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The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO 2 oxidation at different levels of air pollution. The Δ 33 S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ 33 S signatures (0‰ < Δ 33 S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO 2 oxidation and stratospheric SO 2 photolysis. The negative Δ 33 S signatures (-0.236‰ < Δ 33 S < ∼0‰) observed during winter haze events (PM 2.5 > 200 μg/m 3 ) are mainly attributed to SO 2 oxidation by H 2 O 2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H 2 O 2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO 2 can produce significant negative Δ 33 S values in sulfate aerosols.

The heterogeneous oxidation of SO 2 by NO 2 has been extensively proposed as an important pathway of sulfate production during haze events in China. However, the kinetics and mechanism of oxidation of SO 2 by NO 2 on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO 2 and NO 2 on diesel black carbon (DBC) under light irradiation. The experimental results prove that DBC photochemistry can not only significantly promote the heterogeneous reduction of NO 2 to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO 2 as well as NO 2 itself greatly promote the heterogeneous oxidation of SO 2 on DBC. NO 2 oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO 2 . The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO 2 uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO 2 (∼10%). This work highlights the significant synergistic roles of DBC, NO 2 , and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.

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 Ca2+. 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. [ABSTRACT FROM AUTHOR]

Numerical models have been developed to elucidate air pollution caused by sulfate aerosols (SO 4 2- ). However, typical models generally underestimate SO 4 2- , and oxidation processes have not been validated. This study improves the modeling of SO 4 2- formation processes using the mass-independent oxygen isotopic composition [ 17 O-excess; Δ 17 O(SO 4 2- )], which reflects pathways from sulfur dioxide (SO 2 ) to SO 4 2- , at the background site in Japan throughout 2015. The standard setting in the Community Multiscale Air Quality (CMAQ) model captured SO 4 2- concentration, whereas Δ 17 O(SO 4 2- ) was underestimated, suggesting that oxidation processes were not correctly represented. The dust inline calculation improved Δ 17 formation in the region downstream of China. It was estimated that the remaining mismatch of Δ 4 2- ) because dust-derived increases in cloud-water pH promoted acidity-driven SO 4 2- production, but Δ 17 O(SO 4 2- ) was still overestimated during winter as a result. Increasing solubilities of the transition-metal ions, such as iron, which are a highly uncertain modeling parameter, decreased the overestimated Δ 17 O(SO 4 2- ) in winter. Thus, dust and high metal solubility are essential factors for SO 4 2- formation in the region downstream of China. It was estimated that the remaining mismatch of Δ 17 O(SO 4 2- ) between the observation and model can be explained by the proposed SO 4 2- formation mechanisms in Chinese pollution. These accurately modeled SO 4 2- formation mechanisms validated by Δ 17 O(SO 4 2- ) will contribute to emission regulation strategies required for better air quality and precise climate change predictions over East Asia.

This study examined the associations of fine particulate matter (PM 2.5 ) and its chemical constituents with risks of small for gestational age (SGA) and large for gestational age (LGA). Based on the China Labor and Delivery Survey, we included 70,206 birth records from 24 provinces in China. Concentrations of PM 2.5 mass and six main constituents were estimated using satellite-based models. Logistic regression analysis was used to examine the associations, adjusted for sociodemographic characteristics and time trends. We found that an interquartile range increase in PM 2.5 exposure during pregnancy was associated with 16% (95% confidence interval [CI]: 3-30%) and 11% (95% CI: 1-22%) higher risk of SGA and LGA, respectively. Elevated risk of SGA was associated with exposure to black carbon [odds ratio (OR) = 1.15, 95% CI: 1.00-1.32], ammonium (OR = 1.12, 95% CI: 1.01-1.25), and sulfate (OR = 1.12, 95% CI: 1.04-1.21); while increased risk of LGA was associated with exposure to black carbon (OR = 1.13, 95% CI: 1.02-1.26), ammonium (OR = 1.13, 95% CI: 1.03-1.24), sulfate (OR = 1.08, 95% CI: 1.01-1.15), and nitrate (OR = 1.14, 95% CI: 1.03-1.27). Our findings provide evidence that PM 2.5 exposure was associated with increased risks of SGA and LGA, and constituents related to emissions from anthropogenic sources may play important roles in these associations.

Sulfate (SO 4 2- ) is a major species in atmospheric fine particles (PM 2.5 ), inducing haze formation and influencing Earth's climate. In this study, the δ 34 S values in PM 2.5 sulfate (δ 34 ) were measured in Hangzhou, east China, from 2015 September to 2016 October. The result showed that the δ 4 2- ) were measured in Hangzhou, east China, from 2015 September to 2016 October. The result showed that the δ 34 S-SO 4 2- values varied from 1.6 to 6.4‰ with the higher values in the winter. The estimated fractionation factor (α 34 S g→p ) from SO 2 to SO 4 2- averaged at 3.9 ± 1.6‰. The higher α 34 S g→p values in the winter were mainly attributed to the decrease of ambient temperature. We further compared the quantified source apportionments of sulfate by isotope techniques with and without the consideration of fractionation factors. The result revealed that the partitioned emission sources to sulfate with the consideration of the fractionation effects were more logical, highlighting that fractionation effects should be considered in partitioning emission sources to sulfate using sulfur isotope techniques. With considering the fractionation effects, coal burning was the dominant source to sulfate (85.5%), followed by traffic emissions (12.8%) and oil combustion (1.7%). However, the coal combustion for residential heating contributed only 0.9% to sulfate on an annual basis in this megacity.

Because of active exchange between surface and groundwater of a karstic hydrological system, the groundwater of Guiyang, the capital city of Guizhou Province, southwest China, has been seriously polluted by anthropogenic inputs of NO3-, SO42-, CI-, and Na+. In this work, δ37CI of chloride and δ34S variations of sulfate in the karstic surface/groundwater system were studied, with a main focus to identify contaminant sources, including their origins. The surface, ground, rain, and sewage water studied showed variable δ37CI and δ34S values, in the range of -4.1 to +2.0%‰ and -20.4 to +20.9‰ for δ37CI and δ34S (SO42-), respectively. The rainwater samples yielded the lowest δ37CI values among those observed to date for aerosols and rainwater. Chloride in the Guiyang area rain waters emanated from anthropogenic sources rather than being of marine origin, probably derived from HCI (g) emitted by coal combustion. By plotting 1/SO42- vs δ34S and 1/Cl- vs δ37CI, respectively, we were able to identify some clusters of data, which were assigned as atmospheric deposition (acid rain component), discharge from municipal sewage, paleo-brine components in clastic sedimentary rocks, dissolution of gypsum mainly in dolomite, oxidation of sulfide minerals in coal-containing clastic rocks, and possibly degradation of chlorine-containing organic matter We conclude that human activities give a significant input of sulfate and chloride ions, as well as other contaminants, into the studied groundwater system through enhanced atmospheric deposition and municipal sewage, and that multiple isotopic tracers constitute a powerful tool to ascertain geochemical characteristics and origin of complex contaminants in groundwater. [ABSTRACT FROM AUTHOR]

Photolysis of iron chlorides is a well-known photolytic source of Cl • in environmental waters. However, the role of particulate chlorine radicals (Cl • and Cl 2 •- ) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl • /Cl 2 •- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO 2 by Cl • /Cl 2 , comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γ •- were performed. Fast sulfate production from SO 2 oxidation was observed with reactive uptake coefficients of ∼10 -5 , comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γ SO2 , and the Cl • production rate, d[Cl • ]/dt, as γ SO2 = 5.3 × 10 -6 × log(d[Cl • ]/dt) + 4.9 × 10 -5 . When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe 3+ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

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 SO 2 and photosensitizing materials. In this study, we find that the excited triplet states of photosensitizers could induce a direct photosensitized oxidation of hydrated SO 2 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.

Heterogeneous oxidation of SO 2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO 2 oxidation by OH and NO 2 produced during 250 nm nitrate photolysis ( Environ. Sci. Technol. Lett . 2019 , 6 , 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO 2 - or HNO 2 , N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO 2 , γ SO 2 , can be expressed as γ SO 2 = 1.64 × p NO3- , where p NO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10 -5 M s -1 . Our kinetic model with the p NO3- predicts that N(III) is the main contributor to the SO 2 oxidation, followed by NO 2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m -3 h -1 at pH 4-6 and p NO3- = 10 -5 M s -1 . The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO 2 by reactive nitrogen (NO 2 - /HNO 2 and NO 2 ) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

Organosulfates (OSs) have been observed as substantial constituents of atmospheric organic aerosol (OA) in a wide range of environments; however, the chemical composition, sources, and formation mechanism of OSs are still not well understood. In this study, we first created an "OS precursor map" based on the elemental composition of previous OS chamber experiments. Then, according to this "OS precursor map", we estimated the possible sources and molecular structures of OSs in atmospheric PM 2.5 (particles with aerodynamic diameter ≤ 2.5 μm) samples, which were collected in urban areas of Beijing (China) and Mainz (Germany) and analyzed by ultrahigh-performance liquid chromatography (UHPLC) coupled with an Orbitrap mass spectrometer. On the basis of the "OS precursor map", together with the polarity information provided by UHPLC, OSs in Mainz samples are suggested to be mainly derived from isoprene/glyoxal or other unknown small polar organic compounds, while OSs in Beijing samples were generated from both isoprene/glyoxal and anthropogenic sources (e.g., long-chain alkanes and aromatics). The nitrooxy-OSs in the clean aerosol samples were mainly derived from monoterpenes, while much fewer monoterpene-derived nitrooxy-OSs were obtained in the polluted aerosol samples, showing that nitrooxy-OS formation is affected by different precursors in clean and polluted air conditions.

Haze events in China megacities involve the rapid oxidation of SO 2 to sulfate aerosol. Given the weak photochemistry that takes place in these optically thick hazes, it has been hypothesized that SO 2 is mostly oxidized by NO 2 emissions in the bulk of pH > 5.5 aerosols. Because NO 2 (g) dissolution in water is very slow and aerosols are more acidic, we decided to test such a hypothesis. Herein, we report that > 95% of NO 2 (g) disproportionates [2NO 2 (g) + H 2 O(l) = H + + NO 3 - (aq) + HONO (R1)] upon hitting the surface of NaHSO 3 aqueous microjets for < 50 μs, thereby giving rise to strong NO 3 - ( m/ z 62) signals detected by online electrospray mass spectrometry, rather than oxidizing HSO 3 - ( m/ z 81) to HSO 4 - ( m/ z 97) in the relevant pH 3-6 range. Because NO 2 (g) will be consumed via R1 on the surface of typical aerosols, the oxidation of S(IV) may in fact be driven by the HONO/NO 2 - generated therein. S(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity. Whether aerosol acidity affects the oxidation of S(IV) by HONO/NO 2 - remains to be elucidated.