Pathway For Sulfate Formation Research Articles

Can we reach consensus on the dominant sulfate formation pathway in China's haze?

Atmospheric sulfate aerosols contribute significantly to air pollution and climate change. Sulfate formation mechanisms during winter haze events in northern China have recently received considerable attention, with more than 10 studies published in high-impact journals. However, the conclusions from in-field measurements, laboratory studies, and numerical simulations are inconsistent and even contradictory. Here, we propose a physically based yet simple method to clarify the debate on the dominant sulfate formation pathway. Based on the hazes evolving in the synoptic scale, first, a characteristic sulfate formation rate is derived using the Eulerian mass conservation equation constrained by in situ observations. Then, this characteristic value is treated as a guideline to determine the dominant sulfate formation pathway with a 0D chemical box model. Our observation-derived results establish a linkage between studies from laboratory experiments and chemical transport model simulations. A convergent understanding could therefore be reached on sulfate formation mechanisms in China's wintertime haze. This method is universal and can be applied to various haze conditions and different secondary products.

Detection and quantification of organosulfur species in the Tagish Lake Meteorite by highly sensitive LC‐MS

AbstractWe analyzed the methanol extracts of six pristine specimens of the Tagish Lake meteorite (TL1, TL4, TL5A, TL6, TL7, and TL10a) and heated and unheated samples of Allende using high‐performance liquid chromatography coupled with high‐resolution, accurate mass–mass spectrometry (HPLC‐HRAM‐MS). All samples contained ppm levels of sulfate and methyl sulfate. The most abundant organosulfur compound in the methanol extracts of the Tagish Lake and Allende samples was methyl sulfate, which was likely formed primarily via an esterification reaction between intrinsic sources of methanol and sulfate. A homologous series of polythionic acids was also observed in the extracts of the Tagish Lake specimens and Allende. The polythionic acids were the most abundant soluble inorganic sulfur species found in the meteorites. Our results were confirmed using retention time, accurate mass, isotope matching, and tandem mass spectrometry (MS/MS). Hydroxymethanesulfonic acid, previously reported in Tagish Lake, was found only in an unheated Allende sample and in low abundance. Here, we propose possible sulfate formation pathways that begin with interstellar dimethyl sulfide, dimethyl disulfide, methyl sulfide, or methanethiol via cold, nebular processes within the interstellar medium and continue via MSA as an intermediary compound ending within planetary bodies with sulfate and methyl sulfate as the final products.

Measurements of particulate methanesulfonic acid above the remote Arctic Ocean using a high resolution aerosol mass spectrometer

Methanesulfonic acid (MSA) is an important product from the oxidation of dimethyl sulfide (DMS), and thus is often used as a tracer for marine biogenic sources and secondary organic aerosol. MSA also contributes to aerosol mass and potentially to the formation of cloud condensation nuclei and new particles. However, measurements of MSA at high temporal resolution in the remote Arctic are scarce, which limits our understanding of its formation, climate change impact and regional transport. Here, we applied a validated quantification method to determine the mass concentration of MSA and non-sea salt sulfate (nss-SO4) in PM2.5 in the marine boundary layer, using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) during a research cruise to the Arctic and North Atlantic Ocean, between 55 °N and 68 °N (26th May to June 23, 2022). With this method, the concentrations of MSA in the remote Arctic marine boundary layer were determined for the first time. Results show that the average MSA concentration was 0.025 ± 0.03 μg m−3, ranging from <0.01 to 0.32 μg m−3. The lowest MSA level was found towards the northern leg of the cruise (near Sisimut (67 °N)) with air masses from sea ice over the northern polar region, and the highest MSA concentrations were observed over the Atlantic open ocean. The diurnal cycles of gas MSA, particulate MSA and nss-SO4 peaked in the afternoon, about one hour later than that of peak of solar radiation, which suggests that photochemical process is an important mechanism for the conversion of DMS into MSA above the remote ocean. The mass ratio of MSA to nss-SO4 (MSA/nss-SO4) presents a temperature dependence, which indicates that the addition branching pathway favors MSA formation, while thermal decay of intermediate radicals could be a possible pathway for sulfate formation. Finally, we found that the MSA/nss-SO4 ratio is around 0.22-0.25 in the remote northern marine atmosphere.

Quantifying SO2 oxidation pathways to atmospheric sulfate using stable sulfur and oxygen isotopes: laboratory simulation and field observation

Abstract. The formation of secondary sulfate in the atmosphere remains controversial, and it is an urgent need to seek a new method to quantify different sulfate formation pathways. Thus, SO2 and PM2.5 samples were collected from 4 to 22 December 2019 in the Nanjing region. Sulfur and oxygen isotopic compositions were synchronously measured to study the contribution of SO2 homogeneous and heterogeneous oxidation to sulfate. Meanwhile, the correlation of δ18O values between H2O and sulfate from SO2 oxidation by H2O2 and Fe3+ / O2 was simulatively investigated in the laboratory. Based on isotope mass equilibrium equations, the ratios of different SO2 oxidation pathways were quantified. The results showed that secondary sulfate constituted higher than 80 % of total sulfate in PM2.5 during the sampling period. Laboratory simulation experiments indicated that the δ18O value of sulfate was linearly dependent on the δ18O value of water, and the slopes of linear curves for SO2 oxidation by H2O2 and Fe3+ / O2 were 0.43 and 0.65, respectively. The secondary sulfate in PM2.5 was mainly ascribed to SO2 homogeneous oxidation by OH radicals and heterogeneous oxidation by H2O2 and Fe3+ / O2. SO2 heterogeneous oxidation was generally dominant during sulfate formation, and SO2 oxidation by H2O2 predominated in SO2 heterogeneous oxidation reactions, with an average ratio around 54.6 %. This study provided an insight into precisely evaluating sulfate formation by combining stable sulfur and oxygen isotopes.

Primary Sulfate Is the Dominant Source of Particulate Sulfate during Winter in Fairbanks, Alaska.

Within and surrounding high-latitude cities, poor air quality disturbs Arctic ecosystems, influences the climate, and harms human health. The Fairbanks North Star Borough has wintertime particulate matter (PM) concentrations that exceed the Environmental Protection Agency's (EPA) threshold for public health. Particulate sulfate (SO4 2-) is the most abundant inorganic species and contributes approximately 20% of the total PM mass in Fairbanks, but air quality models underestimate observed sulfate concentrations. Here we quantify sulfate sources using size-resolved δ34S(SO4 2-), δ18O(SO4 2-), and Δ17O(SO4 2-) of particulate sulfate in Fairbanks from January 18th to February 25th, 2022 using a Bayesian isotope mixing model. Primary sulfate contributes 62 ± 12% of the total sulfate mass on average. Most primary sulfate is found in the size bin with a particle diameter < 0.7 μm, which contains 90 ±5% of total sulfate mass and poses the greatest risk to human health. Oxidation by all secondary formation pathways combined contributes 38 ± 12% of total sulfate mass on average, indicating that secondary sulfate formation is inefficient in this cold, dark environment. On average, the dominant secondary sulfate formation pathways are oxidation by H2O2 (13 ± 6%), O3 (8 ± 4%), and NO2 (8 ± 3%). These findings will inform mitigation strategies to improve air quality and public health in Fairbanks and possibly other high-latitude urban areas during winter.

Multiphase Reactions between Organic Peroxides and Sulfur Dioxide in Internally Mixed Inorganic and Organic Particles: Key Roles of Particle Phase Separation and Acidity.

Organic peroxides (POs) are ubiquitous in the atmosphere and particularly reactive toward dissolved sulfur dioxide (SO2), yet the reaction kinetics between POs and SO2, especially in complex inorganic-organic mixed particles, remain poorly constrained. Here, we report the first investigation of the multiphase reactions between SO2 and POs in monoterpene-derived secondary organic aerosol internally mixed with different inorganic salts (ammonium sulfate, ammonium bisulfate, or sodium nitrate). We find that when the particles are phase-separated, the PO-S(IV) reactivity is consistent with that measured in pure SOA and depends markedly on the water content in the organic shell. However, when the organic and inorganic phases are miscible, the PO-S(IV) reactivity varies substantially among different aerosol systems, mainly driven by their distinct acidities (not by ionic strength). The second-order PO-S(IV) rate constant decreases monotonically from 5 × 105 to 75 M-1 s-1 in the pH range of 0.1-5.6. Both proton catalysis and general acid catalysis contribute to S(IV) oxidation, with their corresponding third-order rate constants determined to be (6.4 ± 0.7) × 106 and (6.9 ± 4.6) × 104 M-2 s-1 at pH 2-6, respectively. The measured kinetics imply that the PO-S(IV) reaction in aerosol is an important sulfate formation pathway, with the reaction kinetics dominated by general acid catalysis at pH > 3 under typical continental atmospheric conditions.

The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO2 in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO2, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H2O2 aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H2O2 pathway. H2O2 and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO2 pathway was more significant at low secondary source and high coal combustion (higher contribution of NO2 pathway appeared in winter, 24.7 %). While high formation rate of the O3 pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.

A critical review of sulfate aerosol formation mechanisms during winter polluted periods

Sulfate aerosol contributes to particulate matter pollution and plays a key role in aerosol radiative forcing, impacting human health and climate change. Atmospheric models tend to substantially underestimate sulfate concentrations during haze episodes, indicating that there are still missing mechanisms not considered by the models. Despite recent good progress in understanding the missing sulfate sources, knowledge on different sulfate formation pathways during polluted periods still involves large uncertainties and the dominant mechanism is under heated debate, calling for more field, laboratory, and modeling work. Here, we review the traditional sulfate formation mechanisms in cloud water and also discuss the potential factors affecting multiphase S(Ⅳ) oxidation. Then recent progress in multiphase S(Ⅳ) oxidation mechanisms is summarized. Sulfate formation rates by different prevailing oxidation pathways under typical winter-haze conditions are also calculated and compared. Based on the literature reviewed, we put forward control of the atmospheric oxidation capacity as a means to abate sulfate aerosol pollution. Finally, we conclude with a concise set of research priorities for improving our understanding of sulfate formation mechanisms during polluted periods.

The new inspiration from the theoretical re-exploration of traditional autoxidation pathways leading to sulfate formation in the haze episode

The high concentration and fast accumulation of sulfate have been identified in most of the haze episodes’ events. It is well known that the autoxidation process is one of the most common and essential routes for sulfate formation. However, the detailed mechanism underlying sulfate formation, especially that from oxygen (O2) involved autoxidation process, has not been fully established since there is no direct experimental proof available till now. In this work, we thoroughly explored the possible sulfate formation pathways involving O2 through high-level quantum chemical calculation investigation. It is revealed that in the autoxidation process, the reaction of SO3− with O2 will occur on the doublet potential energy surface to form SO5−. We further proposed a pH dependent reaction mechanism in which SO5− will react with different forms of SO2 (HOSO2−, SO32−) through four different pathways to produce sulfate. More importantly, by calculating the branching ratios for different pathways, we found the most dominant path involving O2 in sulfate formation in almost all the atmospherically relevant pH during pollution period. In this path, SO5− react with SO32− through two steps: a O atom transfer process and a subsequent single electron transfer (SET) process. Detailed calculation results emphasize the importance of autoxidation mechanism in explaining sulfate formation during pollution period, and therefore provide some important and detailed information for understanding the possible reason of haze episode.

Sulfate Formation Apportionment during Winter Haze Events in North China.

There is a large gap between the simulated and observed sulfate concentrations during winter haze events in North China. Although multiphase sulfate formation mechanisms have been proposed, they have not been evaluated using chemical transport models. In this study, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to apportion sulfate formation. It was found that Mn-catalyzed oxidation on aerosol surfaces was the dominant sulfate formation pathway, accounting for 92.3 ± 3.5% of the sulfate formation during haze events. Gas-phase oxidation contributed 3.1 ± 0.5% to the sulfate formation due to the low OH levels. The H2O2 oxidation in aerosol water accounted for 4.2 ± 3.6% of the sulfate formation, caused by the rapid consumption of H2O2. The contributions of O3, NO2 oxidation, and transition metal ion-catalyzed reactions in aerosol water could be negligible owing to the low aerosol water content, low pH, and high ionic strength. The contributions from in-cloud reactions were negligible due to the barrier provided by stable stratification during winter haze events.

Elevated particle acidity enhanced the sulfate formation during the COVID-19 pandemic in Zhengzhou, China

The significant reduction in PM2.5 mass concentration after the outbreak of COVID-19 provided a unique opportunity further to study the formation mechanism of secondary inorganic aerosols. Hourly data of chemical components in PM2.5, gaseous pollutants, and meteorological data were obtained from January 1 to 23, 2020 (pre-lockdown) and January 24 to February 17, 2020 (COVID-lockdown) in Zhengzhou, China. Sulfate, nitrate, and ammonium were the main components of PM2.5 during both the pre-lockdown and COVID-lockdown periods. Compared with the pre-lockdown period, even though the concentration and proportion of nitrate decreased, nitrate was the dominant component in PM2.5 during the COVID-lockdown period. Moreover, nitrate production was enhanced by the elevated O3 concentration, which was favorable for the homogeneous and hydrolysis nitrate formation despite the drastic decrease of NO2. The proportion of sulfate during the COVID-lockdown period was higher than that before. Aqueous-phase reactions of H2O2 and transition metal (TMI) catalyzed oxidations were the major pathways for sulfate formation. During the COVID-lockdown period, TMI-catalyzed oxidation became the dominant pathway for aqueous-phase sulfate formation because the elevated acidity favored the dissolution of TMI. Therefore, the enhanced TMI-catalyzed oxidation affected by the elevated particle acidity dominated the sulfate formation, resulting in the slight increase of sulfate concentration during the COVID-lockdown period in Zhengzhou.

Significant decrease in SO2 emission and enhanced atmospheric oxidation trigger changes in sulfate formation pathways in China during 2008–2016

Understanding the changes in the sulfate (SO42−) formation pathway is important to further control particle pollution owing to the significant reduction of SO2 emission and enhanced atmospheric oxidation. This study applied the Community Multiscale Air Quality (CMAQ) model coupled with the sulfur tracking model (STM) to quantify sources of atmospheric SO42− in China and their changes during 2008–2016. The results showed that the surface SO42− concentration decreased by 51.1%, 40.8%, 37.5%, and 36.2% for Beijing–Tianjin–Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB), respectively. In addition, the SO42− formation pathways also changed in a sustained manner. The gas-phase reaction of SO2 with OH (SO4_GAS) and the reaction catalyzed by Fe3+ and Mn2+ (SO4_FEMN) decreased significantly (63.4% and 61.7%), whereas the aqueous-phase reaction of SO2 with O3 (SO4_O3) increased by 39.9% from 2008 to 2016. Primary SO42− emission (SO4_EMIS) played an important role in autumn and winter with over 30% contribution, whereas SO4_GAS and the aqueous-phase reaction of SO2 with H2O2 (SO4_H2O2) dominated in spring and summer, respectively. The sensitivity simulation results showed that sharp SO2 reduction significantly decreased SO4_GAS (over 60%); however, it was less sensitive to SO4_O3 (8%–11%). In addition, the SO2 emission reduction resulted in a large increase in atmospheric NH3 concentrations due to less neutralization reaction, thus increasing NO3− concentration in some regions. These results suggest that multi-pollutants control for SO2-NOx-VOC should be implemented to prevent the enhanced atmospheric oxidation furtherly deteriorating secondary air pollution, particularly in spring and summer. In addition, limiting primary emissions from residential combustion in winter is important to achieve coordinated control of SO2 and SO42− in northern China.

Reaction between a NO2 Dimer and Dissolved SO2: A New Mechanism for ONSO3- Formation and its Fate in Aerosol.

Experimental observations indicate that sulfate formation in aerosol is sensitive to the concentrations of nitric oxide (NO2). While it also widely exists as a dimer in the gas phase, previous studies focus on the monomer of NO2. In this study, we employ quantum chemical calculations and ab initio molecular dynamics simulations to investigate the reaction between the NO2 dimer (ONONO2) and sulfite (HSO3-/SO32-) in the gas phase and in an aerosol. Gas-phase reactions turn out to be barrierless. In an aerosol, the reaction between adsorbed ONONO2 and HSO3- to form ONSO3- follows a stepwise mechanism with proton and electron transfer processes. The reaction between ONONO2 and SO32- is more straightforward. Nevertheless, both reactions occur at a picosecond time scale. Decomposition of ONSO3- can form an NO molecule and SO3-, which gives a complementary pathway for sulfate formation in an aerosol. Hydrolysis of ONSO3- to form HNO and HSO4- is highly impossible in an aerosol, which calls for a revisit of the atmospheric N2O formation mechanism. The results presented in this study deepen our understanding of the interaction between NO2 and SO2 pollutants in the atmosphere.

A comprehensive observation-based multiphase chemical model analysis of sulfur dioxide oxidations in both summer and winter

Abstract. Sulfate is one of the main components of haze fine particles, and its formation mechanism remains controversial. A lack of detailed and comprehensive field data hinders the accurate evaluation of relative roles of prevailing sulfate formation pathways. Here, we analyzed the sulfate production rates using a state-of-the-art multiphase model constrained to the observed concentrations of transition metal, nitrogen dioxide, ozone, hydrogen peroxide and other important parameters in winter and summer in the North China Plain. Our results showed that aqueous transition metal ion (TMI)-catalyzed oxidation was the most important pathway followed by the surface oxidation of Mn in both winter and summer while the hydroxyl and Criegee radical oxidations contribute significantly in summer. In addition, we also modeled the published cases for the fog and cloud conditions. It is found that nitrogen dioxide oxidation is the dominant pathway for the fog in a higher pH range while hydroperoxide and ozone oxidations dominated for the cloud.

Isotopic constraints on atmospheric sulfate formation pathways in the Mt. Everest region, southern Tibetan Plateau

Abstract. As an important atmosphere constituent, sulfate aerosols exert profound impacts on climate, the ecological environment, and human health. The Tibetan Plateau (TP), identified as the “Third Pole”, contains the largest land ice masses outside the poles and has attracted widespread attention for its environment and climatic change. However, the mechanisms of sulfate formation in this specific region still remain poorly characterized. An oxygen-17 anomaly (Δ17O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the Δ17O values in atmospheric sulfate collected at a remote site in the Mt. Everest region to decipher the possible formation mechanisms of sulfate in such a pristine environment. Throughout the sampling campaign (April–September 2018), the Δ17O in non-dust sulfate show an average of 1.7 ‰±0.5 ‰, which is higher than most existing data on modern atmospheric sulfate. The seasonality of Δ17O in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in Δ17O for both sulfate and nitrate (i.e., minima in the warm season and maxima in the cold season) observed from diverse geographic sites. This high Δ17O in non-dust sulfate found in this region clearly indicates the important role of the S(IV)+O3 pathway in atmospheric sulfate formation promoted by conditions of high cloud water pH. Overall, our study provides an observational constraint on atmospheric acidity in altering sulfate formation pathways, particularly in dust-rich environments, and such identification of key processes provides an important basis for a better understanding of the sulfur cycle in the TP.

Sulfate formation is dominated by manganese-catalyzed oxidation of SO2 on aerosol surfaces during haze events

The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO2 and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO2 on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change.

Atmospheric Photosensitization: A New Pathway for Sulfate Formation.

Northern China is regularly subjected to intense wintertime "haze events", with high levels of fine particles that threaten millions of inhabitants. While sulfate is a known major component of these fine haze particles, its formation mechanism remains unclear especially under highly polluted conditions, with state-of-the-art air quality models unable to reproduce or predict field observations. These haze conditions are generally characterized by simultaneous high emissions of SO2 and photosensitizing materials. In this study, we find that the excited triplet states of photosensitizers could induce a direct photosensitized oxidation of hydrated SO2 and bisulfite into sulfate S(VI) through energy transfer, electron transfer, or hydrogen atom abstraction. This photosensitized pathway appears to be a new and ubiquitous chemical route for atmospheric sulfate production. Compared to other aqueous-phase sulfate formation pathways with ozone, hydrogen peroxide, nitrogen dioxide, or transition-metal ions, the results also show that this photosensitized oxidation of S(IV) could make an important contribution to aerosol sulfate formation in Asian countries, particularly in China.

Isotopic constraints on heterogeneous sulfate production in Beijing haze

Abstract. Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42−)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM2.5 concentrations ranged from 16 to 323 µg m−3 with a mean of (141 ± 88 (1σ)) µg m−3, with SO42− representing 8–25 % of PM2.5 mass. The observed Δ17O(SO42−) varied from 0.1 to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰. Δ17O(SO42−) increased with PM2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM2.5 ≥ 75 µg m−3) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III–V, heterogeneous sulfate production (Phet) was estimated to contribute 41–54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggested S(IV) ( = SO2 ⚫ H2O + HSO3− + SO32−) oxidation by H2O2 in aerosol water accounted for 5–13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42−). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21–22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42−), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 via a radical chain mechanism, contributed the remaining 66–73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2. Our local atmospheric conditions-based calculations suggest sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O2 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.

Sulfate Formation Enhanced by a Cocktail of High NOx, SO2, Particulate Matter, and Droplet pH during Haze-Fog Events in Megacities in China: An Observation-Based Modeling Investigation.

In recent years in a few Chinese megacities, fog events lasting one to a few days have been frequently associated with high levels of aerosol loading characterized by high sulfate (as high as 30 μg m(-3)), therefore termed as haze-fog events. The concomitant pollution characteristics include high gas-phase mixing ratios of SO2 (up to 71 ppbv) and NO2 (up to 69 ppbv), high aqueous phase pH (5-6), and smaller fog droplets (as low as 2 μm), resulting from intense emissions from fossil fuel combustion and construction activities supplying abundant Ca(2+). In this work, we use an observation-based model for secondary inorganic aerosols (OBM-SIA) to simulate sulfate formation pathways under conditions of haze-fog events encountered in Chinese megacities. The OBM analysis has identified, at a typical haze-fogwater pH of 5.6, the most important pathway to be oxidation of S(IV) by dissolved NO2, followed by the heterogeneous reaction of SO2 on the aerosol surface. The aqueous phase oxidation of S(IV) by H2O2 is a very minor formation pathway as a result of the high NOx conditions suppressing H2O2 formation. The model results indicate that the unique cocktail of high fogwater pH, high concentrations of NO2, SO2, and PM, and small fog droplets are capable of greatly enhancing sulfate formation. Such haze-fog conditions could lead to rapid sulfate production at night and subsequently high PM2.5 in the morning when the fog evaporates. Sulfate formation is simulated to be highly sensitive to fogwater pH, PM, and precursor gases NO2 and SO2. Such insights on major contributing factors imply that reduction of road dust and NOx emissions could lessen PM2.5 loadings in Chinese megacities during fog events.

Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust.

The heterogeneous reactions of SO2 and NH3 on typical mineral oxides were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A new sulfate formation pathway was proposed where NH3 accelerated the formation of sulfate species. The results revealed that surface hydroxyls and oxygen played principal roles in the conversion of SO2 to sulfate. It was proposed that NH3 adsorbed onto Lewis acid sites, and hydroxyls and water molecules adsorbed on the surfaces of mineral dust. The enhancement of surface Lewis basicity by NH3 induced more SO2 molecules to adsorb on the surface, which were further oxidized to sulfate by interacting with surface hydroxyls and oxygen atoms. The formation of sulfate, in turn, contributed to the adsorption of NH3, mainly as NH4(+) due to enhanced Brønsted acid sites. The IC results showed that the synergistic effect between SO2 and NH3 was more significant on acidic oxides like γ-Al2O3 and α-Fe2O3 compared to basic oxides like MgO.