soluble trace gases with size-resolved aerosols in near-surface continental air over northern Colorado, USA, during winter. J. Geophys. Res.: Atmos. 2013, 118, 9414– 9427,  DOI: 10.1002/jgrd.50655
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  15. 15
    Wang, G. H.; Zhang, R. Y.; Gomez, M. E.; Yang, L. X.; Zamora, M. L.; Hu, M.; Lin, Y.; Peng, J. F.; Guo, S.; Meng, J. J.; Li, J. J.; Cheng, C. L.; Hu, T. F.; Ren, Y. Q.; Wang, Y. S.; Gao, J.; Cao, J. J.; An, Z. S.; Zhou, W. J.; Li, G. H.; Wang, J. Y.; Tian, P. F.; Marrero-Ortiz, W.; Secrest, J.; Du, Z. F.; Zheng, J.; Shang, D. J.; Zeng, L. M.; Shao, M.; Wang, W. G.; Huang, Y.; Wang, Y.; Zhu, Y. J.; Li, Y. X.; Hu, J. X.; Pan, B.; Cai, L.; Cheng, Y. T.; Ji, Y. M.; Zhang, F.; Rosenfeld, D.; Liss, P. S.; Duce, R. A.; Kolb, C. E.; Molina, M. J. Persistent sulfate formation from London Fog to Chinese haze. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 13630– 13635,  DOI: 10.1073/pnas.1616540113
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    15
    Persistent sulfate formation from London Fog to Chinese haze
    Wang, Gehui; Zhang, Renyi; Gomez, Mario E.; Yang, Lingxiao; Zamora, Misti Levy; Hu, Min; Lin, Yun; Peng, Jianfei; Guo, Song; Meng, Jingjing; Li, Jianjun; Cheng, Chunlei; Hu, Tafeng; Ren, Yanqin; Wang, Yuesi; Gao, Jian; Cao, Junji; An, Zhisheng; Zhou, Weijian; Li, Guohui; Wang, Jiayuan; Tian, Pengfei; Marrero-Ortiz, Wilmarie; Secrest, Jeremiah; Du, Zhuofei; Zheng, Jing; Shang, Dongjie; Zeng, Limin; Shao, Min; Wang, Weigang; Huang, Yao; Wang, Yuan; Zhu, Yujiao; Li, Yixin; Hu, Jiaxi; Pan, Bowen; Cai, Li; Cheng, Yuting; Ji, Yuemeng; Zhang, Fang; Rosenfeld, Daniel; Liss, Peter S.; Duce, Robert A.; Kolb, Charles E.; Molina, Mario J.
    Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (48), 13630-13635CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atm. models consistently underpredict sulfate levels under diverse environmental conditions. From atm. measurements in two Chinese megacities and complementary lab. expts., we show that the aq. oxidn. of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atm. conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidn. process leads to large sulfate prodn. rates and promotes formation of nitrate and org. matter on aq. particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addn. to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate prodn. mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.
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  16. 16
    Giulianelli, L.; Gilardoni, S.; Tarozzi, L.; Rinaldi, M.; Decesari, S.; Carbone, C.; Facchini, M.; Fuzzi, S. Fog occurrence and chemical composition in the Po valley over the last twenty years. Atmos. Environ. 2014, 98, 394– 401,  DOI: 10.1016/j.atmosenv.2014.08.080
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    16
    Fog occurrence and chemical composition in the Po valley over the last twenty years
    Giulianelli, L.; Gilardoni, S.; Tarozzi, L.; Rinaldi, M.; Decesari, S.; Carbone, C.; Facchini, M. C.; Fuzzi, S.
    Atmospheric Environment (2014), 98 (), 394-401CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    Frequency of fog events together with fog water chem. compn., pH, cond. and liq. water content have systematically been measured from the end of the 1980's at the field station of San Pietro Capofiume, in the eastern Po Valley, Northern Italy. In agreement with what has been obsd. in other regions in Europe, fog frequency (visibility < 1 km) has decreased over the last three decades. Ionic strength and cond. of fog samples also decreased over the period indicating a redn. of the ionic load of the droplets. Specifically, the three major inorg. ions (NH+4, SO2-4, NO-3), accounting for 86% of the total fog water ionic strength, show a decreasing trend in concn. over the period, which can be linked to the decreasing trend of NH3, SO2 and NOx emissions registered in northern Italy over the same period. Sulfate exhibits the highest relative decrease (76%). Seasonal vol.-weighted means of pH show an increasing trend over the obsd. period. The available data of total water-sol. org. matter concns. indicate that org. compds. represent a considerable fraction (25% on av.) of the total solute mass of fog water. Fog water samples often contain suspended insol. particles, which were collected by filtering fog water through quartz fiber filters. EC-OC anal. performed on the filters collected over a four-year period, show that the sum of elemental carbon (EC) and water-insol. org. mass accounts on av. for 46%-56% of the total suspended material mass. Insol. carbonaceous material is composed mainly of org. matter, with EC accounting on av. only for 19% of the insol. carbon.
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  17. 17
    Fuzzi, S.; Orsi, G.; Mariotti, M. Radiation fog liquid water acidity at a field station in the Po Valley. J. Aerosol Sci. 1983, 14, 135– 138,  DOI: 10.1016/0021-8502(83)90037-X
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    17
    Radiation fog liquid water acidity at a field station in the Po Valley
    Fuzzi, S.; Orsi, G.; Mariotti, M.
    Journal of Aerosol Science (1983), 14 (2), 135-8CODEN: JALSB7; ISSN:0021-8502.
    The problem of fog water acidity is examd. in this paper. The fog occurrence in some sites of the Po Valley (northern Italy) during the winter months is very high (∼30% of the time), so that the problem of fog acidity is very important in the degrdn. of materials, agriculture, and human health. The anal. of the pH trend of 3 individual fog events is presented. From these data it appears that, although pH follows the same trend of fog liq. water content, the evolving microphys. fog structure cannot alone account for pH variations during a fog event. The anal. of SO42-, NO3- and Cl- concn. in fog water by means of the ion chromatograph, suggests that SO2 and NOx oxidn. to H2SO4 and HNO3 is responsible for the excess acidity not accounted for by condensation nuclei chem. compn. and microphys. fog structure evolution (condensation/evapn. processes).
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  18. 18
    Fuzzi, S.; Orsi, G.; Mariotti, M. Wet deposition due to fog in the Po Valley, Italy. J. Atmos. Chem. 1985, 3, 289– 296,  DOI: 10.1007/BF00210501
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    18
    Wet deposition due to fog in the Po Valley, Italy
    Fuzzi, Sandro; Orsi, Giordano; Mariotti, Mauro
    Journal of Atmospheric Chemistry (1985), 3 (2), 289-96CODEN: JATCE2; ISSN:0167-7764.
    Wet deposition due to radiation fog in the Po Valley is characterized by both a high fog occurrence during the fall-winter months and fog water solns. of high ionic concn. and acidity. Estd. wet deposition for NH4+, NO3-, and SO42- ions due to fog droplet settling to the ground accounts for 13.2, 12.1, and 5.3%, resp., with respect to bulk pptns. over the same period: Jan.-Mar. and Oct.-Dec. (fog season). Fog decompn. rates show that this process can be a important pathway of trace gases and particle loss from the air. First indicative results of fog removal efficiency with respect to air particulate matter are presented. Dry deposition parameters should be taken into account in evaluating the potential effect of fog droplet deposition on vegetation.
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  19. 19
    Fuzzi, S.; Orsi, G.; Nardini, G.; Facchini, M. C.; McLaren, S.; McLaren, E.; Mariotti, M. Heterogeneous processes in the Po Valley radiation fog. J. Geophys. Res.: Atmos. 1988, 93, 11141– 11151,  DOI: 10.1029/JD093iD09p11141
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    Winiwarter, W.; Puxbaum, H.; Fuzzi, S.; Facchini, M. C.; Orsi, G.; Beltz, N.; Enderle, K.; Jaeschke, W. Organic acid gas and liquid-phase measurements in Po Valley fall-winter conditions in the presence of fog. Tellus B 1988, 40B, 348– 357,  DOI: 10.1111/j.1600-0889.1988.tb00109.x
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    20
    Organic acid gas and liquid-phase measurements in Po Valley fall-winter conditions in the presence of fog
    Winiwarter, Wilfried; Puxbaum, Hans; Fuzzi, Sandro; Facchini, Maria Cristina; Orsi, Giordano; Beltz, Norbert; Jaeschke, Wolfgang; Enderle, Karlheinz; Jaeschke, Wolfgang
    Tellus, Series B: Chemical and Physical Meteorology (1988), 40B (5), 348-57CODEN: TSBMD7; ISSN:0280-6509.
    Concurrent gas and liq. phase org. acid measurements during radiation fog episodes are reported. Gas phase concns. of HCO2H and AcOH exhibit large variations from the detection limit (5 nmol/m3, 0.1 ppb) to 150 nmol/m3 (3.5 ppb). Liq. phase concn. ranges of 11-175 and 10-269 μM were obsd. for HCO2H and AcOH, resp. Large deviations from Henry's law equil. were obsd. for HCO2H when the pH of fog droplets was >5. In this range, the bulk sample pH might not be representative of the acidity of individual droplets in equil. with the gas phase. The org. acids in the Po Valley under fall-winter conditions seem to originate from anthropogenic processes.
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    Fuzzi, S.; Orsi, G.; Bonforte, G.; Zardini, B.; Franchini, P. L. An automated fog water collector suitable for deposition networks: design, operation and field tests. Water, Air, Soil Pollut. 1997, 93, 383– 394,  DOI: 10.1007/BF02404768
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    21
    An automated fog water collector suitable for deposition networks: design, operation and field tests
    Fuzzi, Sandro; Orsi, Giordano; Bonforte, Giuseppe; Zardini, Bruno; Franchini, Pier Luigi
    Water, Air, and Soil Pollution (1997), 93 (1-4), 383-394CODEN: WAPLAC; ISSN:0049-6979. (Kluwer)
    The study of fog water chem. compn. and the contribution of fog droplets of total chem. deposition has become a relevant environmental subject over the past few years. This paper describes a fog water collector suitable for deposition network operation, due to its complete automation and to the facility of remote acquisition of sampling information. Sampling of fog droplets on teflon strings is activated by an optical fog detector according to a particular protocol operated by a microprocessor controller. Multiple sample collection, also microprocessor controlled, is possible with this instrument. The problem of fog droplet sampling in sub-freezing conditions is overcome using a sampling schedule implemented by the microprocessor controller which alternates between sampling periods and stand-by periods during which melting of the rime collected on the strings is allowed. Field tests on the reliability and reproducibility of the sampling operations are presented in the paper. Side by side operation of the fog collector with a PVM-100 fog liq. water content meter shows that the amt. of water per unit vol. of air collected by the sampling instrument is proportional to the fog liq. water content averaged over the period of an entire fog event.
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  22. 22
    Tomasi, C.; Tampieri, F. Features of the proportionality coefficient in the relationship between visibility and liquid water content in haze and fog. Atmosphere 1976, 61– 76,  DOI: 10.1080/00046973.1976.9648403
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    Matta, E.; Facchini, M. C.; Decesari, S.; Mircea, M.; Cavalli, F.; Fuzzi, S.; Putaud, J. P.; Dell’Acqua, A. Mass closure on the chemical species in size-segregated atmospheric aerosol collected in an urban area of the Po Valley, Italy. Atmos. Chem. Phys. 2003, 3, 623– 637,  DOI: 10.5194/acp-3-623-2003
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    23
    Mass closure on the chemical species in size-segregated atmospheric aerosol collected in an urban area of the Po Valley, Italy
    Matta, E.; Facchini, M. C.; Decesari, S.; Mircea, M.; Cavalli, F.; Fuzzi, S.; Putaud, J. -P.; Dell'Acqua, A.
    Atmospheric Chemistry and Physics (2003), 3 (3), 623-637CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
    A complete size segregated chem. characterization was carried out for aerosol samples collected in the urban area of Bologna over a period of one year, using five-stage low pressure Berner impactors. An original dual-substrate technique was adopted to obtain samples suitable for a complete chem. characterization. Total mass, inorg., and org. components were analyzed as a function of size, and a detailed characterization of the water sol. org. compds. was also performed by means of a previously developed methodol., based on HPLC sepn. of org. compds. according to their acid character and functional group anal. by proton NMR. Chem. mass closure of the collected samples was reached to within a few percent on av. in the submicron aerosol range, while a higher unknown fraction in the coarse aerosol range was attributed to soil-derived species not analyzed in this expt. Comparison of the functional group anal. results with model results simulating water sol. org. compd. prodn. by gas-to-particle conversion of anthropogenic VOCs showed that this pathway provides a minor contribution to the org. compn. of the aerosol samples in the urban area of Bologna.
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  24. 24
    Ricciardelli, I.; Bacco, D.; Rinaldi, M.; Bonafe, G.; Scotto, F.; Trentini, A.; Bertacci, G.; Ugolini, P.; Zigola, C.; Rovere, F.; Maccone, C.; Pironi, C.; Poluzzi, V. A three-year investigation of daily PM2:5 main chemical components in four sites: the routine measurement program of the Supersito Project (Po Valley, Italy). Atmos. Environ. 2017, 152, 418– 430,  DOI: 10.1016/j.atmosenv.2016.12.052
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    24
    A three-year investigation of daily PM2.5 main chemical components in four sites: the routine measurement program of the Supersito Project (Po Valley, Italy)
    Ricciardelli, Isabella; Bacco, Dimitri; Rinaldi, Matteo; Bonafe, Giovanni; Scotto, Fabiana; Trentini, Arianna; Bertacci, Giulia; Ugolini, Pamela; Zigola, Claudia; Rovere, Flavio; Maccone, Claudio; Pironi, Claudia; Poluzzi, Vanes
    Atmospheric Environment (2017), 152 (), 418-430CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    The Supersito Project (www.supersito-er.it) has been active in the Emilia-Romagna region, southern part of the Po Valley (Italy), since 2011. Focal aim of the project is to enhance the knowledge on atm. aerosol and its impact on human health. In the framework of Supersito, major chem. components of daily PM2.5 were investigated over a period of more than three years at four sampling sites, representative of dissimilar territorial conditions: one rural background (SPC) and three urban background sites in the coastal (RN), central (MS) and inner area (PR) of the region. In all the sites, org. and elemental carbon and water sol. inorg. ions accounted for more than 70% of PM2.5 mass, during all seasons. Nitrate and org. carbon (OC) were the main components of winter PM2.5, while summer aerosol was mainly contributed by OC and sulfate. OC was dominated by primary sources, with a potentially important contribution from biomass burning, in winter, while secondary processes dominated OC prodn. in summer. A substantial homogeneity was obsd. on a regional scale in terms of spatial distribution of pollutants, with EC only presenting significant differences between urban and rural areas during winters. Nonetheless, differences were obsd. between the coastal and the inner part of the region, with the former being systematically characterized by higher concns. of carbonaceous compds. and lower concns. of ammonium nitrate. The coastal area was likely influenced by the aged OC from the Po Valley outflow in addn. to local sources, while the scarcity of local sources of ammonia limited the formation of ammonium nitrate. In the studied area, local and regional meteorol. - mostly governed by geog. collocation and orog. - was responsible for PM2.5 mass and compn. no less than local and regional emission sources.
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  25. 25
    Guo, H.; Sullivan, A. P.; Campuzano-Jost, P.; Schroder, J. C.; Lopez-Hilfiker, F. D.; Dibb, J. E.; Jimenez, J. L.; Thornton, J. A.; Brown, S. S.; Nenes, A.; Weber, R. J. Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States. J. Geophys. Res.: Atmos. 2016, 121, 10355– 10376,  DOI: 10.1002/2016jd025311
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    25
    Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States
    Guo, Hongyu; Sullivan, Amy P.; Campuzano-Jost, Pedro; Schroder, Jason C.; Lopez-Hilfiker, Felipe D.; Dibb, Jack E.; Jimenez, Jose L.; Thornton, Joel A.; Brown, Steven S.; Nenes, Athanasios; Weber, Rodney J.
    Journal of Geophysical Research: Atmospheres (2016), 121 (17), 10355-10376CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
    Particle pH is a crit. but poorly constrained quantity that affects many aerosol processes and properties, including aerosol compn., concns., and toxicity. We assess PM1 pH as a function of geog. location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 Feb. to 15 March 2015). Particle pH and water were predicted with the ISORROPIA-II thermodn. model and validated by comparing predicted to obsd. partitioning of inorg. nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH obsd. particle nitrate was higher than predicted, possibly due to org.-inorg. phase sepns. or nitrate measurement uncertainties assocd. with low concns. (nitrate < 1 μg m-3). Including refractory ions in the pH calcns. did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between -0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern U.S. and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH-dependent aerosol components, such as PM1 nitrate, over a wide range of temps. (-21 to 19°C), RH (20 to 95%), inorg. gas, and particle concns. and also provides further evidence that particles with low pH are ubiquitous.
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  26. 26
    Guo, H.; Liu, J.; Froyd, K. D.; Roberts, J. M.; Veres, P. R.; Hayes, P. L.; Jimenez, J. L.; Nenes, A.; Weber, R. J. Fine particle pH and gas–particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign. Atmos. Chem. Phys. 2017, 17, 5703– 5719,  DOI: 10.5194/acp-17-5703-2017
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    26
    Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign
    Guo, Hongyu; Liu, Jiumeng; Froyd, Karl D.; Roberts, James M.; Veres, Patrick R.; Hayes, Patrick L.; Jimenez, Jose L.; Nenes, Athanasios; Weber, Rodney J.
    Atmospheric Chemistry and Physics (2017), 17 (9), 5703-5719CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    PH is a fundamental aerosol property that affects ambient particle concn. and compn., linking pH to all aerosol environmental impacts. Here, PM1 and PM2.5 pH are calcd. based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodn. model and validated by comparing predicted to measured gas-particle partitioning of inorg. nitrate, ammonium, and chloride. The study mean ± std. deviation PM1 pH was 1.9 ± 0.5 for the SO2-4-NO-3-NH+4-HNO3-NH3 system. For PM2.5, internal mixing of sea salt components (SO2-4-NO-3-NH+4-Na+-Cl--K+-HNO3-NH3-HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2.5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2.5 pH. A mean PM1 pH of 1.9 at Pasadena was approx. one unit higher than what we have reported in the southeastern US, despite similar temp., relative humidity, and sulfate ranges, and is due to higher total nitrate concns. (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concns. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by diln., further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.
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  27. 27
    Guo, H.; Otjes, R.; Schlag, P.; Kiendler-Scharr, A.; Nenes, A.; Weber, R. J. Effectiveness of ammonia reduction on control of fine particle nitrate. Atmos. Chem. Phys. 2018, 18, 12241– 12256,  DOI: 10.5194/acp-18-12241-2018
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    27
    Effectiveness of ammonia reduction on control of fine particle nitrate
    Guo, Hongyu; Otjes, Rene; Schlag, Patrick; Kiendler-Scharr, Astrid; Nenes, Athanasios; Weber, Rodney J.
    Atmospheric Chemistry and Physics (2018), 18 (16), 12241-12256CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    In some regions, reducing aerosol ammonium nitrate (NH4NO3) concns. may substantially improve air quality. This can be accomplished by redns. in precursor emissions, such as nitrogen oxides (NOx) to lower nitric acid (HNO3) that partitions to the aerosol, or redns. in ammonia (NH3) to lower particle pH and keep HNO3 in the gas phase. Using the ISORROPIA-II thermodn. aerosol model and detailed observational data sets, we explore the sensitivity of aerosol NH4NO3 to gas-phase NH3 and NOx controls for a no. of contrasting locations, including Europe, the United States, and China. NOx control is always effective, whereas the aerosol response to NH3 control is highly nonlinear and only becomes effective at a thermodn. sweet spot. The anal. provides a conceptual framework and fundamental evaluation on the relative value of NOx vs. NH3 control and demonstrates the relevance of pH as an air quality parameter. We find that, regardless of the locations examd., it is only when ambient particle pH drops below an approx. crit. value of 3 (slightly higher in warm and slightly lower in cold seasons) that NH3 redn. leads to an effective response in PM2.5 mass. The required amt. of NH3 redn. to reach the crit. pH and efficiently decrease NH4NO3 at different sites is assessed. Owing to the linkage between NH3 emissions and agricultural productivity, the substantial NH3 redn. required in some locations may not be feasible. Finally, controlling NH3 emissions to increase aerosol acidity and evap. NH4NO3 will have other effects, beyond redn. of PM2.5 NH4NO3, such as increasing aerosol toxicity and potentially altering the deposition patterns of nitrogen and trace nutrients.
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  28. 28
    Vasilakos, P.; Russell, A.; Weber, R.; Nenes, A. Understanding nitrate formation in a world with less sulfate. Atmos. Chem. Phys. 2018, 18, 12765– 12775,  DOI: 10.5194/acp-18-12765-2018
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    28
    Understanding nitrate formation in a world with less sulfate
    Vasilakos, Petros; Russell, Armistead; Weber, Rodney; Nenes, Athanasios
    Atmospheric Chemistry and Physics (2018), 18 (17), 12765-12775CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    SO2 emission controls, combined with modestly increasing ammonia, have been thought to generate aerosol with significantly reduced acidity for cases in which sulfate is partially substituted by nitrate. However, neither expectation agrees with decadal observations in the southeastern USA, suggesting that a fundamentally different response of aerosol pH to emissions changes is occurring. We postulate that this nitrate substitution paradox arises from a pos. bias in aerosol pH in model simulations. This bias can elevate pH to a level at which nitrate partitioning is readily promoted, leading to behavior consistent with nitrate substitution. CMAQ simulations are used to investigate this hypothesis; modeled PM2.5 pH using 2001 emissions compare favorably with pH inferred from obsd. species concns. Using 2011 emissions, however, leads to simulated pH increases of one unit, which is inconsistent with observations from that year. Nonvolatile cations (K+, Na+, Ca+2, and Mg+2) in the fine mode are found to be responsible for the erroneous predicted increase in aerosol pH of about 1 unit on av. over the USA. Such an increase can induce a nitrate bias of 1-2 μg m-3, which may further increase in future projections, reaffirming an otherwise incorrect expectation of a significant nitrate substitution. Evaluation of predicted aerosol pH against thermodn. anal. of observations is therefore a critically important, but overlooked, aspect of model evaluation for a robust emissions policy.
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  29. 29
    Battaglia, M. A., Jr.; Weber, R. J.; Nenes, A.; Hennigan, C. J. Effects of water-soluble organic carbon on aerosol pH. Atmos. Chem. Phys. 2019, 19, 14607– 14620,  DOI: 10.5194/acp-19-14607-2019
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    29
    Effects of water-soluble organic carbon on aerosol pH
    Battaglia, Michael A.; Weber, Rodney J.; Nenes, Athanasios; Hennigan, Christopher J.
    Atmospheric Chemistry and Physics (2019), 19 (23), 14607-14620CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Water-sol. org. carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter. Despite advances in aerosol thermodn. equil. models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH). We address this limitation by studying submicron aerosols that represent the two extremes in acidity levels found in the atm.: strongly acidic aerosol from Baltimore, MD, and weakly acidic conditions characteristic of Beijing, China. These cases are then used to construct mixed inorg.-org. single-phase aq. particles and thermodynamically analyzed by the Extended Aerosol Inorgs. Model (E-AIM) and ISORROPIA models in combination with activity coeff. model AIOMFAC (Aerosol Inorg.-Org. Mixts. Functional groups Activity Coeff.) to evaluate the effects of WSOC on the H+ ion activity coeffs. (γ H+) and activity (pH). We find that addn. of org. acids and nonacid org. species concurrently increases γ H+ and aerosol liq. water. Under the highly acidic conditions typical of the eastern US (inorg.-only pH ∼ 1), these effects mostly offset each other, giving pH changes of < 0.5 pH units even at org. aerosol dry mass fractions in excess of 60%. Under conditions with weaker acidity typical of Beijing (inorg.-only pH ∼ 4:5), the nonacidic WSOC compds. had similarly minor effects on aerosol pH, but org. acids imparted the largest changes in pH compared to the inorg.-only simulations. Org. acids affect pH in the order of their pKa values (oxalic acid > malonic acid > glutaric acid). Although the inorg.-only pH was above the pKa value of all three org. acids investigated, pH changes in excess of 1 pH unit were only obsd. at unrealistic org. acid levels (aerosol org. acid concns. > 35μgm-3) in Beijing. The model simulations were run at 70%, 80%, and 90% relative humidity (RH) levels and the effect of WSOC was inversely related to RH. At 90% RH, WSOC altered aerosol pH by up to ∼ 0:2 pH units, though the effect was up to ∼ 0:6 pH units at 70% RH. The somewhat offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorg. constituents alone under conditions where liq.-liq. phase sepn. is not anticipated to occur.
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  30. 30
    Facchini, M. C.; Fuzzi, S.; Zappoli, S.; Andracchio, A.; Gelencsér, A.; Kiss, G.; Krivácsy, Z.; Mészáros, E.; Hansson, H.-C.; Alsberg, T.; Zebühr, Y. Partitioning of the organic aerosol component between fog droplets and interstitial air. J. Geophys. Res.: Atmos. 1999, 104, 26821– 26832,  DOI: 10.1029/1999JD900349
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    Partitioning of the organic aerosol component between fog droplets and interstitial air
    Facchini, Maria Cristina; Fuzzi, Sandro; Zappoli, Sergio; Andracchio, Antonella; Gelencser, Andras; Kiss, Gyula; Krivacsy, Zoltan; Meszaros, Erno; Hansson, Hans-Christen; Alsberg, Tomas; Zebuhr, Yngve
    Journal of Geophysical Research, [Atmospheres] (1999), 104 (D21), 26821-26832CODEN: JGRDE3 ISSN:. (American Geophysical Union)
    Limited information is available on the nature of org. compds. in tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nucleation (CCN) ability. Here we analyze samples of liq. droplets and interstitial aerosol, concurrently collected during fog episodes, to det. how the org. compds. are partitioned between the two reservoirs. By comparing the nature and concn. of different org. carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon species on the basis of their chem. properties, with polar water-sol. species representing the greater part of total C within fog droplets, and water-insol. C species preferentially found in the interstitial reservoir. Water-sol. org. species are scavenged by fog droplets to a extent comparable to major inorg. ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water-sol. org. carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol anal. are aliph. dicarboxylic acids, sugars, aliph. alcs., and aliph. carboxylic acids. However, such species, ∼120 individual compds., only account for a few percent (<5% on av.) of total WSOC. A new class of water-sol. macromol. compds. (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (∼40% on av.) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water-sol. species identified. More than 50% of WSOC still remains undetd.
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    Pandis, S. N.; Seinfeld, J. H. Should bulk cloudwater samplesobey Henry’s law?. J. Geophys. Res. 1991, 96, 10791– 10798,  DOI: 10.1029/91JD01031
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    Ricci, L.; Fuzzi, S.; Laj, P.; Lazzari, A.; Orsi, G.; Berner, A.; Günther, A.; Jaeschke, W.; Wendisch, M.; Arends, B. Gas-liquid equilibria in polluted fog. Contrib. Atmos. Phys. 1998, 71, 159
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    Fountoukis, C.; Nenes, A.; Sullivan, A.; Weber, R.; Van Reken, T.; Fischer, M.; Matías, E.; Moya, M.; Farmer, D.; Cohen, R. C. Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006. Atmos. Chem. Phys. 2009, 9, 2141– 2156,  DOI: 10.5194/acp-9-2141-2009
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    Thermodynamic characterization of Mexico city aerosol during MILAGRO 2006
    Fountoukis, C.; Nenes, A.; Sullivan, A.; Weber, R.; Van Reken, T.; Fischer, M.; Matias, E.; Moya, M.; Farmer, D.; Cohen, R. C.
    Atmospheric Chemistry and Physics (2009), 9 (6), 2141-2156CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
    Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodn. equil. model are used to study the partitioning of semivolatile inorg. species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. For particle sizes up to 1 μm diam., semi-volatile partitioning requires 15-30 min to equilibrate; longer time is typically required during the night and early morning hours. Aerosol and gas-phase speciation always exhibits substantial temporal variability, so that aerosol compn. measurements (bulk or size-resolved) obtained over large integration periods are not reflective of its true state. When the aerosol sulfate-to-nitrate molar ratio is less than unity, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equiv. sodium" (rather than explicitly) in the thermodn. equil. calcns. introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramatically. This suggests that explicitly considering crustals in the thermodn. calcns. is required to accurately predict the partitioning and phase state of aerosols.
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    Hess, A.; Iyer, H.; Malm, W. Linear trend analysis: a comparison of methods. Atmos. Environ. 2001, 35, 5211– 5222,  DOI: 10.1016/S1352-2310(01)00342-9
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    Linear trend analysis: a comparison of methods
    Hess, A.; Iyer, H.; Malm, W.
    Atmospheric Environment (2001), 35 (30), 5211-5222CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
    In this paper, we present an overview of statistical approaches available for detecting and estg. linear trends in environmental data. We evaluate seven methods of trend detection and make recommendations based on a simulation study. We also illustrate the methods using real data.
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    Rosenfeld, D.; Zhu, Y.; Wang, M.; Zheng, Y.; Goren, T.; Yu, S. Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds. Science 2019, 363, eaav0566  DOI: 10.1126/science.aav0566
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    Paglione, M.; Gilardoni, S.; Rinaldi, M.; Decesari, S.; Zanca, N.; Sandrini, S.; Giulianelli, L.; Bacco, D.; Ferrari, S.; Poluzzi, V.; Scotto, F.; Trentini, A.; Poulain, L.; Herrmann, H.; Wiedensohler, A.; Canonaco, F.; Prévôt, A. S. H.; Massoli, P.; Carbone, C.; Facchini, M. C.; Fuzzi, S. The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy. Atmos. Chem. Phys. 2020, 20, 1233– 1254,  DOI: 10.5194/acp-20-1233-2020
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    36
    The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy
    Paglione, Marco; Gilardoni, Stefania; Rinaldi, Matteo; Decesari, Stefano; Zanca, Nicola; Sandrini, Silvia; Giulianelli, Lara; Bacco, Dimitri; Ferrari, Silvia; Poluzzi, Vanes; Scotto, Fabiana; Trentini, Arianna; Poulain, Laurent; Herrmann, Hartmut; Wiedensohler, Alfred; Canonaco, Francesco; Prevot, Andre S. H.; Massoli, Paola; Carbone, Claudio; Facchini, Maria Cristina; Fuzzi, Sandro
    Atmospheric Chemistry and Physics (2020), 20 (3), 1233-1254CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    The Po Valley (Italy) is a well-known air quality hotspot characterized by particulate matter (PM) levels well above the limit set by the European Air Quality Directive and by the World Health Organization, esp. during the colder season. In the framework of Emilia-Romagna regional project "Supersito", the southern Po Valley submicron aerosol chem. compn. was characterized by means of high-resoln. aerosol mass spectroscopy (HR-AMS) with the specific aim of org. aerosol (OA) characterization and source apportionment. Eight intensive observation periods (IOPs) were carried out over 4 years (from 2011 to 2014) at two different sites (Bologna, BO, urban background, and San Pietro Capofiume, SPC, rural background), to characterize the spatial variability and seasonality of the OA sources, with a special focus on the cold season. On the multi-year basis of the study, the AMS observations show that OA accounts for avs. of 45 ± 8% (ranging from 33% to 58%) and 46 ± 7% (ranging from 36% to 50%) of the total non-refractory submicron particle mass (PM1-NR) at the urban and rural sites, resp. Primary org. aerosol (POA) comprises biomass burning (23±13% of OA) and fossil fuel (12±7%) contributions with a marked seasonality in concn. As expected, the biomass burning contribution to POA is more significant at the rural site (urban / rural concn. ratio of 0.67), but it is also an important source of POA at the urban site during the cold season, with contributions ranging from 14% to 38% of the total OA mass. Secondary org. aerosol (SOA) contributes to OA mass to a much larger extent than POA at both sites throughout the year (69 ± 16% and 83 ± 16% at the urban and rural sites, resp.), with important implications for public health. Within the secondary fraction of OA, the measurements highlight the importance of biomass burning aging products during the cold season, even at the urban background site. This biomass burning SOA fraction represents 14%-44% of the total OA mass in the cold season, indicating that in this region a major contribution of combustion sources to PM mass is mediated by environmental conditions and atm. reactivity. Among the environmental factors controlling the formation of SOA in the Po Valley, the availability of liq. water in the aerosol was shown to play a key role in the cold season. We est. that the org. fraction originating from aq. reactions of biomass burning products ("bb-aqSOA") represents 21% (14%-28%) and 25% (14%-35%) of the total OA mass and 44% (32%-56%) and 61% (21%- 100%) of the SOA mass at the urban and rural sites, resp.
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  37. 37
    Nenes, A.; Pandis, S. N.; Weber, R. J.; Russell, A. Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability. Atmos. Chem. Phys. 2020, 20, 3249– 3258,  DOI: 10.5194/acp-20-3249-2020
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    37
    Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability
    Nenes, Athanasios; Pandis, Spyros N.; Weber, Rodney J.; Russell, Armistead
    Atmospheric Chemistry and Physics (2020), 20 (5), 3249-3258CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concns. worldwide. The response of PM to changes in the emissions of both compds. is typically studied on a case-by-case basis, owing in part to the complex thermodn. interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chem. domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liq. water content. From our anal., four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the "chem. regime" of PM sensitivity to NH3 and HNO3 availability is directly detd. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chem. transport models. With this extended understanding, aerosol pH and assocd. liq. water content naturally emerge as previously ignored state parameters that drive PM formation.
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  38. 38
    Nenes, A.; Pandis, S. N.; Kanakidou, M.; Russell, A.; Song, S.; Vasilakos, P.; Weber, R. J. Aerosol acidity and liquid water content regulate the dry deposition of inorganic reactive nitrogen. Atmos. Chem. Phys. Discuss. 2020, 1– 25,  DOI: 10.5194/acp-2020-266
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  1. Ruijun Dang, Daniel J. Jacob, Shixian Zhai, Laura Hyesung Yang, Drew C. Pendergrass, Pierre Coheur, Lieven Clarisse, Martin Van Damme, Jin-soo Choi, Jin-soo Park, Zirui Liu, Peifu Xie, Hong Liao. A Satellite-Based Indicator for Diagnosing Particulate Nitrate Sensitivity to Precursor Emissions: Application to East Asia, Europe, and North America. Environmental Science & Technology 2024, 58 (45) , 20101-20113. https://doi.org/10.1021/acs.est.4c08082
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Cite this: Environ. Sci. Technol. 2021, 55, 11, 7307–7315
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  • Abstract

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    Figure 1

    Figure 1. Evaluation of the ISORROPIA-II ammonia/ammonium concentrations: simulated concentrations of ammonia (a) and ammonium (b) are plotted against the corresponding observed/estimated values. Red markers refer to the measured NH3(g) and PM2.5 aerosol composition; green markers represent the partitioning obtained from the fog water composition and the estimated NH3 inside the fog.

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    Figure 2

    Figure 2. pH values calculated using ISORROPIA-II from the PM2.5 composition (AerPM2.5 pH) and from the fog water composition (AerFOG pH). (a) Time series of AerPM2.5 (dashed lines) and AerFOG pH (markers) for the evaluation dataset. (b) The pH values of AerPM2.5 (y-axis) and AerFOG (x-axis) for parallel samples. The dashed lines indicate ΔpH = 1, 0, and −1. In both panels, black line and markers represent the calculation made without gaseous ammonia (NH3(g)) as input, while red line and markers represent the results using particle/fog NH4(p)+ + gaseous ammonia measured (NH3(g)) or estimated by fog composition (NH3(fog)), respectively.

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    Figure 3

    Figure 3. Fog and aerosol pH trends as measured directly in fog water or calculated using ISORROPIA-II. Black circles are the annual average of the pH measured; red squares are the annual average values of the aerosol pH calculated using ISORROPIA-II starting from the fog ionic composition with the gaseous estimated NH3(fog) as input (pH AerFOG + NH3(fog)). Green triangles are the annual average values of the aerosol pH calculated using ISORROPIA-II starting from PM2.5 ionic composition with the measured gaseous NH3 as input (pH AerPM2.5 + NH3(g)). Error bars represent the standard deviations of the measured/calculated pH values.

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    Figure 4

    Figure 4. Distribution of aerosol pH versus aerosol liquid water content from both fog composition (blue points) and PM2.5 composition (brown points) inside the chemical domains of aerosol sensitivity to NH3 and NOx emissions as introduced by Nenes et al. (37) The blue line defines the characteristic pH when the aerosol is sensitive to changes in the available nitrate and the red line in ammonia. These are calculated for the average temperature of the winter periods (280 K). PM2.5 data are for 2017–2018 wintertime samples for which both NH4(p)+ and NH3(g) measurements are available. Bigger markers indicate the average values of the corresponding multiyear period reported below each of them: white symbols refer to calculations based on the fog water data and the orange circles refer to the aerosol and gas-phase ammonia data.

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    Figure 5

    Figure 5. Seasonality of the distribution of aerosol pH versus liquid water content inside the chemical domains of aerosol sensitivity introduced by Nenes et al. (37) Characteristic pH for defining when the aerosol is sensitive to changes in the available nitrate (blue lines) and ammonia (red lines) are calculated for the annual average temperature (=287 K, solid lines) ± its standard deviation (=295 K, dotted lines, and =278 K, dashed lines, respectively).

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  • References

    This article references 38 other publications.

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      Weber, R. J.; Guo, H.; Russell, A. G.; Nenes, A. High aerosol acidity despite declining atmospheric sulfate concentrations over the past 15 years. Nat. Geosci. 2016, 9, 282– 285,  DOI: 10.1038/ngeo2665
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      High aerosol acidity despite declining atmospheric sulfate concentrations over the past 15 years
      Weber, Rodney J.; Guo, Hongyu; Russell, Armistead G.; Nenes, Athanasios
      Nature Geoscience (2016), 9 (4), 282-285CODEN: NGAEBU; ISSN:1752-0894. (Nature Publishing Group)
      Particle acidity affects aerosol concns., chem. compn. and toxicity. Sulfate is often the main acid component of aerosols, and largely dets. the acidity of fine particles under 2.5 μm in diam., PM2.5. Over the past 15 years, atm. sulfate concns. in the southeastern United States have decreased by 70%, whereas ammonia concns. have been steady. Similar trends are occurring in many regions globally. Aerosol ammonium nitrate concns. were assumed to increase to compensate for decreasing sulfate, which would result from increasing neutrality. Here we use obsd. gas and aerosol compn., humidity, and temp. data collected at a rural southeastern US site in June and July 2013 (ref. 1), and a thermodn. model that predicts pH and the gas-particle equil. concns. of inorg. species from the observations to show that PM2.5 at the site is acidic. PH buffering by partitioning of ammonia between the gas and particle phases produced a relatively const. particle pH of 0-2 throughout the 15 years of decreasing atm. sulfate concns., and little change in particle ammonium nitrate concns. We conclude that the redns. in aerosol acidity widely anticipated from sulfur redns., and expected acidity-related health and climate benefits, are unlikely to occur until atm. sulfate concns. reach near pre-anthropogenic levels.
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      Pye, H. O. T.; Nenes, A.; Alexander, B.; Ault, A. P.; Barth, M. C.; Clegg, S. L.; Collett, J. L., Jr.; Fahey, K. M.; Hennigan, C. J.; Herrmann, H.; Kanakidou, M.; Kelly, J. T.; Ku, I.-T.; McNeill, V. F.; Riemer, N.; Schaefer, T.; Shi, G.; Tilgner, A.; Walker, J. T.; Wang, T.; Weber, R.; Xing, J.; Zaveri, R. A.; Zuend, A. The acidity of atmospheric particles and clouds. Atmos. Chem. Phys. 2020, 20, 4809– 4888,  DOI: 10.5194/acp-20-4809-2020
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      The acidity of atmospheric particles and clouds
      Pye, Havala O. T.; Nenes, Athanasios; Alexander, Becky; Ault, Andrew P.; Barth, Mary C.; Clegg, Simon L.; Collett, Jeffrey L., Jr.; Fahey, Kathleen M.; Hennigan, Christopher J.; Herrmann, Hartmut; Kanakidou, Maria; Kelly, James T.; Ku, I-Ting; McNeill, V. Faye; Riemer, Nicole; Schaefer, Thomas; Shi, Guoliang; Tilgner, Andreas; Walker, John T.; Wang, Tao; Weber, Rodney; Xing, Jia; Zaveri, Rahul A.; Zuend, Andreas
      Atmospheric Chemistry and Physics (2020), 20 (8), 4809-4888CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Acidity, defined as pH, is a central component of aq. chem. In the atm., the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and org. acids and bases as well as chem. reaction rates. It has implications for the atm. lifetime of pollutants, deposition, and human health. Despite its fundamental role in atm. processes, only recently has this field seen a growth in the no. of studies on particle acidity. Even with this growth, many fine-particle pH ests. must be based on thermodn. model calcns. since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH ests. are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively const. due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atm. condensed phases, specifically particles and cloud droplets. It includes recommendations for estg. acidity and pH, std. nomenclature, a synthesis of current pH ests. based on observations, and new model calcns. on the local and global scale.
      >> More from SciFinder ®
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      Jang, M.; Sun, S.; Winslow, R.; Han, S.; Yu, Z. In situ aerosol acidity measurements using a UV–Visible micro-spectrometer and its application to the ambient air. Aerosol Sci. Technol. 2020, 54, 446– 461,  DOI: 10.1080/02786826.2020.1711510
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      In situ aerosol acidity measurements using a UV-Visible micro-spectrometer and its application to the ambient air
      Jang, Myoseon; Sun, Shiqi; Winslow, Ryan; Han, Sanghee; Yu, Zechen
      Aerosol Science and Technology (2020), 54 (4), 446-461CODEN: ASTYDQ; ISSN:0278-6826. (Taylor & Francis, Inc.)
      An in situ anal. method was demonstrated to measure the proton concn. ([H+]C-RUV) of an aerosol particle by using colorimetry integrated with a Reflectance UV-Visible spectrometer (C-RUV). Acidic particles comprising ammonium, sulfate, and water were generated in a flow tube under varying humidity and employed to calibrate the method using the inorg. thermodn. models (i.e., E-AIM and ISORROPIA). The predictive [H+]C-RUV equation derived using strongly acidic compns. was then extended to ammonia-rich aerosols, which were lacking in the database of the thermodn. models. The predictive [H+]C-RUV equation was also expanded to aerosols composed of sodium, ammonium, and sulfate. [H+]C-RUV generally agrees with both E-AIM predicted [H+] and ISORROPIA predicted [H+] for highly acidic aerosols, or aerosols at high humidity. For ammonia-rich aerosols under low humidity, [H+]C-RUV disagrees with that predicted from inorg. thermodn. models. C-RUV was feasible for ambient aerosols because colorimetry is specific to aerosol acidity. Most aerosols collected at the University of Florida between 2018 and 2019 were acidic. Sodium ions appeared during the spring and summer, as coastal sea breezes traveled inland. The concns. of ammonium and nitrate were high in the winter due to the phase partitioning of nitric acid and ammonia gases. The fraction of non-electrolytic dialkyl-organosulfate (diOS) to total sulfate is estd. by comparing the actual particle [H+] measured by C-RUV to the [H+] predicted using the inorg. compn. and the inorg. thermodn. models. The diOS fraction varied from 0% to 60% and was higher in the summer months when [H+] is high.
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      Lei, Z.; Bliesner, S. E.; Mattson, C. N.; Cooke, M. E.; Olson, N. E.; Chibwe, K.; Albert, J.N.L.; Ault, A. P. Aerosol acidity sensing via polymer degradation. Anal. Chem. 2020, 92, 6502– 6511,  DOI: 10.1021/acs.analchem.9b05766
      4
      Aerosol Acidity Sensing via Polymer Degradation
      Lei, Ziying; Bliesner, Samuel E.; Mattson, Claire N.; Cooke, Madeline E.; Olson, Nicole E.; Chibwe, Kaseba; Albert, Julie N. L.; Ault, Andrew P.
      Analytical Chemistry (Washington, DC, United States) (2020), 92 (9), 6502-6511CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Atm. aerosol acidity is a crit. property affecting atm. chem. and compn. Key multi-phase chem. reactions are pH-dependent, impact processes such as secondary org. aerosol formation, and must be understood at a single particle level due to differences in particle-to-particle compn. which impact climate and health. The anal. challenge of measuring aerosol acidity of individual particles has limited pH measurements for fine (<2.5μm) and coarse (2.5-10μm) particles. This led to a reliance on indirect methods or thermodn. modeling, focusing on av., not individual, particle pH. Hence, new approaches are needed to probe single particle pH. A method to measure pH was assessed using degrdn. of a pH-sensitive polymer, poly(ε-caprolactone), PCL, to det. acidity of individual sub-micron particles. Sub-micron particles of known pH (0 or 6) were deposited on a polymer film (21-25 nm thick) and allowed to react. Particles were then rinsed off, and polymer degrdn. was characterized using at. force microscopy and Raman microspectroscopy. Following degrdn., holes in PCL films exposed to pH 0 were obsd., and loss of the carbonyl stretch was monitored at 1723 cm-1. As particle size decreased, polymer degrdn. increased, indicating an increase in aerosol acidity for smaller particle diams. This work describes an approach to det. individual particle acidity and moves toward addressing a key measurement gap related to understanding atm. aerosol impacts on climate and health.
      >> More from SciFinder ®
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    5. 5
      Wei, H.; Vejerano, E. P.; Leng, W.; Huang, Q.; Willner, M. R.; Marr, L. C.; Vikesland, P. J. Aerosol microdroplets exhibit a stable pH gradient. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 7272– 7277,  DOI: 10.1073/pnas.1720488115
      5
      Aerosol microdroplets exhibit a stable pH gradient
      Wei, Haoran; Vejerano, Eric P.; Leng, Weinan; Huang, Qishen; Willner, Marjorie R.; Marr, Linsey C.; Vikesland, Peter J.
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (28), 7272-7277CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Suspended aq. aerosol droplets (<50 μm) are microreactors for many important atm. reactions. In droplets and other aquatic environments, pH is arguably the key parameter dictating chem. and biol. processes. The nature of the droplet air/water interface has the potential to significantly alter droplet pH relative to bulk water. Historically, it has been challenging to measure the pH of individual droplets because of their inaccessibility to conventional pH probes. In this study, we scanned droplets contg. 4-mercaptobenzoic acid-functionalized gold nanoparticle pH nanoprobes by 2D and 3D laser confocal Raman microscopy. Using surface-enhanced Raman scattering, we acquired the pH distribution inside approx. 20-μm-diam. phosphate-buffered aerosol droplets and found that the pH in the core of a droplet is higher than that of bulk soln. by up to 3.6 pH units. This finding suggests the accumulation of protons at the air/water interface and is consistent with recent thermodn. model results. The existence of this pH shift was corroborated by the observation that a catalytic reaction that occurs only under basic conditions (i.e., dimerization of 4-aminothiophenol to produce dimercaptoazobenzene) occurs within the high pH core of a droplet, but not in bulk soln. Our nanoparticle probe enables pH quantification through the cross-section of an aerosol droplet, revealing a spatial gradient that has implications for acid-base-catalyzed atm. chem.
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    6. 6
      Hennigan, C. J.; Izumi, J.; Sullivan, A. P.; Weber, R. J.; Nenes, A. A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles. Atmos. Chem. Phys. 2015, 15, 2775– 2790,  DOI: 10.5194/acp-15-2775-2015
      6
      A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles
      Hennigan, C. J.; Izumi, J.; Sullivan, A. P.; Weber, R. J.; Nenes, A.
      Atmospheric Chemistry and Physics (2015), 15 (5), 2775-2790CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Given significant challenges with available measurements of aerosol acidity, proxy methods are frequently used to est. the acidity of atm. particles. In this study, four of the most common aerosol acidity proxies are evaluated and compared: (1) the ion balance method, (2) the molar ratio method, (3) thermodn. equil. models, and (4) the phase partitioning of ammonia. All methods are evaluated against predictions of thermodn. models and against direct observations of aerosol-gas equil. partitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratio methods assume that any deficit in inorg. cations relative to anions is due to the presence of H+ and that a higher H+ loading and lower cation/anion ratio both correspond to increasingly acidic particles (i.e., lower pH). Based on the MILAGRO measurements, no correlation is obsd. between H+ levels inferred with the ion balance and aerosol pH predicted by the thermodn. models and NH3-NH+4 partitioning. Similarly, no relationship is obsd. between the cation/anion molar ratio and predicted aerosol pH. Using only measured aerosol chem. compn. as inputs without any constraint for the gas phase, the E-AIM (Extended Aerosol Inorgs. Model) and ISORROPIA-II thermodn. equil. models tend to predict aerosol pH levels that are inconsistent with the obsd. NH3-NH+4 partitioning. The modeled pH values from both E-AIM and ISORROPIA-II run with gasCaerosol inputs agreed well with the aerosol pH predicted by the phase partitioning of ammonia. It appears that (1) thermodn. models constrained by gas + aerosol measurements and (2) the phase partitioning of ammonia provide the best available predictions of aerosol pH. Furthermore, neither the ion balance nor the molar ratio can be used as surrogates for aerosol pH, and previously published studies with conclusions based on such acidity proxies may need to be reevaluated. Given the significance of acidity for chem. processes in the atm., the implications of this study are important and far reaching.
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    7. 7
      Song, S. J.; Gao, M.; Xu, W. Q.; Shao, J. Y.; Shi, G. L.; Wang, S. X.; Wang, Y. X.; Sun, Y. L.; McElroy, M. B. Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models. Atmos. Chem. Phys. 2018, 18, 7423– 7438,  DOI: 10.5194/acp-18-7423-2018
      7
      Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models
      Song, Shaojie; Gao, Meng; Xu, Weiqi; Shao, Jingyuan; Shi, Guoliang; Wang, Shuxiao; Wang, Yuxuan; Sun, Yele; McElroy, Michael B.
      Atmospheric Chemistry and Physics (2018), 18 (10), 7423-7438CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      PH is an important property of aerosol particles but is difficult to measure directly. Several studies have estd. the pH values for fine particles in northern China winter haze using thermodn. models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calcd. pH values using these models with different assumptions with regard to model inputs and particle phase states.We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calcns. using only aerosol-phase compn. as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between -2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calcns. using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calcns. in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in detg. this property. The assumed particle phase state, either stable (solid plus liq.) or metastable (only liq.), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the std. ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study.
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    8. 8
      Guo, H.; Xu, L.; Bougiatioti, A.; Cerully, K. M.; Capps, S. L.; Hite, J. R.; Carlton, A. G.; Lee, S. H.; Bergin, M. H.; Ng, N. L.; Nenes, A.; Weber, R. J. Fine-particle water and pH in the southeastern United States. Atmos. Chem. Phys. 2015, 15, 5211– 5228,  DOI: 10.5194/acp-15-5211-2015
      8
      Fine-particle water and pH in the southeastern United States
      Guo, H.; Xu, L.; Bougiatioti, A.; Cerully, K. M.; Capps, S. L.; Hite, J. R., Jr.; Carlton, A. G.; Lee, S.-H.; Bergin, M. H.; Ng, N. L.; Nenes, A.; Weber, R. J.
      Atmospheric Chemistry and Physics (2015), 15 (9), 5211-5228CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Particle water and pH are predicted using meteorol. observations (relative humidity (RH), temp. (T)), gas/particle compn., and thermodn. modeling (ISORROPIA-II). A comprehensive uncertainty anal. is included, and the model is validated. We investigate mass concns. of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiol. (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aq. concn. and depends on both the presence of ions and amt. of particle liq. water. Levels of particle water, in turn, are detd. through water uptake by both the ionic species and org. compds. Thermodn. calcns. based on measured ion concns. can predict both pH and liq. water but may be biased since contributions of org. species to liq. water are not considered. In this study, contributions of both the inorg. and org. fractions to aerosol liq. water were considered, and predictions were in good agreement with measured liq. water based on differences in ambient and dry light scattering coeffs. (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg m-3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concns. (slope = 1.07, intercept = -0.12 μg m-3, R2 = 0.76). Based on this study, org. species on av. contributed 35% to the total water, with a substantially higher contribution (50 %) at night. However, not including contributions of org. water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of org. water could be sufficient for the purposes of aq. secondary org. aerosol (SOA) chem. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liq. water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas-aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissocn. state of individual ions or particle liq. water levels and do not correlate with particle pH.
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    9. 9
      Fountoukis, C.; Nenes, A. ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+–Ca2+–Mg2+–NH4+–Na+–SO42––NO3––Cl––H2O aerosols. Atmos. Chem. Phys. 2007, 7, 4639– 4659,  DOI: 10.5194/acp-7-4639-2007
      9
      ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosols
      Fountoukis, C.; Nenes, A.
      Atmospheric Chemistry and Physics (2007), 7 (17), 4639-4659CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
      This study presents ISORROPIA II, a thermodn. equil. model for the K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosol system. A comprehensive evaluation of its performance is conducted against water uptake measurements for lab. aerosol and predictions of the SCAPE2 thermodn. module over a wide range of atmospherically relevant conditions. The two models agree well, to within 13% for aerosol water content and total PM mass, 16% for aerosol nitrate and 6% for aerosol chloride and ammonium. Largest discrepancies were found under conditions of low RH, primarily from differences in the treatment of water uptake and solid state compn. In terms of computational speed, ISORROPIA II was more than an order of magnitude faster than SCAPE2, with robust and rapid convergence under all conditions. The addn. of crustal species does not slow down the thermodn. calcns. (compared to the older ISORROPIA code) because of optimizations in the activity coeff. calcn. algorithm. Based on its computational rigor and performance, ISORROPIA II appears to be a highly attractive alternative for use in large scale air quality and atm. transport models.
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    10. 10
      Song, S.; Nenes, A.; Gao, M.; Zhang, Y.; Liu, P. F.; Shao, J. Y.; Ye, D. C.; Xu, W. Q.; Lei, L.; Sun, Y. L.; Liu, B. X.; Wang, S. X.; McElroy, M. B. Thermodynamic modeling suggests declines in water uptake and acidity of inorganic aerosols in Beijing winter haze events during 2014/2015–2018/2019. Environ. Sci. Technol. Lett. 2019, 6, 752– 760,  DOI: 10.1021/acs.estlett.9b00621
      10
      Thermodynamic Modeling Suggests Declines in Water Uptake and Acidity of Inorganic Aerosols in Beijing Winter Haze Events during 2014/2015-2018/2019
      Song, Shaojie; Nenes, Athanasios; Gao, Meng; Zhang, Yuzhong; Liu, Pengfei; Shao, Jingyuan; Ye, Dechao; Xu, Weiqi; Lei, Lu; Sun, Yele; Liu, Baoxian; Wang, Shuxiao; McElroy, Michael B.
      Environmental Science & Technology Letters (2019), 6 (12), 752-760CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      In recent years, aggressive air pollution mitigation in northern China resulted in considerable changes in gas and aerosol chem. compn.; however, it is unclear whether aerosol water content and acidity responded to these changes. These two parameters have been shown to affect heterogeneous prodn. of winter haze aerosols. The authors performed thermodn. equil. modeling using chem. and meteorol. data in urban Beijing for four recent winter seasons and quantified changes of inorg. aerosols mass growth factor and pH. They focused on high relative humidity (>60%) conditions when sub-micron particles have been shown to be in a liq. state. From 2014/2015 to 2018/2019, modeled mass growth factor decreased ∼9-17% due to changes in aerosol compn. (more NO3-, less SO42- and Cl-); modeled pH increased ∼0.3-0.4 std. units, mainly due to increasing NH3. A buffer equation, derived from semi-volatile NH3 partitioning, helped understand the pH sensitivity to meteorol. and chem. variables. Results provide implications to evaluate potential chem. feedback in secondary aerosol prodn. and the effectiveness of NH3 control as a measure to alleviate winter haze.
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    11. 11
      Ibikunle, I.; Beyersdorf, A.; Campuzano-Jost, P.; Corr, C.; Crounse, J. D.; Dibb, J.; Diskin, G.; Huey, G.; Jimenez, J.-L.; Kim, M. J.; Nault, B. A.; Scheuer, E.; Teng, A.; Wennberg, P. O.; Anderson, B.; Crawford, J.; Weber, R.; Nenes, A. Fine particle pH and sensitivity to NH3 and HNO3 over summertime South Korea during KORUS-AQ. Atmos. Chem. Phys. Discuss. 2020, 1– 42,  DOI: 10.5194/acp-2020-501
      There is no corresponding record for this reference.
    12. 12
      Bougiatioti, A.; Nikolaou, P.; Stavroulas, I.; Kouvarakis, G.; Weber, R.; Nenes, A.; Kanakidou, M.; Mihalopoulos, N. Particle water and pH in the eastern Mediterranean: source variability and implications for nutrient availability. Atmos. Chem. Phys. 2016, 16, 4579– 4591,  DOI: 10.5194/acp-16-4579-2016
      12
      Particle water and pH in the eastern mediterranean: source variability and implications for nutrient availability
      Bougiatioti, Aikaterini; Nikolaou, Panayiota; Stavroulas, Iasonas; Kouvarakis, Giorgos; Weber, Rodney; Nenes, Athanasios; Kanakidou, Maria; Mihalopoulos, Nikolaos
      Atmospheric Chemistry and Physics (2016), 16 (7), 4579-4591CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Particle water (liq. water content, LWC) and aerosol pH are important parameters of the aerosol phase, affecting heterogeneous chem. and bioavailability of nutrients that profoundly impact cloud formation, atm. compn., and atm. fluxes of nutrients to ecosystems. Few measurements of in situ LWC and pH, however, exist in the published literature. Using concurrent measurements of aerosol chem. compn., cloud condensation nuclei activity, and tandem light scattering coeffs., the particle water mass concns. assocd. with the aerosol inorg. (Winorg) and org. (Worg) components are detd. for measurements conducted at the Finokalia atm. observation station in the eastern Mediterranean between June and Nov. 2012. These data are interpreted using the ISORROPIA-II thermodn. model to predict the pH of aerosols originating from the various sources that influence air quality in the region. On av., closure between predicted aerosol water and that detd. by comparison of ambient with dry light scattering coeffs. was achieved to within 8% (slope=0.92, R2 = 0.8, n = 5201 points). Based on the scattering measurements, a parameterization is also derived, capable of reproducing the hygroscopic growth factor (f(RH)) within 15% of the measured values. The highest aerosol water concns. are obsd. during night time, when relative humidity is highest and the collapse of the boundary layer increases the aerosol concn. A significant diurnal variability is found for Worg with morning and afternoon av. mass concns. being 10-15 times lower than night time concns., thus rendering Winorg the main form of particle water during daytime. The av. value of total aerosol water was 2.19±1.75 μgm-3, contributing on av. up to 33% of the total submicron mass concn. Av. aerosol water assocd. with orgs., Worg, was equal to 0.56±0.37 μgm-3; thus, orgs. contributed about 27.5% to the total aerosol water, mostly during early morning, late evening, and night time hours. The aerosol was found to be highly acidic with calcd. aerosol pH varying from 0.5 to 2.8 throughout the study period. Biomass burning aerosol presented the highest values of pH in the submicron fraction and the lowest values in total water mass concn. The low pH values obsd. in the submicron mode and independently of air mass origin could increase nutrient availability and esp. P soly., which is the nutrient limiting sea water productivity of the eastern Mediterranean.
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    13. 13
      Gilardoni, S.; Massoli, P.; Giulianelli, L.; Rinaldi, M.; Paglione, M.; Pollini, F.; Lanconelli, C.; Poluzzi, V.; Carbone, S.; Hillamo, R.; Russell, L. M.; Facchini, M. C.; Fuzzi, S. Fog scavenging of organic and inorganic aerosol in the Po Valley. Atmos. Chem. Phys. 2014, 14, 6967– 6981,  DOI: 10.5194/acp-14-6967-2014
      13
      Fog scavenging of organic and inorganic aerosol in the Po Valley
      Gilardoni, S.; Massoli, P.; Giulianelli, L.; Rinaldi, M.; Paglione, M.; Pollini, F.; Lanconelli, C.; Poluzzi, V.; Carbone, S.; Hillamo, R.; Russell, L. M.; Facchini, M. C.; Fuzzi, S.
      Atmospheric Chemistry and Physics (2014), 14 (13), 6967-6981, 15 pp.CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      The interaction of aerosol with atm. water affects the processing and wet removal of atm. particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol-water interaction at high relative humidity during fog events in the Po Valley within the framework of the Agenzia Regionale per la Prevenzione e l'Ambiente (ARPA) - Emilia Romagna supersite project. For the first time in this area, the changes in particle chem. compn. caused by fog are discussed along with changes in particle microphysics. During the expt., 14 fog events were obsd. The av. mass scavenging efficiency was 70% for nitrate, 68% for ammonium, 61% for sulfate, 50% for orgs., and 39% for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm Dva (vacuum aerodynamic diam.) and enriched in carbonaceous aerosol, mainly black carbon and water-insol. org. aerosol. For each fog event, the size-segregated scavenging efficiency of nitrate and org. aerosol (OA) was calcd. by comparing chem. species size distribution before and after fog formation. For both nitrate and OA, the size-segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was obsd., indicating that 44-51% of the variability of nitrate scavenging in smaller particles (below 300 nm Dva) was explained by changes in particle chem. compn. The size-segregated scavenging curves of OA followed those of nitrate, suggesting that org. scavenging was controlled by mixing with water-sol. species. In particular, functional group compn. and OA elemental anal. indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of org. functional group compn. during the expt. did not allow us to discriminate the effect of different org. functionalities on OA scavenging.
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    14. 14
      Young, A. H.; Keene, W. C.; Pszenny, A. A. P.; Sander, R.; Thornton, J. A.; Riedel, T. P.; Maben, J. R. Phase partitioning of soluble trace gases with size-resolved aerosols in near-surface continental air over northern Colorado, USA, during winter. J. Geophys. Res.: Atmos. 2013, 118, 9414– 9427,  DOI: 10.1002/jgrd.50655
      There is no corresponding record for this reference.
    15. 15
      Wang, G. H.; Zhang, R. Y.; Gomez, M. E.; Yang, L. X.; Zamora, M. L.; Hu, M.; Lin, Y.; Peng, J. F.; Guo, S.; Meng, J. J.; Li, J. J.; Cheng, C. L.; Hu, T. F.; Ren, Y. Q.; Wang, Y. S.; Gao, J.; Cao, J. J.; An, Z. S.; Zhou, W. J.; Li, G. H.; Wang, J. Y.; Tian, P. F.; Marrero-Ortiz, W.; Secrest, J.; Du, Z. F.; Zheng, J.; Shang, D. J.; Zeng, L. M.; Shao, M.; Wang, W. G.; Huang, Y.; Wang, Y.; Zhu, Y. J.; Li, Y. X.; Hu, J. X.; Pan, B.; Cai, L.; Cheng, Y. T.; Ji, Y. M.; Zhang, F.; Rosenfeld, D.; Liss, P. S.; Duce, R. A.; Kolb, C. E.; Molina, M. J. Persistent sulfate formation from London Fog to Chinese haze. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 13630– 13635,  DOI: 10.1073/pnas.1616540113
      15
      Persistent sulfate formation from London Fog to Chinese haze
      Wang, Gehui; Zhang, Renyi; Gomez, Mario E.; Yang, Lingxiao; Zamora, Misti Levy; Hu, Min; Lin, Yun; Peng, Jianfei; Guo, Song; Meng, Jingjing; Li, Jianjun; Cheng, Chunlei; Hu, Tafeng; Ren, Yanqin; Wang, Yuesi; Gao, Jian; Cao, Junji; An, Zhisheng; Zhou, Weijian; Li, Guohui; Wang, Jiayuan; Tian, Pengfei; Marrero-Ortiz, Wilmarie; Secrest, Jeremiah; Du, Zhuofei; Zheng, Jing; Shang, Dongjie; Zeng, Limin; Shao, Min; Wang, Weigang; Huang, Yao; Wang, Yuan; Zhu, Yujiao; Li, Yixin; Hu, Jiaxi; Pan, Bowen; Cai, Li; Cheng, Yuting; Ji, Yuemeng; Zhang, Fang; Rosenfeld, Daniel; Liss, Peter S.; Duce, Robert A.; Kolb, Charles E.; Molina, Mario J.
      Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (48), 13630-13635CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atm. models consistently underpredict sulfate levels under diverse environmental conditions. From atm. measurements in two Chinese megacities and complementary lab. expts., we show that the aq. oxidn. of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atm. conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidn. process leads to large sulfate prodn. rates and promotes formation of nitrate and org. matter on aq. particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addn. to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate prodn. mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.
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    16. 16
      Giulianelli, L.; Gilardoni, S.; Tarozzi, L.; Rinaldi, M.; Decesari, S.; Carbone, C.; Facchini, M.; Fuzzi, S. Fog occurrence and chemical composition in the Po valley over the last twenty years. Atmos. Environ. 2014, 98, 394– 401,  DOI: 10.1016/j.atmosenv.2014.08.080
      16
      Fog occurrence and chemical composition in the Po valley over the last twenty years
      Giulianelli, L.; Gilardoni, S.; Tarozzi, L.; Rinaldi, M.; Decesari, S.; Carbone, C.; Facchini, M. C.; Fuzzi, S.
      Atmospheric Environment (2014), 98 (), 394-401CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      Frequency of fog events together with fog water chem. compn., pH, cond. and liq. water content have systematically been measured from the end of the 1980's at the field station of San Pietro Capofiume, in the eastern Po Valley, Northern Italy. In agreement with what has been obsd. in other regions in Europe, fog frequency (visibility < 1 km) has decreased over the last three decades. Ionic strength and cond. of fog samples also decreased over the period indicating a redn. of the ionic load of the droplets. Specifically, the three major inorg. ions (NH+4, SO2-4, NO-3), accounting for 86% of the total fog water ionic strength, show a decreasing trend in concn. over the period, which can be linked to the decreasing trend of NH3, SO2 and NOx emissions registered in northern Italy over the same period. Sulfate exhibits the highest relative decrease (76%). Seasonal vol.-weighted means of pH show an increasing trend over the obsd. period. The available data of total water-sol. org. matter concns. indicate that org. compds. represent a considerable fraction (25% on av.) of the total solute mass of fog water. Fog water samples often contain suspended insol. particles, which were collected by filtering fog water through quartz fiber filters. EC-OC anal. performed on the filters collected over a four-year period, show that the sum of elemental carbon (EC) and water-insol. org. mass accounts on av. for 46%-56% of the total suspended material mass. Insol. carbonaceous material is composed mainly of org. matter, with EC accounting on av. only for 19% of the insol. carbon.
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    17. 17
      Fuzzi, S.; Orsi, G.; Mariotti, M. Radiation fog liquid water acidity at a field station in the Po Valley. J. Aerosol Sci. 1983, 14, 135– 138,  DOI: 10.1016/0021-8502(83)90037-X
      17
      Radiation fog liquid water acidity at a field station in the Po Valley
      Fuzzi, S.; Orsi, G.; Mariotti, M.
      Journal of Aerosol Science (1983), 14 (2), 135-8CODEN: JALSB7; ISSN:0021-8502.
      The problem of fog water acidity is examd. in this paper. The fog occurrence in some sites of the Po Valley (northern Italy) during the winter months is very high (∼30% of the time), so that the problem of fog acidity is very important in the degrdn. of materials, agriculture, and human health. The anal. of the pH trend of 3 individual fog events is presented. From these data it appears that, although pH follows the same trend of fog liq. water content, the evolving microphys. fog structure cannot alone account for pH variations during a fog event. The anal. of SO42-, NO3- and Cl- concn. in fog water by means of the ion chromatograph, suggests that SO2 and NOx oxidn. to H2SO4 and HNO3 is responsible for the excess acidity not accounted for by condensation nuclei chem. compn. and microphys. fog structure evolution (condensation/evapn. processes).
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    18. 18
      Fuzzi, S.; Orsi, G.; Mariotti, M. Wet deposition due to fog in the Po Valley, Italy. J. Atmos. Chem. 1985, 3, 289– 296,  DOI: 10.1007/BF00210501
      18
      Wet deposition due to fog in the Po Valley, Italy
      Fuzzi, Sandro; Orsi, Giordano; Mariotti, Mauro
      Journal of Atmospheric Chemistry (1985), 3 (2), 289-96CODEN: JATCE2; ISSN:0167-7764.
      Wet deposition due to radiation fog in the Po Valley is characterized by both a high fog occurrence during the fall-winter months and fog water solns. of high ionic concn. and acidity. Estd. wet deposition for NH4+, NO3-, and SO42- ions due to fog droplet settling to the ground accounts for 13.2, 12.1, and 5.3%, resp., with respect to bulk pptns. over the same period: Jan.-Mar. and Oct.-Dec. (fog season). Fog decompn. rates show that this process can be a important pathway of trace gases and particle loss from the air. First indicative results of fog removal efficiency with respect to air particulate matter are presented. Dry deposition parameters should be taken into account in evaluating the potential effect of fog droplet deposition on vegetation.
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    19. 19
      Fuzzi, S.; Orsi, G.; Nardini, G.; Facchini, M. C.; McLaren, S.; McLaren, E.; Mariotti, M. Heterogeneous processes in the Po Valley radiation fog. J. Geophys. Res.: Atmos. 1988, 93, 11141– 11151,  DOI: 10.1029/JD093iD09p11141
      There is no corresponding record for this reference.
    20. 20
      Winiwarter, W.; Puxbaum, H.; Fuzzi, S.; Facchini, M. C.; Orsi, G.; Beltz, N.; Enderle, K.; Jaeschke, W. Organic acid gas and liquid-phase measurements in Po Valley fall-winter conditions in the presence of fog. Tellus B 1988, 40B, 348– 357,  DOI: 10.1111/j.1600-0889.1988.tb00109.x
      20
      Organic acid gas and liquid-phase measurements in Po Valley fall-winter conditions in the presence of fog
      Winiwarter, Wilfried; Puxbaum, Hans; Fuzzi, Sandro; Facchini, Maria Cristina; Orsi, Giordano; Beltz, Norbert; Jaeschke, Wolfgang; Enderle, Karlheinz; Jaeschke, Wolfgang
      Tellus, Series B: Chemical and Physical Meteorology (1988), 40B (5), 348-57CODEN: TSBMD7; ISSN:0280-6509.
      Concurrent gas and liq. phase org. acid measurements during radiation fog episodes are reported. Gas phase concns. of HCO2H and AcOH exhibit large variations from the detection limit (5 nmol/m3, 0.1 ppb) to 150 nmol/m3 (3.5 ppb). Liq. phase concn. ranges of 11-175 and 10-269 μM were obsd. for HCO2H and AcOH, resp. Large deviations from Henry's law equil. were obsd. for HCO2H when the pH of fog droplets was >5. In this range, the bulk sample pH might not be representative of the acidity of individual droplets in equil. with the gas phase. The org. acids in the Po Valley under fall-winter conditions seem to originate from anthropogenic processes.
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    21. 21
      Fuzzi, S.; Orsi, G.; Bonforte, G.; Zardini, B.; Franchini, P. L. An automated fog water collector suitable for deposition networks: design, operation and field tests. Water, Air, Soil Pollut. 1997, 93, 383– 394,  DOI: 10.1007/BF02404768
      21
      An automated fog water collector suitable for deposition networks: design, operation and field tests
      Fuzzi, Sandro; Orsi, Giordano; Bonforte, Giuseppe; Zardini, Bruno; Franchini, Pier Luigi
      Water, Air, and Soil Pollution (1997), 93 (1-4), 383-394CODEN: WAPLAC; ISSN:0049-6979. (Kluwer)
      The study of fog water chem. compn. and the contribution of fog droplets of total chem. deposition has become a relevant environmental subject over the past few years. This paper describes a fog water collector suitable for deposition network operation, due to its complete automation and to the facility of remote acquisition of sampling information. Sampling of fog droplets on teflon strings is activated by an optical fog detector according to a particular protocol operated by a microprocessor controller. Multiple sample collection, also microprocessor controlled, is possible with this instrument. The problem of fog droplet sampling in sub-freezing conditions is overcome using a sampling schedule implemented by the microprocessor controller which alternates between sampling periods and stand-by periods during which melting of the rime collected on the strings is allowed. Field tests on the reliability and reproducibility of the sampling operations are presented in the paper. Side by side operation of the fog collector with a PVM-100 fog liq. water content meter shows that the amt. of water per unit vol. of air collected by the sampling instrument is proportional to the fog liq. water content averaged over the period of an entire fog event.
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    22. 22
      Tomasi, C.; Tampieri, F. Features of the proportionality coefficient in the relationship between visibility and liquid water content in haze and fog. Atmosphere 1976, 61– 76,  DOI: 10.1080/00046973.1976.9648403
      There is no corresponding record for this reference.
    23. 23
      Matta, E.; Facchini, M. C.; Decesari, S.; Mircea, M.; Cavalli, F.; Fuzzi, S.; Putaud, J. P.; Dell’Acqua, A. Mass closure on the chemical species in size-segregated atmospheric aerosol collected in an urban area of the Po Valley, Italy. Atmos. Chem. Phys. 2003, 3, 623– 637,  DOI: 10.5194/acp-3-623-2003
      23
      Mass closure on the chemical species in size-segregated atmospheric aerosol collected in an urban area of the Po Valley, Italy
      Matta, E.; Facchini, M. C.; Decesari, S.; Mircea, M.; Cavalli, F.; Fuzzi, S.; Putaud, J. -P.; Dell'Acqua, A.
      Atmospheric Chemistry and Physics (2003), 3 (3), 623-637CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
      A complete size segregated chem. characterization was carried out for aerosol samples collected in the urban area of Bologna over a period of one year, using five-stage low pressure Berner impactors. An original dual-substrate technique was adopted to obtain samples suitable for a complete chem. characterization. Total mass, inorg., and org. components were analyzed as a function of size, and a detailed characterization of the water sol. org. compds. was also performed by means of a previously developed methodol., based on HPLC sepn. of org. compds. according to their acid character and functional group anal. by proton NMR. Chem. mass closure of the collected samples was reached to within a few percent on av. in the submicron aerosol range, while a higher unknown fraction in the coarse aerosol range was attributed to soil-derived species not analyzed in this expt. Comparison of the functional group anal. results with model results simulating water sol. org. compd. prodn. by gas-to-particle conversion of anthropogenic VOCs showed that this pathway provides a minor contribution to the org. compn. of the aerosol samples in the urban area of Bologna.
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    24. 24
      Ricciardelli, I.; Bacco, D.; Rinaldi, M.; Bonafe, G.; Scotto, F.; Trentini, A.; Bertacci, G.; Ugolini, P.; Zigola, C.; Rovere, F.; Maccone, C.; Pironi, C.; Poluzzi, V. A three-year investigation of daily PM2:5 main chemical components in four sites: the routine measurement program of the Supersito Project (Po Valley, Italy). Atmos. Environ. 2017, 152, 418– 430,  DOI: 10.1016/j.atmosenv.2016.12.052
      24
      A three-year investigation of daily PM2.5 main chemical components in four sites: the routine measurement program of the Supersito Project (Po Valley, Italy)
      Ricciardelli, Isabella; Bacco, Dimitri; Rinaldi, Matteo; Bonafe, Giovanni; Scotto, Fabiana; Trentini, Arianna; Bertacci, Giulia; Ugolini, Pamela; Zigola, Claudia; Rovere, Flavio; Maccone, Claudio; Pironi, Claudia; Poluzzi, Vanes
      Atmospheric Environment (2017), 152 (), 418-430CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
      The Supersito Project (www.supersito-er.it) has been active in the Emilia-Romagna region, southern part of the Po Valley (Italy), since 2011. Focal aim of the project is to enhance the knowledge on atm. aerosol and its impact on human health. In the framework of Supersito, major chem. components of daily PM2.5 were investigated over a period of more than three years at four sampling sites, representative of dissimilar territorial conditions: one rural background (SPC) and three urban background sites in the coastal (RN), central (MS) and inner area (PR) of the region. In all the sites, org. and elemental carbon and water sol. inorg. ions accounted for more than 70% of PM2.5 mass, during all seasons. Nitrate and org. carbon (OC) were the main components of winter PM2.5, while summer aerosol was mainly contributed by OC and sulfate. OC was dominated by primary sources, with a potentially important contribution from biomass burning, in winter, while secondary processes dominated OC prodn. in summer. A substantial homogeneity was obsd. on a regional scale in terms of spatial distribution of pollutants, with EC only presenting significant differences between urban and rural areas during winters. Nonetheless, differences were obsd. between the coastal and the inner part of the region, with the former being systematically characterized by higher concns. of carbonaceous compds. and lower concns. of ammonium nitrate. The coastal area was likely influenced by the aged OC from the Po Valley outflow in addn. to local sources, while the scarcity of local sources of ammonia limited the formation of ammonium nitrate. In the studied area, local and regional meteorol. - mostly governed by geog. collocation and orog. - was responsible for PM2.5 mass and compn. no less than local and regional emission sources.
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    25. 25
      Guo, H.; Sullivan, A. P.; Campuzano-Jost, P.; Schroder, J. C.; Lopez-Hilfiker, F. D.; Dibb, J. E.; Jimenez, J. L.; Thornton, J. A.; Brown, S. S.; Nenes, A.; Weber, R. J. Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States. J. Geophys. Res.: Atmos. 2016, 121, 10355– 10376,  DOI: 10.1002/2016jd025311
      25
      Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States
      Guo, Hongyu; Sullivan, Amy P.; Campuzano-Jost, Pedro; Schroder, Jason C.; Lopez-Hilfiker, Felipe D.; Dibb, Jack E.; Jimenez, Jose L.; Thornton, Joel A.; Brown, Steven S.; Nenes, Athanasios; Weber, Rodney J.
      Journal of Geophysical Research: Atmospheres (2016), 121 (17), 10355-10376CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
      Particle pH is a crit. but poorly constrained quantity that affects many aerosol processes and properties, including aerosol compn., concns., and toxicity. We assess PM1 pH as a function of geog. location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 Feb. to 15 March 2015). Particle pH and water were predicted with the ISORROPIA-II thermodn. model and validated by comparing predicted to obsd. partitioning of inorg. nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH obsd. particle nitrate was higher than predicted, possibly due to org.-inorg. phase sepns. or nitrate measurement uncertainties assocd. with low concns. (nitrate < 1 μg m-3). Including refractory ions in the pH calcns. did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between -0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern U.S. and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH-dependent aerosol components, such as PM1 nitrate, over a wide range of temps. (-21 to 19°C), RH (20 to 95%), inorg. gas, and particle concns. and also provides further evidence that particles with low pH are ubiquitous.
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    26. 26
      Guo, H.; Liu, J.; Froyd, K. D.; Roberts, J. M.; Veres, P. R.; Hayes, P. L.; Jimenez, J. L.; Nenes, A.; Weber, R. J. Fine particle pH and gas–particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign. Atmos. Chem. Phys. 2017, 17, 5703– 5719,  DOI: 10.5194/acp-17-5703-2017
      26
      Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign
      Guo, Hongyu; Liu, Jiumeng; Froyd, Karl D.; Roberts, James M.; Veres, Patrick R.; Hayes, Patrick L.; Jimenez, Jose L.; Nenes, Athanasios; Weber, Rodney J.
      Atmospheric Chemistry and Physics (2017), 17 (9), 5703-5719CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      PH is a fundamental aerosol property that affects ambient particle concn. and compn., linking pH to all aerosol environmental impacts. Here, PM1 and PM2.5 pH are calcd. based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodn. model and validated by comparing predicted to measured gas-particle partitioning of inorg. nitrate, ammonium, and chloride. The study mean ± std. deviation PM1 pH was 1.9 ± 0.5 for the SO2-4-NO-3-NH+4-HNO3-NH3 system. For PM2.5, internal mixing of sea salt components (SO2-4-NO-3-NH+4-Na+-Cl--K+-HNO3-NH3-HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2.5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2.5 pH. A mean PM1 pH of 1.9 at Pasadena was approx. one unit higher than what we have reported in the southeastern US, despite similar temp., relative humidity, and sulfate ranges, and is due to higher total nitrate concns. (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concns. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by diln., further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.
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    27. 27
      Guo, H.; Otjes, R.; Schlag, P.; Kiendler-Scharr, A.; Nenes, A.; Weber, R. J. Effectiveness of ammonia reduction on control of fine particle nitrate. Atmos. Chem. Phys. 2018, 18, 12241– 12256,  DOI: 10.5194/acp-18-12241-2018
      27
      Effectiveness of ammonia reduction on control of fine particle nitrate
      Guo, Hongyu; Otjes, Rene; Schlag, Patrick; Kiendler-Scharr, Astrid; Nenes, Athanasios; Weber, Rodney J.
      Atmospheric Chemistry and Physics (2018), 18 (16), 12241-12256CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      In some regions, reducing aerosol ammonium nitrate (NH4NO3) concns. may substantially improve air quality. This can be accomplished by redns. in precursor emissions, such as nitrogen oxides (NOx) to lower nitric acid (HNO3) that partitions to the aerosol, or redns. in ammonia (NH3) to lower particle pH and keep HNO3 in the gas phase. Using the ISORROPIA-II thermodn. aerosol model and detailed observational data sets, we explore the sensitivity of aerosol NH4NO3 to gas-phase NH3 and NOx controls for a no. of contrasting locations, including Europe, the United States, and China. NOx control is always effective, whereas the aerosol response to NH3 control is highly nonlinear and only becomes effective at a thermodn. sweet spot. The anal. provides a conceptual framework and fundamental evaluation on the relative value of NOx vs. NH3 control and demonstrates the relevance of pH as an air quality parameter. We find that, regardless of the locations examd., it is only when ambient particle pH drops below an approx. crit. value of 3 (slightly higher in warm and slightly lower in cold seasons) that NH3 redn. leads to an effective response in PM2.5 mass. The required amt. of NH3 redn. to reach the crit. pH and efficiently decrease NH4NO3 at different sites is assessed. Owing to the linkage between NH3 emissions and agricultural productivity, the substantial NH3 redn. required in some locations may not be feasible. Finally, controlling NH3 emissions to increase aerosol acidity and evap. NH4NO3 will have other effects, beyond redn. of PM2.5 NH4NO3, such as increasing aerosol toxicity and potentially altering the deposition patterns of nitrogen and trace nutrients.
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    28. 28
      Vasilakos, P.; Russell, A.; Weber, R.; Nenes, A. Understanding nitrate formation in a world with less sulfate. Atmos. Chem. Phys. 2018, 18, 12765– 12775,  DOI: 10.5194/acp-18-12765-2018
      28
      Understanding nitrate formation in a world with less sulfate
      Vasilakos, Petros; Russell, Armistead; Weber, Rodney; Nenes, Athanasios
      Atmospheric Chemistry and Physics (2018), 18 (17), 12765-12775CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      SO2 emission controls, combined with modestly increasing ammonia, have been thought to generate aerosol with significantly reduced acidity for cases in which sulfate is partially substituted by nitrate. However, neither expectation agrees with decadal observations in the southeastern USA, suggesting that a fundamentally different response of aerosol pH to emissions changes is occurring. We postulate that this nitrate substitution paradox arises from a pos. bias in aerosol pH in model simulations. This bias can elevate pH to a level at which nitrate partitioning is readily promoted, leading to behavior consistent with nitrate substitution. CMAQ simulations are used to investigate this hypothesis; modeled PM2.5 pH using 2001 emissions compare favorably with pH inferred from obsd. species concns. Using 2011 emissions, however, leads to simulated pH increases of one unit, which is inconsistent with observations from that year. Nonvolatile cations (K+, Na+, Ca+2, and Mg+2) in the fine mode are found to be responsible for the erroneous predicted increase in aerosol pH of about 1 unit on av. over the USA. Such an increase can induce a nitrate bias of 1-2 μg m-3, which may further increase in future projections, reaffirming an otherwise incorrect expectation of a significant nitrate substitution. Evaluation of predicted aerosol pH against thermodn. anal. of observations is therefore a critically important, but overlooked, aspect of model evaluation for a robust emissions policy.
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    29. 29
      Battaglia, M. A., Jr.; Weber, R. J.; Nenes, A.; Hennigan, C. J. Effects of water-soluble organic carbon on aerosol pH. Atmos. Chem. Phys. 2019, 19, 14607– 14620,  DOI: 10.5194/acp-19-14607-2019
      29
      Effects of water-soluble organic carbon on aerosol pH
      Battaglia, Michael A.; Weber, Rodney J.; Nenes, Athanasios; Hennigan, Christopher J.
      Atmospheric Chemistry and Physics (2019), 19 (23), 14607-14620CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Water-sol. org. carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter. Despite advances in aerosol thermodn. equil. models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH). We address this limitation by studying submicron aerosols that represent the two extremes in acidity levels found in the atm.: strongly acidic aerosol from Baltimore, MD, and weakly acidic conditions characteristic of Beijing, China. These cases are then used to construct mixed inorg.-org. single-phase aq. particles and thermodynamically analyzed by the Extended Aerosol Inorgs. Model (E-AIM) and ISORROPIA models in combination with activity coeff. model AIOMFAC (Aerosol Inorg.-Org. Mixts. Functional groups Activity Coeff.) to evaluate the effects of WSOC on the H+ ion activity coeffs. (γ H+) and activity (pH). We find that addn. of org. acids and nonacid org. species concurrently increases γ H+ and aerosol liq. water. Under the highly acidic conditions typical of the eastern US (inorg.-only pH ∼ 1), these effects mostly offset each other, giving pH changes of < 0.5 pH units even at org. aerosol dry mass fractions in excess of 60%. Under conditions with weaker acidity typical of Beijing (inorg.-only pH ∼ 4:5), the nonacidic WSOC compds. had similarly minor effects on aerosol pH, but org. acids imparted the largest changes in pH compared to the inorg.-only simulations. Org. acids affect pH in the order of their pKa values (oxalic acid > malonic acid > glutaric acid). Although the inorg.-only pH was above the pKa value of all three org. acids investigated, pH changes in excess of 1 pH unit were only obsd. at unrealistic org. acid levels (aerosol org. acid concns. > 35μgm-3) in Beijing. The model simulations were run at 70%, 80%, and 90% relative humidity (RH) levels and the effect of WSOC was inversely related to RH. At 90% RH, WSOC altered aerosol pH by up to ∼ 0:2 pH units, though the effect was up to ∼ 0:6 pH units at 70% RH. The somewhat offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorg. constituents alone under conditions where liq.-liq. phase sepn. is not anticipated to occur.
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    30. 30
      Facchini, M. C.; Fuzzi, S.; Zappoli, S.; Andracchio, A.; Gelencsér, A.; Kiss, G.; Krivácsy, Z.; Mészáros, E.; Hansson, H.-C.; Alsberg, T.; Zebühr, Y. Partitioning of the organic aerosol component between fog droplets and interstitial air. J. Geophys. Res.: Atmos. 1999, 104, 26821– 26832,  DOI: 10.1029/1999JD900349
      30
      Partitioning of the organic aerosol component between fog droplets and interstitial air
      Facchini, Maria Cristina; Fuzzi, Sandro; Zappoli, Sergio; Andracchio, Antonella; Gelencser, Andras; Kiss, Gyula; Krivacsy, Zoltan; Meszaros, Erno; Hansson, Hans-Christen; Alsberg, Tomas; Zebuhr, Yngve
      Journal of Geophysical Research, [Atmospheres] (1999), 104 (D21), 26821-26832CODEN: JGRDE3 ISSN:. (American Geophysical Union)
      Limited information is available on the nature of org. compds. in tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nucleation (CCN) ability. Here we analyze samples of liq. droplets and interstitial aerosol, concurrently collected during fog episodes, to det. how the org. compds. are partitioned between the two reservoirs. By comparing the nature and concn. of different org. carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon species on the basis of their chem. properties, with polar water-sol. species representing the greater part of total C within fog droplets, and water-insol. C species preferentially found in the interstitial reservoir. Water-sol. org. species are scavenged by fog droplets to a extent comparable to major inorg. ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water-sol. org. carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol anal. are aliph. dicarboxylic acids, sugars, aliph. alcs., and aliph. carboxylic acids. However, such species, ∼120 individual compds., only account for a few percent (<5% on av.) of total WSOC. A new class of water-sol. macromol. compds. (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (∼40% on av.) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water-sol. species identified. More than 50% of WSOC still remains undetd.
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    31. 31
      Pandis, S. N.; Seinfeld, J. H. Should bulk cloudwater samplesobey Henry’s law?. J. Geophys. Res. 1991, 96, 10791– 10798,  DOI: 10.1029/91JD01031
      There is no corresponding record for this reference.
    32. 32
      Ricci, L.; Fuzzi, S.; Laj, P.; Lazzari, A.; Orsi, G.; Berner, A.; Günther, A.; Jaeschke, W.; Wendisch, M.; Arends, B. Gas-liquid equilibria in polluted fog. Contrib. Atmos. Phys. 1998, 71, 159
      There is no corresponding record for this reference.
    33. 33
      Fountoukis, C.; Nenes, A.; Sullivan, A.; Weber, R.; Van Reken, T.; Fischer, M.; Matías, E.; Moya, M.; Farmer, D.; Cohen, R. C. Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006. Atmos. Chem. Phys. 2009, 9, 2141– 2156,  DOI: 10.5194/acp-9-2141-2009
      33
      Thermodynamic characterization of Mexico city aerosol during MILAGRO 2006
      Fountoukis, C.; Nenes, A.; Sullivan, A.; Weber, R.; Van Reken, T.; Fischer, M.; Matias, E.; Moya, M.; Farmer, D.; Cohen, R. C.
      Atmospheric Chemistry and Physics (2009), 9 (6), 2141-2156CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
      Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodn. equil. model are used to study the partitioning of semivolatile inorg. species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. For particle sizes up to 1 μm diam., semi-volatile partitioning requires 15-30 min to equilibrate; longer time is typically required during the night and early morning hours. Aerosol and gas-phase speciation always exhibits substantial temporal variability, so that aerosol compn. measurements (bulk or size-resolved) obtained over large integration periods are not reflective of its true state. When the aerosol sulfate-to-nitrate molar ratio is less than unity, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equiv. sodium" (rather than explicitly) in the thermodn. equil. calcns. introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramatically. This suggests that explicitly considering crustals in the thermodn. calcns. is required to accurately predict the partitioning and phase state of aerosols.
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    34. 34
      Hess, A.; Iyer, H.; Malm, W. Linear trend analysis: a comparison of methods. Atmos. Environ. 2001, 35, 5211– 5222,  DOI: 10.1016/S1352-2310(01)00342-9
      34
      Linear trend analysis: a comparison of methods
      Hess, A.; Iyer, H.; Malm, W.
      Atmospheric Environment (2001), 35 (30), 5211-5222CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
      In this paper, we present an overview of statistical approaches available for detecting and estg. linear trends in environmental data. We evaluate seven methods of trend detection and make recommendations based on a simulation study. We also illustrate the methods using real data.
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    35. 35
      Rosenfeld, D.; Zhu, Y.; Wang, M.; Zheng, Y.; Goren, T.; Yu, S. Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds. Science 2019, 363, eaav0566  DOI: 10.1126/science.aav0566
      There is no corresponding record for this reference.
    36. 36
      Paglione, M.; Gilardoni, S.; Rinaldi, M.; Decesari, S.; Zanca, N.; Sandrini, S.; Giulianelli, L.; Bacco, D.; Ferrari, S.; Poluzzi, V.; Scotto, F.; Trentini, A.; Poulain, L.; Herrmann, H.; Wiedensohler, A.; Canonaco, F.; Prévôt, A. S. H.; Massoli, P.; Carbone, C.; Facchini, M. C.; Fuzzi, S. The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy. Atmos. Chem. Phys. 2020, 20, 1233– 1254,  DOI: 10.5194/acp-20-1233-2020
      36
      The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy
      Paglione, Marco; Gilardoni, Stefania; Rinaldi, Matteo; Decesari, Stefano; Zanca, Nicola; Sandrini, Silvia; Giulianelli, Lara; Bacco, Dimitri; Ferrari, Silvia; Poluzzi, Vanes; Scotto, Fabiana; Trentini, Arianna; Poulain, Laurent; Herrmann, Hartmut; Wiedensohler, Alfred; Canonaco, Francesco; Prevot, Andre S. H.; Massoli, Paola; Carbone, Claudio; Facchini, Maria Cristina; Fuzzi, Sandro
      Atmospheric Chemistry and Physics (2020), 20 (3), 1233-1254CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      The Po Valley (Italy) is a well-known air quality hotspot characterized by particulate matter (PM) levels well above the limit set by the European Air Quality Directive and by the World Health Organization, esp. during the colder season. In the framework of Emilia-Romagna regional project "Supersito", the southern Po Valley submicron aerosol chem. compn. was characterized by means of high-resoln. aerosol mass spectroscopy (HR-AMS) with the specific aim of org. aerosol (OA) characterization and source apportionment. Eight intensive observation periods (IOPs) were carried out over 4 years (from 2011 to 2014) at two different sites (Bologna, BO, urban background, and San Pietro Capofiume, SPC, rural background), to characterize the spatial variability and seasonality of the OA sources, with a special focus on the cold season. On the multi-year basis of the study, the AMS observations show that OA accounts for avs. of 45 ± 8% (ranging from 33% to 58%) and 46 ± 7% (ranging from 36% to 50%) of the total non-refractory submicron particle mass (PM1-NR) at the urban and rural sites, resp. Primary org. aerosol (POA) comprises biomass burning (23±13% of OA) and fossil fuel (12±7%) contributions with a marked seasonality in concn. As expected, the biomass burning contribution to POA is more significant at the rural site (urban / rural concn. ratio of 0.67), but it is also an important source of POA at the urban site during the cold season, with contributions ranging from 14% to 38% of the total OA mass. Secondary org. aerosol (SOA) contributes to OA mass to a much larger extent than POA at both sites throughout the year (69 ± 16% and 83 ± 16% at the urban and rural sites, resp.), with important implications for public health. Within the secondary fraction of OA, the measurements highlight the importance of biomass burning aging products during the cold season, even at the urban background site. This biomass burning SOA fraction represents 14%-44% of the total OA mass in the cold season, indicating that in this region a major contribution of combustion sources to PM mass is mediated by environmental conditions and atm. reactivity. Among the environmental factors controlling the formation of SOA in the Po Valley, the availability of liq. water in the aerosol was shown to play a key role in the cold season. We est. that the org. fraction originating from aq. reactions of biomass burning products ("bb-aqSOA") represents 21% (14%-28%) and 25% (14%-35%) of the total OA mass and 44% (32%-56%) and 61% (21%- 100%) of the SOA mass at the urban and rural sites, resp.
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    37. 37
      Nenes, A.; Pandis, S. N.; Weber, R. J.; Russell, A. Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability. Atmos. Chem. Phys. 2020, 20, 3249– 3258,  DOI: 10.5194/acp-20-3249-2020
      37
      Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability
      Nenes, Athanasios; Pandis, Spyros N.; Weber, Rodney J.; Russell, Armistead
      Atmospheric Chemistry and Physics (2020), 20 (5), 3249-3258CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concns. worldwide. The response of PM to changes in the emissions of both compds. is typically studied on a case-by-case basis, owing in part to the complex thermodn. interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chem. domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liq. water content. From our anal., four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the "chem. regime" of PM sensitivity to NH3 and HNO3 availability is directly detd. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chem. transport models. With this extended understanding, aerosol pH and assocd. liq. water content naturally emerge as previously ignored state parameters that drive PM formation.
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    38. 38
      Nenes, A.; Pandis, S. N.; Kanakidou, M.; Russell, A.; Song, S.; Vasilakos, P.; Weber, R. J. Aerosol acidity and liquid water content regulate the dry deposition of inorganic reactive nitrogen. Atmos. Chem. Phys. Discuss. 2020, 1– 25,  DOI: 10.5194/acp-2020-266
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    • Deeper descriptions of the modeling methods (e.g., estimation of gaseous ammonia from fog water composition; NH3 estimation effect on the aerosol pH variability and RH sensitivity test); statistical analysis of the pH trend; details on the trends of pollutants and meteorological parameters; details on the results of the multiple linear regression analysis to study the drivers of aerosol pH reduction (PDF)


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