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Recent studies reveal that organosulfates at the particle surface can be oxidized by gas-phase OH radicals with significant rates. Inorganic sulfur species, such as the bisulfate ion (HSO4-) and sulfate ion (SO42-), can be formed upon these heterogeneous oxidation processes through the formation and subsequent reactions of sulfate radical anions (SO4·-) in the particle phase. However, the amount of inorganic sulfur species produced in these heterogeneous oxidation reactions is not known. We investigate the heterogeneous OH oxidation of sodium methyl sulfate (CH3SO4Na), the smallest organosulfate detected in atmospheric particles, using an oxidation flow reactor at a relative humidity of 75%. We quantify the kinetics by measuring the decay of CH3SO4Na and the amount of HSO4- and SO42- formed upon oxidation using ion chromatography. Kinetic measurements determine the heterogeneous OH reaction rate to be (5.72 ± 0.14) × 10-13 cm3 molecule-1 s-1, with an effective OH uptake coefficient, γeff, of 0.31 ± 0.06. The molar yield of inorganic sulfur species, defined as the total number of moles of HSO4- and SO42- formed per mole of CH3SO4Na consumed upon oxidation, is found to be significant and has an average value of 0.62 ± 0.18 upon oxidation. A kinetic model is developed to describe the kinetics and inorganic sulfur species formation upon oxidation. Model simulations suggest that CH3SO4Na tends to decompose rapidly into formaldehyde and SO4·-, and the reaction of SO4·- with CH3SO4Na plays a significant role in both governing the kinetics and the formation of inorganic sulfur species.

The potential impact of transitioning from conventional fossil fuel to a non-fossil-fuel vehicle fleet was investigated by measuring primary emissions via extractive sampling of bus plumes and assessing secondary mass formation using the Gothenburg Potential Aerosol Mass (Go:PAM) reactor from 76 in-use transit buses. Online chemical characterization of gaseous and particulate emissions from these buses was conducted using chemical ionization mass spectrometry (CIMS) with acetate as the reagent ion, coupled with the Filter Inlet for Gases and AEROsols (FIGAERO). Acetate reagent ion chemistry selectively ionizes acidic compounds, including organic and inorganic acids, as well as nitrated and sulfated organics. A significant reduction (48 %–98 %) in fresh particle emissions was observed in buses utilizing compressed natural gas (CNG), biodiesels like rapeseed methyl ester (RME) and hydrotreated vegetable oil (HVO), and hybrid-electric HVO (HVOHEV) compared to diesel (DSL). However, secondary particle formation from photooxidation of emissions was substantial across all the fuel types. The median ratio of particle mass emission factors of aged to fresh emissions increased in the following order: DSL buses at 4.0, HVO buses at 6.7, HVOHEV buses at 10.5, RME buses at 10.8, and CNG buses at 84. Of the compounds that can be identified by CIMS, fresh gaseous emissions from all Euro V/EEV (Enhanced Environmentally friendly Vehicle) buses, regardless of fuel type, were dominated by nitrogen-containing compounds such as nitrous acid (HONO), nitric acid (HNO3), and isocyanic acid (HNCO), alongside small monoacids (C1-C3). Notably, the emission of nitrogen-containing compounds was lower in Euro VI buses equipped with more advanced emission control technologies. Secondary gaseous organic acids correlated strongly with gaseous HNO3 signals (R2=0.85 –0.99) in Go:PAM, but their moderate to weak correlations with post-photooxidation secondary particle mass suggest that they are not reliable tracers of secondary organic aerosol formation from bus exhaust. Our study highlights that non-regulated compounds and secondary pollutant formation, not currently addressed in legislation, are crucial considerations in the evaluation of environmental impacts of future fuel and engine technology shifts. [ABSTRACT FROM AUTHOR]

To investigate the combined impacts of anthropogenic and biogenic emissions on the formation of secondary organic aerosols (SOA), SOA molecular tracers, their corresponding volatile organic compound precursors, and other air pollutants were measured online during the winter and summer seasons of 2022 in an industrial city, Zibo, China. The results indicate that the average concentrations of SOA tracers were 16.1 ± 9.8 ng m−3 in winter and 99.4 ± 57.2 ng m−3 in summer. During winter, anthropogenic SOA (ASOA, the sum of SOA derived from naphthalene and mono‐aromatic volatile organic compounds) dominated, whereas isoprene SOA (SOAI) prevailed in summer. Correlation analysis between SO42− and both SOAI and high‐order monoterpene SOA tracers (SOAM‐H) (R = 0.46–0.72, p < 0.001) revealed that higher aerosol acidity facilitated the formation of SOAI and SOAM‐H, with SO2 emissions playing a significant role in leading to higher acidity. Most biogenic SOA (BSOA) tracers exhibited a significant positive correlation with NO3−, particularly in winter, implying the remarkable influence of NOx emissions on BSOA formation. The levels of BSOA tracers increased with NH3, indicating that NH3 can enhance the formation of BSOA. In summer, SOA formation correlated with Ox (Ox = O3 + NO2), indicating the substantial impact of atmospheric oxidizing capacity on SOA formation. During winter, aerosol liquid water content (ALWC) correlated well with SOAI tracers (i.e., 3‐hydroxyglutaric acid (3‐HGA) and 3‐hydroxy‐4,4‐dimethylglutaric acid (3‐HDMGA)), and 2,3‐dihydroxy‐4‐oxopentanoic acid (DHOPA) (R > 0.5, p < 0.001), indicating the important contribution of aqueous‐phase formation of SOA. These findings underscore the significant role of anthropogenic pollutant emissions in the formation of ASOA and BSOA in urban environments. Plain Language Summary: Air pollution and climate change are influenced by secondary organic aerosol (SOA), which are formed through a series of chemical reactions that could involve both anthropogenic and natural emissions. However, there is limited research on the relationship between the precursors of SOA and the resulting SOA at the molecular level. To address this gap, we investigated the influence of anthropogenic emissions on the formation of SOA in a typical industrial city (Zibo, China) based on online measurements of SOA molecular tracers and corresponding volatile organic compound precursors, and other air pollutants, at a high‐time resolution during the winter and summer of 2022. Our findings demonstrate the important role of human‐generated pollutants in the formation of anthropogenic SOA and biogenic SOA. These results enhance our understanding of SOA formation in industrial cities and provide valuable insights for the formulation of SOA management and control strategies in such areas. Key Points: Anthropogenic secondary organic tracers were the predominant contributors during winter, whereas isoprene secondary organic aerosols (SOA) tracers were predominant during summerAnthropogenic pollutants have an important impact on the formation of SOAHigh correlation was observed between SOA tracers and Ox in summer and aerosol liquid water content in winter [ABSTRACT FROM AUTHOR]

Fog processing has a significant impact on sulfur chemistry in the atmosphere. This study analyzed three winter fog events in Hong Kong using single‐particle aerosol mass spectrometry (SPAMS) and a Monitor for AeRosols and GAses in ambient air (MARGA). Black carbon (BC)‐related carbonaceous particles with substantial sulfate amounts comprised the largest particle number fraction (56.7%). Sulfate mass concentration decreased during fog due to the cloud's effective scavenging, but fog processing notably promoted sulfate formation at the single‐particle level (average peak area increases of 31.2%). Hydroxymethanesulfonate (HMS), an important S(IV) compound and fog tracer, was identified accounting for up to 12% by particle number fraction. Although pH showed a positive correlation (r = 0.53–0.69) with HMS particles in each fog scenario, a negative overall correlation (r = −0.51) was observed. Further analysis revealed that the higher aerosol acidity (pH 0.65–3.11), promoted Fe dissolution, leading to 49% of HMS particles being mixed with Fe, which potentially facilitated sulfate formation via the Fenton reaction. Additionally, around 40% of HMS‐Fe particles are mixed with oxalate, thereby warranting further attention for their potential to cause more intricate sulfur oxidation processes. This study reveals the initial identification of a high mixed‐state of HMS‐Fe, which could potentially serve as a crucial avenue for the formation of sulfate on individual particulate matter. Considering the persistent augmentation of aerosol acidity in the Asian region, this phenomenon necessitates further investigation and attention. Plain Language Summary: Cloud and fog processes are crucial for atmospheric chemistry and important pathways in sulfate formation. However, current models often underestimate sulfate production in the atmosphere. While several potential pathways have been proposed, the influence of organic compounds on hydroxymethanesulfonate (HMS) formation and its subsequent sulfate generation is yet to be fully explored. This study, utilizing a single‐particle perspective, reveals that higher aerosol acidity in coastal areas can lead to the dissolution of iron (Fe) in coastal aerosols, thereby promoting the formation of HMS‐Fe. Additionally, as a transition metal, Fe has the potential to catalyze the conversion of HMS to sulfates. This finding challenges the prevailing notion that HMS hinders sulfate production by being less susceptible to oxidation by hydrogen peroxide (H2O2) and other radicals. The significance of this process is particularly pronounced in coastal or remote marine atmospheres, where cloud and fog processes exhibit higher aerosol acidity and limited ammonia sources. Furthermore, considering the current global scenario of increased aerosol acidity, the impact of transition metals on sulfate formation in HMS is likely to be more prominent. Consequently, this phenomenon necessitates further investigation and attention. [ABSTRACT FROM AUTHOR]

Observation-based data of primary and secondary organic carbon in ambient particulate matter (PM) are essential for model evaluation, climate and air quality research, health effect assessments, and mitigation policy development. Since there are no direct measurement tools available to quantify primary organic (POC) and secondary organic carbon (SOC) as separate quantities, their estimation relies on inference approaches using relevant measurable PM constituents. In this study, we measured hourly carbonaceous components and major ions in PM 2.5 for a year and a half in suburban Hong Kong from July 2020 to December 2021. We differentiated POC and SOC using a novel Bayesian inference approach. The hourly POC and SOC data allowed us to examine temporal characteristics varying from diurnal and weekly patterns to seasonal variations, as well as their evolution characteristics during individual PM 2.5 episodes. A total of 65 city-wide PM 2.5 episodes were identified throughout the entire study period, with SOC contributions during individual episodes varying from 10 % to 66 %. In summertime typhoon episodes, elevated SOC levels were observed during daytime hours, and high temperature and NOx levels were identified as significant factors contributing to episodic SOC formation. Winter haze episodes exhibited high SOC levels, likely due to persistent influences from regional transport originating from the northern region to the sampling site. Enhanced SOC formation was observed with increase in the nocturnal NO3 radical (indicated by the surrogate quantity of [ NO2 ][ O3 ]) and under conditions characterized by high water content and strong acidity. These results suggest that both NO3 chemistry and acid-catalyzed aqueous-phase reactions likely make notable contributions to SOC formation during winter haze episodes. The methodology employed in this study for estimating POC and SOC provides practical guidance for other locations with similar monitoring capabilities in place. The availability of hourly POC and SOC data is invaluable for evaluating and improving atmospheric models, as well as understanding the evolution processes of PM pollution episodes. This, in turn, leads to more accurate model predictions and a better understanding of the contributing sources and processes. [ABSTRACT FROM AUTHOR]

Ambient air measurements of volatile organic compounds (VOCs) and oxygenated volatile organic compounds (OVOCs) were conducted and characterised during a two-year grid study in the Pearl River Delta (PRD) region of southern China. The present grid study pioneered the systematic investigation of the nature and characteristics of complex VOC and OVOC sources at a regional scale. The largest contributing VOCs, accounting over 80% of the total VOCs mixing ratio, were toluene, ethane, ethyne, propane, ethene, butane, benzene, pentane, ethylbenzene, and xylenes. Sub-regional VOC spatial characteristics were identified, namely: i) relatively fresh pollutants, consistent with elevated vehicular and industrial activities, around the PRD estuary; and ii) a concentration gradient with higher mixing ratios of VOCs in the west as compared with the eastern part of PRD. Based on alkyl nitrate aging determination, a high hydroxyl radical (OH) concentration favoured fast hydrocarbon reactions and formation of locally produced ozone. The photochemical reactivity analysis showed aromatic hydrocarbons and alkenes together consisted of around 80% of the ozone formation potential (OFP) among the key VOCs. We also found that the OFP from OVOCs should not be neglected since their OFP contribution was more than one-third of that from VOCs alone. These findings support the choice of current air pollution control policy which focuses on vehicular sources but warrants further controls. Industrial emissions and VOCs emitted by solvents should be the next targets for ground-level ozone abatement. © 2012 Elsevier Ltd.

Positive matrix factorization (PMF) has been widely used to apportion the sources of fine particulate matter (PM2.5) by utilizing PM chemical speciation data measured at the receptor site(s). Traditional PMF, which typically relies on long-term observational datasets of daily or lower time resolution to meet the required sample size, has its reliability undermined by changes in source profiles; thus, it is inherently ill-suited for apportioning sporadic sources or ephemeral pollution events. In this study, we explored short-term source apportionment of PM2.5 using a set of bihourly chemical speciation data over a period of 37 d in the winter of 2019–2020. PMF run with campaign-wide data as input (PMFref) was initially conducted to obtain reference profiles for the primary source factors. Subsequently, short-term PMF analysis was performed using the Source Finder Professional (SoFi Pro). The analysis sets a window length of 18 d and constrained the primary source profiles using the a -value approach embedded in SoFi Pro software. Rolling PMF was then conducted with a fixed window length of 18 d and a step of 1 d using the remaining dataset. By applying the a -value constraints to the primary sources, the rolling PMF effectively reproduced the individual primary sources, as evidenced by the slope values close to unity (i.e., 0.9–1.0). However, the estimation for the firework emission factor in the rolling PMF was lower compared with PMFref (slope: 0.8). These results suggest the unique advantage of short-term PMF analysis in accurately apportioning sporadic sources. Although the total secondary sources were well modeled (slope: 1.0), larger biases were observed for individual secondary sources. The variation in source profiles indicated higher variabilities for the secondary sources, with average relative differences ranging from 42 % to 173 %, while the primary source profiles exhibited much smaller variabilities (relative differences of 8 %–26 %). This study suggests that short-term PMF analysis with the a -value constraints in SoFi Pro can be utilized to apportion primary sources accurately, while future efforts are needed to improve the prediction of individual secondary sources. Additionally, future rapid source apportionment analysis can benefit from utilizing a library of source profiles derived from existing measurement data, thereby significantly reducing the time lag associated with receptor modeling source apportionment techniques. [ABSTRACT FROM AUTHOR]

Continuing control efforts in recent decade have notably brought down fine‐particulate‐matter (PM2.5) pollution to compliance with the annual average air quality objective in Hong Kong. However, short‐term (several hours to few days) PM2.5 pollution sporadically occurred in winter and has become a major focus of attention, calling research in characterizing sources and their variations at hourly timescale. This study reports hourly measurements of PM2.5 and its chemical speciation, including molecular and elemental tracers in suburban Hong Kong in 2020 winter. Hourly PM2.5 was 20.3 ± 7.33 μg m−3, dominated by organics and sulfate. We deployed tracer‐based source apportionment using positive matrix factorization, which resolved six secondary and seven primary sources. Sulfate‐rich and nitrate‐rich factors were the leading PM2.5 contributors. Variation in source contributions under different air masses revealed the Pearl River Delta region and coastal areas to the north are main origins. Elevation in secondary sources was the driving factor in causing high PM2.5 polluted hours. During Chinese New Year holiday, firework emissions partially offset the reductions in anthropogenic sources, even leading to an increased PM2.5. Hour‐by‐hour examination of most polluted and firework event hours enhanced understanding on dynamic variations of haze and firework burning evolution. The mass concentration of firework‐derived PM2.5 was notably lower under "prohibition" than "restriction" control scenario (avg. 3.5 vs. 8.4 μg m−3). Our results highlight the effectiveness of tracer‐based source apportionment in revealing hourly PM2.5 source variations and the necessity of regional joint management to lessen the severity of episodic PM2.5 pollution and health risks. Plain Language Summary: Episodic PM2.5 pollution elicits acute health impacts and has attracted increasing attention of the public in recent years, thus impelling the need for understanding PM2.5 source variations at the hourly timescale. We present a comprehensive study of PM2.5 sources based on hourly measurements of PM2.5 in suburban Hong Kong during the winter of 2020. Totally, 20 out of 94 sampling days recorded PM2.5 levels exceeding 25 μg m−3, the World Health Organization IT‐4 air quality guideline for daily PM2.5. Over 200 hourly samples (21% of total) surpassed over 25 μg m−3, which we set as the episode threshold value in this study. Our source apportionment results identified the episodic high PM2.5 hr in Hong Kong was attributed to the large increment in secondary formation processes. Firework emissions had a significant but short‐term impact on air quality during the Chinese New Year holidays (5 consecutive days), and their mass contributions increased by ∼2.4 times when the control policy shifted from "prohibition" to "restriction." The overall findings provide scientific insights for formulating effective control strategies to meet the WHO AQG IT‐4 and to strike a balance between pollution mitigation and the inclusion of fireworks to enhance festive occasions. Key Points: Receptor modeling based on hourly aerosol organic molecular and elemental tracers resolved 13 source factors in suburban Hong KongHourly measurements allowed hourly time‐scale characterization of variations in aerosol chemical composition as well as major sourcesAnalysis of PM2.5 episodes and firework events on an hourly basis offered quantitative source data for devising targeted control measures [ABSTRACT FROM AUTHOR]