Persistent sulfate formation from London Fog to Chinese haze

Characterization of size-resolved aerosol hygroscopicity and liquid water content in Nanjing of the Yangtze River Delta

Aerosol hygroscopicity and liquid water content (ALWC) have important influences on the environmental and climate effect of aerosols. In this study, we measured the hygroscopic growth factors (GF) of particles with dry diameters of 40, 80, 150, and 200 nm during the wintertime in Nanjing. Both the GF-derived hygroscopicity parameter (κgf) and ALWC increased with particle size, but displayed differing diurnal variations, with κgf peaking around the midday, while ALWC peaking in the early morning. Nitrate, ammonium and oxygenated organic aerosols (OOA) were found as the chemical components mostly strongly correlated with ALWC. A closure study suggests that during midday photo-oxidation and nighttime high ALWC periods, the κ of organic aerosols (κorg) was underestimated when using previous parameterizations. Accordingly, we re-constructed parameterizations for κorg and the oxidation level of organics for these periods, which indicates a higher hygroscopicity of photochemically formed OOA than the aqueous OOA, yet both being much higher than the generally assumed OOA hygroscopicity. Additionally, in a typical high ALWC episode, concurrently increased ALWC, nitrate, OOA as well as aerosol surface area and mass concentrations were observed under elevated ambient RH. This strongly indicates a coupled effect that the hygroscopic secondary aerosols, in particular nitrate with strong hygroscopicity, led to large increase in ALWC, which in turn synergistically boosted nitrate and OOA formation by heterogeneous/aqueous reactions. Such interaction may represent an important mechanism contributing to enhanced formation of secondary aerosols and rapid growth of fine particulate matter under relatively high RH conditions.

Review of health effects driven by aerosol acidity: Occurrence and implications for air pollution control

Acidity, generally expressed as pH, plays a crucial role in atmospheric processes and ecosystem evolution. Atmospheric acidic aerosol, triggering severe air pollution in the industrialization process (e.g., London Great Smoke in 1952), has detrimental effects on human health. Despite global endeavors to mitigate air pollution, the variation of aerosol acidity remains unclear and further restricts the knowledge of the acidity-driven toxicity of fine particles (PM2.5) in the atmosphere. Here, we summarize the toxicological effects and mechanisms of inhalable acidic aerosol and its response to air pollution control. The acidity could adjust toxic components (e.g., metals, quinones, and organic peroxides) bonded in aerosol and synergize with oxidant gaseous pollutants (e.g., O3 and NO2) in epithelial lining fluid to induce oxidative stress and inflammation. The inhaled aerosol from the ambient air with higher acidity might elevate airway responsiveness and cause worse pulmonary dysfunction. Furthermore, historical observation data and model simulation indicate that PM2.5 can retain its acidic property despite considerable reductions in acidifying gaseous pollutants (e.g., SO2 and NOx) from anthropogenic emissions, suggesting its continuing adverse impacts on human health. The study highlights that aerosol acidity could partially offset the health benefits of emission reduction, indicating that acidity-related health effects should be considered for future air pollution control policies.

Study on the phase transformation mechanism and influencing factors of inorganic condensable particulate matter from coal-fired power plants

In this study, the concentration of inorganic ions (SO42-, NH4+, NO3- and NO2-) and morphological characteristics of condensable particulate matter (CPM) were investigated to elucidate the formation mechanism of inorganic CPM from ultra-low emission coal-fired power plants. The concentration of inorganic ions increased with the increase of H2O content and concentration of inorganic gaseous contaminants (SO2, NOX and NH3), and decrease of condensation temperature, indicating the enhancement of heterogenous reaction in the saturated flue gas. Furthermore, NOX and SO2 could undergo redox reactions, leading to an elevation in the concentration of SO42- and NO3-. Additionally, the introduction of NH3 resulted in increased concentrations of SO42-, NO3-, and NO2-, highlighting the significant role of NH3 neutralization in CPM nucleation. The condensation of SO3/sulfuric acid aerosols was enhanced under saturation conditions, and SO2 and SO3/sulfuric acid aerosols could contribute synergistically to the formation of SO42-. Moreover, morphological analysis revealed the presence of both well-aggregated solid CPM and dispersed liquid CPM, confirming the formation of inorganic CPM during fast condensation. Furthermore, the detected CPM were composed of S and O, which identified the significant role of sulfates in the inorganic CPM. These findings provide valuable insights for the control of inorganic CPM in flue gas systems.

Challenges in moving towards fog's contribution to spatial patterns of atmospheric deposition fluxes on a national scale

Fog is an important environmental phenomenon affecting, among other things, geochemical cycles via atmospheric deposition pathways. It is generally accepted that fog contributes substantially to atmospheric deposition fluxes especially in mountain forests. Nevertheless, due to intrinsic constraints, fog pathway has thus far been neglected in the quantification of atmospheric deposition and fog pathway has not been accounted for in nation-wide spatial patterns of atmospheric deposition of air pollutants. In this review we explore the causes as to why it is so complex to create a spatial pattern of fog contribution to atmospheric ion deposition fluxes on a national scale. Physical and chemical principles of fog formation are presented and factors influencing the abrupt temporal and spatial changes in both fog occurrence and fog chemistry are elucidated. The focus is on both constituents essential for fog deposition flux quantification, i.e. (i) hydrological input on fog water and (ii) chemistry of fog water.

Atmospheric brown carbon in China haze is dominated by secondary formation

Brown carbon (BrC) is a class of light-absorbing organic aerosols (OA) and has significant influence on atmospheric radiative forcing. However, the current limited understanding of the physicochemical properties of BrC restricts the accurate evaluation of its environmental effects. Here the optical characteristics and chemical composition of BrC during wintertime in the Yangtze River Delta (YRD) region, China were measured by using high-resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectrometry. Our results showed that BrC in PM2.5 during the campaign was dominated by water-soluble organics, which consist of less oxidized oxygenated OA (LO-OOA), more oxidized oxygenated OA (MO-OOA), fossil fuel OA (FFOA) and biomass burning OA (BBOA). MO-OOA and BBOA were the strongest light absorbing BrC at 365 nm (Abs365), followed by LO-OOA and FFOA with a mass absorption coefficient (MAC) being 0.74 ± 0.04, 0.73 ± 0.03, 0.48 ± 0.04 and 0.39 ± 0.06 m2 g-1 during the campaign, respectively. In the low relative humidity (RH < 80 %) haze periods Abs365 of LO-OOA contributed to 44 % of the total light absorption at 365 nm, followed by MO-OOA (31 %), FFOA (21 %) and BBOA (4 %). In contrast, in the high-RH (RH > 80 %) haze periods Abs365 was dominated by MO-OOA, which accounted for 62 % of the total Abs365, followed by LO-OOA (17 %), BBOA (13 %) and FFOA (8 %). Chemical composition analysis further showed that LO-OOA and MO-OOA are produced from gas-phase photooxidation of VOCs and aerosol aqueous reactions, respectively, in which ammonia significantly enhanced the formation and light absorption of BrC in the high RH haze period. On average, >75 % of the total Abs365nm in the YRD region during the haze events was contributed by LO-OOA and MO-OOA, suggesting that atmospheric BrC in China haze periods is predominantly formed by secondary reactions.

Modeling Indoor Inorganic Aerosol Concentrations During the ATHLETIC Campaign with IMAGES

In 2018, the ATHLETIC campaign was conducted at the University of Colorado Dal Ward Athletic Center and characterized dynamic indoor air composition in a gym environment. Among other parameters, inorganic particle and gas-phase species were alternatingly measured in the gym's supply duct and weight room. The Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) uses the inorganic aerosol thermodynamic equilibrium model, ISORROPIA, to estimate the partitioning of inorganic aerosols and corresponding gases. In this study herein, measurements from the ATHLETIC campaign were used to evaluate IMAGES' performance. Ammonia emission rates, nitric acid deposition, and particle deposition velocities were related to observed occupancy, which informed these rates in IMAGES runs. Initially, modeled indoor inorganic aerosol concentrations were not in good agreement with measurements. A parametric investigation revealed that lowering the temperature or raising the relative humidity used in the ISORROPIA model drove the semivolatile species more toward the particle phase, substantially improving modeled-measured agreement. One speculated reason for these solutions is that aerosol water was enhanced by increasing the RH or decreasing the temperature. Another is that thermodynamic equilibrium was not established in this indoor setting or that the thermodynamic parametrizations in ISORROPIA are less accurate for typical indoor settings. This result suggests that applying ISORROPIA indoors requires further careful experimental validation.

Enhanced aqueous formation and neutralization of fine atmospheric particles driven by extreme cold

The prevailing view for aqueous secondary aerosol formation is that it occurs in clouds and fogs, owing to the large liquid water content compared to minute levels in fine particles. Our research indicates that this view may need reevaluation due to enhancements in aqueous reactions in highly concentrated small particles. Here, we show that low temperature can play a role through a unique effect on particle pH that can substantially modulate secondary aerosol formation. Marked increases in hydroxymethanesulfonate observed under extreme cold in Fairbanks, Alaska, demonstrate the effect. These findings provide insight on aqueous chemistry in fine particles under cold conditions expanding possible regions of secondary aerosol formation that are pH dependent beyond conditions of high liquid water.

New insights into black carbon light absorption enhancement: A comprehensive analysis of two differential behaviors

High uncertainty in optical properties of black carbon (BC) involving heterogeneous chemistry has recently attracted increasing attention in the field of atmospheric climatology. To fill the gap in BC optical knowledge so as to estimate more accurate climate effects and serve the response to global warming, it is beneficial to conduct site-level studies on BC light absorption enhancement (Eabs) characteristics. Real-time surface gas and particulate pollutant observations during the summer and winter over Wuhan were utilized for the analysis of Eabs simulated by minimum R squared (MRS), considering two distinct atmospheric conditions (2015 and 2017). In general, differences in aerosol emissions led to Eabs differential behaviors. The summer average of Eabs (1.92 ± 0.55) in 2015 was higher than the winter average (1.27 ± 0.42), while the average (1.11 ± 0.20) in 2017 summer was lower than that (1.67 ± 0.69) in winter. Eabs and RBC (representing the mass ratio of non-refractory constituents to elemental carbon) constraints suggest that Eabs increased with the increase in RBC under the ambient condition enriched by secondary inorganic aerosol (SIA), with a maximum growth rate of 70.6% in 2015 summer. However, Eabs demonstrated a negative trend against RBC in 2017 winter due to the more complicated mixing state. The result arose from the opposite impact of hygroscopic SIA and absorbing OC/irregular distributed coatings on amplifying the light absorbency of BC. Furthermore, sensitivity analysis revealed a robust positive correlation (R > 0.9) between aerosol chemical compositions (including sulfate, nitrate, ammonium and secondary organic carbon), which could be significantly perturbed by only a small fraction of absorbing materials or restructuring BC through gaps filling. The above findings not only deepen the understanding of BC, but also provide useful information for the scientific decision-making in government to mitigate particulate pollution and obtain more precise BC radiative forcing.

Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

Remote sensing estimation of regional PM2.5 based on GTWR model -A case study of southwest China

Air pollution in China has becoming increasingly serious in recent years with frequent incidents of smog. Parts of southwest China still experience high incidents of smog, with PM2.5 (particulate matter with diameter ≤2.5 μm) being the main contributor. Establishing the spatial distribution of PM2.5 in Southwest China is important for safeguarding regional human health, environmental quality, and economic development. This study used remote sensing (RS) and geographical information system (GIS) technologies and aerosol optical depth (AOD), digital elevation model (DEM), normalized difference vegetation index (NDVI), population density, and meteorological data from January to December 2018 for southwest China. PM2.5 concentrations were estimated using ordinary least squares regression (OLS), geographic weighted regression (GWR) and geographically and temporally weighted regression (GTWR). The results showed that: (1) Eight influencing factors showed different correlations to PM2.5 concentrations. However, the R2 values of the correlations all exceeded 0.3, indicating a moderate degree of correlation or more; (2) The correlation R2 values between the measured and remote sensed estimated PM2.5 data by OLS, GWR, and GTWR were 0.554, 0.713, and 0.801, respectively; (3) In general, the spatial distribution of PM2.5 in southwest of China decreases from the Northeast to Northwest, with moderate concentrations in the Southeast and Southwest; (4) The seasonal average PM2.5 concentration is high in winter, low in summer, and moderate in spring and autumn, whereas the monthly average shows a "V" -shaped oscillation change.

Size-resolved water-soluble organic carbon and its significant contribution to aerosol liquid water

Size-segregated aerosols collected in Beijing from 2021 to 2022 were used to investigate the contribution of organic aerosols to the aerosol liquid water content (ALWC), the influencing factors of ALWC, and the concentrations and size distribution characteristics of water-soluble organic carbon (WSOC) after clean air actions. The results showed that the concentration of WSOC in particulate matter (PM)1.8 was 3.52 ± 2.43 μg/m3 during the sampling period. Obvious changes were observed in the size distribution of WSOC after clean air actions, which may be attributed to the enhancement of atmospheric oxidation capacity and the decrease in PM concentration. The contribution of organic aerosols to the ALWC in fine PM was 18.1 % during the sampling period, which was more significant at lower particles concentration and smaller particle size ranges. The ambient relative humidity (RH) and the ratio of NO3-/SO42- had an apparent influence on ALWC. The continuous increase in the nitrate proportion significantly reduced the deliquescence point of the aerosols, making them prone to hygroscopic growth at lower RH. Analysis of the relation among nitrogen oxidation ratio (sulfur oxidation ratio), ALWC and PM1.8 mass concentrations suggests that organic matter has a significant effect on the formation of secondary inorganic aerosols in the initial phase of pollution formation and plays a crucial role in aerosol pollution formation in Beijing. These results are conducive to understanding the formation mechanism of aerosols and provide scientific data and theoretical support for the formulation of more effective emission-reduction measures.

PM2.5 exposure contributes to anxiety and depression-like behaviors via phenyl-containing compounds interfering with dopamine receptor

As a global problem, fine particulate matter (PM2.5) really needs local fixes. Considering the increasing epidemiological relevance to anxiety and depression but inconsistent toxicological results, the most important question is to clarify whether and how PM2.5 causally contributes to these mental disorders and which components are the most dangerous for crucial mitigation in a particular place. In the present study, we chronically subjected male mice to a real-world PM2.5 exposure system throughout the winter heating period in a coal combustion area and revealed that PM2.5 caused anxiety and depression-like behaviors in adults such as restricted activity, diminished exploratory interest, enhanced repetitive stereotypy, and elevated acquired immobility, through behavioral tests including open field, elevated plus maze, marble-burying, and forced swimming tests. Importantly, we found that dopamine signaling was perturbed using mRNA transcriptional profile and bioinformatics analysis, with Drd1 as a potential target. Subsequently, we developed the Drd1 expression-directed multifraction isolating and nontarget identifying framework and identified a total of 209 compounds in PM2.5 organic extracts capable of reducing Drd1 expression. Furthermore, by applying hierarchical characteristic fragment analysis and molecular docking and dynamics simulation, we clarified that phenyl-containing compounds competitively bound to DRD1 and interfered with dopamine signaling, thereby contributing to mental disorders. Taken together, this work provides experimental evidence for researchers and clinicians to identify hazardous factors in PM2.5 and prevent adverse health outcomes and for local governments and municipalities to control source emissions for diminishing specific disease burdens.

Direct aqueous photochemistry of methylglyoxal and its effect on sulfate formation

In recent years, among many oxidation pathways studied for atmospheric sulfate formation, the aqueous phase oxidation pathways of H2O2 and organic hydroperoxides (ROOHs) have attracted great scientific attention. Higher concentrations of H2O2 and ubiquitous ROOHs have been observed in atmospheric aqueous phase environments (cloud water, fog droplets, etc.). However, there are still some gaps in the study of their aqueous phase generation and their influences on sulfate formation. In this study, the aqueous phase photochemical reaction of methylglyoxal, a ubiquitous organic substance in the atmospheric aqueous phase, was studied under UV irradiation, and the generation of H2O2 and ROOHs in this system was investigated. It is found for the first time that the aqueous phase photolysis of methylglyoxal not only produces H2O2 but also produces ROOHs, and UV light and O2 are necessary for the formation of H2O2 and ROOHs. Based on the experimental results, the possible mechanism of aqueous phase photochemistry of methylglyoxal and the generation of H2O2 and ROOHs were proposed. The effect of aqueous phase photolysis of methylglyoxal on sulfate formation under different conditions was also investigated. The results show that the aqueous phase photolysis of methylglyoxal significantly promoted SO2 oxidation and sulfate formation, in which SO2 oxidation was realized by the generated H2O2, ROOHs and •OH radicals, and the importance of the formed ROOHs cannot be ignored. These results fill some gaps in the field of aqueous phase H2O2 and ROOHs production, and to a certain extent confirm the important roles of the aqueous phase photolysis of methylglyoxal and the formed H2O2 and ROOHs in the production of sulfate. The study reveals the new sources of H2O2 and ROOHs, and provides a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.

Nitrogen isotopic characteristics of aerosol ammonium in a Chinese megacity indicate the reduction from vehicle emissions during the lockdown period

The role of agricultural versus vehicle emissions in urban atmospheric ammonia (NH3) remains unclear. The lockdown due to the outbreak of COVID-19 provided an opportunity to assess the role of source emissions on urban NH3. Concentrations and δ15N of aerosol ammonium (NH4+) were measured before (autumn in 2017) and during the lockdown (summer, autumn, and winter in 2020), and source contributions were quantified using SIAR. Despite the insignificant decrease in NH4+ concentrations, significantly lower δ15N-NH4+ was found in 2020 (0.6 ± 1.0‰ in PM2.5 and 1.4 ± 2.1‰ in PM10) than in 2017 (15.2 ± 6.7‰ in PM2.5), which indicates the NH3 from vehicle emissions has decreased by∼50% during the lockdown while other source emissions are less affected. Moreover, a reversed seasonal pattern of δ15N-NH4+ during the lockdown in Changsha has been revealed compared to previous urban studies, which can be explained by the dominant effect of non-fossil fuel emissions due to the reductions of vehicle emissions during the lockdown period. Our results highlight the effects of lockdown on aerosol δ15N-NH4+ and the importance of vehicle emissions to urban atmospheric NH3, providing conclusive evidence that reducing vehicle NH3 emissions could be an effective strategy to reduce PM2.5 in Chinese megacities.

Toxic Potencies of Particulate Matter from Typical Industrial Plants Mediated with Acidity via Metal Dissolution

Acidity is an important property of particulate matter (PM) in the atmosphere, but its association with PM toxicity remains unclear. Here, this study quantitively reports the effect of the acidity level on PM toxicity via pH-control experiments and cellular analysis. Oxidative stress and cytotoxicity potencies of acidified PM samples at pH of 1-2 were up to 2.8-5.2 and 2.1-13.2 times higher than those at pH of 8-11, respectively. The toxic potencies of PM samples from real-world smoke plumes at the pH of 2.3 were 9.1-18.2 times greater than those at the pH of 5.6, demonstrating a trend similar to that of acidified PM samples. Furthermore, the impact of acidity on PM toxicity was manifested by promoting metal dissolution. The dramatic increase by 2-3 orders of magnitude in water-soluble metal content dominated the variation in PM toxicity. The significant correlation between sulfate, the pH value, water-soluble Fe, IC20, and EC1.5 (p < 0.05) suggested that acidic sulfate could enhance toxic potencies by dissolving insoluble metals. The findings uncover the superficial association between sulfate and adverse health outcomes in epidemiological research and highlight the control of wet smoke plume emissions to mitigate the toxicity effects of acidity.

Dominant contribution of combustion-related ammonium during haze pollution in Beijing

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Exploring the interplay between particulate matter capture, wash-off, and leaf traits in green wall species

The study investigated inter-species variation in particulate matter (PM) accumulation, wash-off, and retention on green wall plants, with a focus on leaf characteristics. Ten broadleaf plant species were studied in an experimental green wall. Ambient PM concentrations remained relatively stable throughout the measurement period: PM1: 16.60 ± 9.97 μgm-3, PM2.5: 23.27 ± 11.88 μgm-3, and PM10: 39.59 ± 25.72 μgm-3. Leaf samples were taken before and after three rainfall events, and PM deposition was measured using Scanning Electron Microscopy (SEM). Leaf micromorphological traits, including surface roughness, hair density, and stomatal density, exhibited variability among species and leaf surfaces. Notably, I.sempervirens and H.helix had relatively high PM densities across all size fractions. The study underscored the substantial potential of green wall plants for atmospheric PM removal, with higher Wall Leaf Area Index (WLAI) species like A.maritima and T.serpyllum exhibiting increased PM accumulation at plant level. Rainfall led to significant wash-off for smaller particles, whereas larger particles exhibited lower wash-off rates. Leaf micromorphology impacted PM accumulation, although effects varied among species, and parameters such as surface roughness, stomatal density, and leaf size did not consistently affect PM deposition. The composition of deposited particles encompassed natural, vehicular, salt, and unclassified agglomerates, with minimal changes after rainfall. Air Pollution Tolerance Index (APTI) assessments revealed that I.sempervirens displayed the highest air pollution tolerance, while O.vulgare had the lowest. APTI showed a moderate positive correlation with PM deposition across all fractions. The study concluded that the interplay of macro and micromorphology in green wall plant species determines their PM removal potential. Further research is needed to identify the key leaf characteristics for optimal green wall species selection for effective PM removal.

Revealing the Contribution of Interfacial Processes to Atmospheric Oxidizing Capacity in Haze Chemistry

The atmospheric oxidizing capacity is the most important driving force for the chemical transformation of pollutants in the atmosphere. Traditionally, the atmospheric oxidizing capacity mainly depends on the concentration of O3 and other gaseous oxidants. However, the atmospheric oxidizing capacity based on gas-phase oxidation cannot accurately describe the explosive growth of secondary particulate matter under complex air pollution. From the chemical perspective, the atmospheric oxidizing capacity mainly comes from the activation of O2, which can be achieved in both gas-phase and interfacial processes. In the heterogeneous or multiphase formation pathways of secondary particulate matter, the enhancement of oxidizing capacity ascribed to the O2/H2O-involved interfacial oxidation and hydrolysis processes is an unrecognized source of atmospheric oxidizing capacity. Revealing the enhanced oxidizing capacity due to interfacial processes in high-concentration particulate matter environments and its contribution to the formation of secondary pollution are critical in understanding haze chemistry. The accurate evaluation of atmospheric oxidizing capacity ascribed to interfacial processes is also an important scientific basis for the implementation of PM2.5 and O3 collaborative control in China and around the world.

Elaborations of the influencing factors on the formation of secondary inorganic aerosols in a heavily polluted urban area of China

In this study, online water-soluble inorganic ions were detected to deduce the formation mechanism of secondary inorganic aerosol in Xianyang, China during wintertime. The dominant inorganic ions of sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) (the sum of those is abbreviated as SNA) accounted for 17%, 21%, and 12% of PM2.5 mass, respectively. While the air quality deteriorated from excellent to poor grades, the precursor gas sulfur dioxide (SO2) of SO42- increased and then decreased with a fluctuation, while nitrogen dioxide (NO2) and ammonia (NH3), precursors of NO3- and NH4+, and SNA show increasing trends. Meteorological factors including boundary layer height (BLH), temperature, and wind speed also show decline trends, except relative humidity (RH). Meanwhile, the secondary conversion ratio shows a remarkable increasing trend, indicating that there was a strong secondary transformation. From the perspective of chemical mechanisms, RH is positively correlated with sulfur oxidation ratios (SOR), nitrogen oxidation ratios (NOR), and ammonia conversion ratios, representing that the increase of humidity could promote the generation of SNA. Notably, SOR and NOR were also positively related to the ammonia. On the one hand, the low wind speed and BLH led to the accumulation of pollutants. On the other hand, the increases of RH and ammonia promoted more formations of SNA and PM2.5. The results advance our identification of the contributors to the haze episodes and assist to establish more efficient emission controls in Xianyang, in addition to other cities with similar emission and geographical characteristics.

Aerosol liquid water in PM2.5 and its roles in secondary aerosol formation at a regional site of Yangtze River Delta

Aerosol liquid water content (ALWC) plays an important role in secondary aerosol formation. In this study, a whole year field campaign was conducted at Shanxi in north Zhejiang Province during 2021. ALWC estimated by ISORROPIA-II was then investigated to explore its characteristics and relationship with secondary aerosols. ALWC exhibited a highest value in spring (66.38 µg/m3), followed by winter (45.08 µg/m3), summer (41.64 µg/m3), and autumn (35.01 µg/m3), respectively. It was supposed that the secondary inorganic aerosols (SIA) were facilitated under higher ALWC conditions (RH > 80%), while the secondary organic species tended to form under lower ALWC levels. Higher RH (> 80%) promoted the NO3- formation via gas-particle partitioning, while SO42- was generated at a relative lower RH (> 50%). The ALWC was more sensitive to NO3- (R = 0.94) than SO42- (R = 0.90). Thus, the self-amplifying processes between the ALWC and SIA enhanced the particle mass growth. The sensitivity of ALWC and OX (NO2 + O3) to secondary organic carbon (SOC) varied in different seasons at Shanxi, more sensitive to aqueous-phase reactions (daytime R = 0.84; nighttime R = 0.54) than photochemical oxidation (daytime R = 0.23; nighttime R = 0.41) in wintertime with a high level of OX (daytime: 130-140 µg/m3; nighttime: 100-140 µg/m3). The self-amplifying process of ALWC and SIA and the aqueous-phase formation of SOC will enhance aerosol formation, contributing to air pollution and reduction of visibility.

Interannual evolution of the chemical composition, sources and processes of PM2.5 in Chengdu, China: Insights from observations in four winters

The air quality in China has improved significantly in the last decade and, correspondingly, the characteristics of PM2.5 have also changed. We studied the interannual variation of PM2.5 in Chengdu, one of the most heavily polluted megacities in southwest China, during the most polluted season (winter). Our results show that the mass concentrations of PM2.5 decreased significantly year-by-year, from 195.8 ± 91.0 µg/m3 in winter 2016 to 96.1 ± 39.3 µg/m3 in winter 2020. The mass concentrations of organic matter (OM), SO42-, NH4+ and NO3- decreased by 49.6%, 57.1%, 49.7% and 28.7%, respectively. The differential reduction in the concentrations of chemical components increased the contributions from secondary organic carbon and NO3- and there was a larger contribution from mobile sources. The contribution of OM and NO3- not only increased with increasing levels of pollution, but also increased year-by-year at the same level of pollution. Four sources of PM2.5 were identified: combustion sources, vehicular emissions, dust and secondary aerosols. Secondary aerosols made the highest contribution and increased year-by-year, from 40.6% in winter 2016 to 46.3% in winter 2020. By contrast, the contribution from combustion sources decreased from 14.4% to 8.7%. Our results show the effectiveness of earlier pollution reduction policies and emphasizes that priority should be given to key pollutants (e.g., OM and NO3-) and sources (secondary aerosols and vehicular emissions) in future policies for the reduction of pollution in Chengdu during the winter months.

Secondary inorganic aerosols and aerosol acidity at different PM2.5 pollution levels during winter haze episodes in the Sichuan Basin, China

Wintertime fine particle (PM2.5) pollution remains to be perplexing air quality problems in many parts of China. In this study, PM2.5 compositions and aerosol acidity at different pollution levels at an urban cite in the southwest China's Sichuan Basin were investigated during a sustained winter haze episode. Organic matter was the most abundant component of PM2.5, followed by nitrate, sulfate and ammonium. Shares of organic aerosol in PM2.5 mass decreased with the elevated PM2.5 levels, while the enhancements of sulfate and secondary organic aerosol were much less than that of nitrate and ammonium during heavy pollution with increased ratios of nitrate to sulfate, implying a significant role of nitrate in the haze formation. Results also suggest the nighttime chemistry might contribute substantially to the formation of nitrate under severe pollutions. The daily average aerosol pH showed a decreasing trend with the elevated levels of PM2.5, and this increased aerosl acidity was mainly due to the fast rising secondary inorganic aerosol (SIA) concentration, with the increase in hydronium ion concentration in air (Hair+) surpassing the dilution effect of elevated aerosol liquid water content (LWC). Thermodynamic model calculations revealed that the air environment was NH3-rich with total NHx (NH3 + NH4+) greater than required NHx, and the aerosol pH exponentially declined with the decreasing excess NHx (p < 0.01). This study demonstrated that under air stagnation and NH3-rich environment during winter, the raised relative humidity (RH) would lead to an increase in LWC and thereby facilitate the aqueous chemistry processes with the neutralization capacity of NH3 to form sulfate and nitrate, which would further increase the LWC and lower the pH. This self-amplifying SIA formation might be crucial to the severe PM2.5 pollution and haze events during winter, and therefore cutting both NOx and NH3 emissions would benefit stopping the self-amplification.

Single Droplet Tweezer Revealing the Reaction Mechanism of Mn(II)-Catalyzed SO2 Oxidation

Sulfate aerosol is one of the major components of secondary fine particulate matter in urban haze that has crucial impacts on the social economy and public health. Among the atmospheric sulfate sources, Mn(II)-catalyzed SO2 oxidation on aerosol surfaces has been regarded as a dominating one. In this work, we measured the reaction kinetics of Mn(II)-catalyzed SO2 oxidation in single droplets using an aerosol optical tweezer. We show that the SO2 oxidation occurs at the Mn(II)-active sites on the aerosol surface, per a piecewise kinetic formulation, one that is characterized by a threshold surface Mn(II) concentration and gaseous SO2 concentration. When the surface Mn(II) concentration is lower than the threshold value, the reaction rate is first order with respect to both Mn(II) and SO2, agreeing with our traditional knowledge. But when surface Mn(II) concentration is above the threshold, the reaction rate becomes independent of Mn(II) concentration, and the reaction order with respect to SO2 becomes greater than unity. The measured reaction rate can serve as a tool to estimate sulfate formation based on field observation, and our established parametrization corrects these calculations. This framework for reaction kinetics and parametrization holds promising potential for generalization to various heterogeneous reaction pathways.

High heterogeneity and aging state of mineral particles in a slowly-moving dust plume on the southwestern coast of Japan

Aerosol particles in two size ranges, namely 0.18-1.4 μm (fine) and larger than 1.4 μm (coarse), were collected in the pre-dust, in-dust, and post-dust air during the passage of a slowly-moving dust event at a coastal site in southwestern Japan. We identified the composition and size of individual particles using a scanning electron microscope to investigate the variations during dust passage. The particles could be classified as mineral-seasalt mixtures, non-mixture minerals, sulfur-containing minerals, and seasalt particles, and the number fractions of these type particles in the two size ranges exhibited significant variation across the three periods. In the coarse size range, mixture particles accounted for 17.6 %, 26.8 %, and 37.8 % of the particles in the pre-dust, in-dust, and post-dust air, respectively. Non-mixture particles made up 36.8 %, 29.2 %, and 24.3 % in the same respective periods. In the in-dust air, the average relative ratio of sulfur content in sulfur-containing mineral particles in the coarse range was 5.5 %, whereas in the fine range, it was 17.2 %. The aging state of sea salt components, described by the Cl loss and reflecting the changes in particles due to chemical reactions, exhibited significant differences in the two size ranges. In the fine range, the aging of >90 % particles was predominantly influenced by sulfate formation in the in-dust air. In contrast, nitrate likely played a certain role in both the pre-dust and post-dust air. In the coarse range, the aging was independent of sulfate formation. These results indicate the close dependence of the aging of dust particles on their size and the notable variations of the aged states, underscoring the essentiality to treat dust particles properly according to time and space for a better understanding on their roles in the marine atmosphere.

Significantly Accelerated Photosensitized Formation of Atmospheric Sulfate at the Air-Water Interface of Microdroplets

The multiphase oxidation of sulfur dioxide (SO2) to form sulfate is a complex and important process in the atmosphere. While the conventional photosensitized reaction mainly explored in the bulk medium is reported to be one of the drivers to trigger atmospheric sulfate production, how this scheme functionalizes at the air-water interface (AWI) of aerosol remains an open question. Herein, employing an advanced size-controllable microdroplet-printing device, surface-enhanced Raman scattering (SERS) analysis, nanosecond transient adsorption spectrometer, and molecular level theoretical calculations, we revealed the previously overlooked interfacial role in photosensitized oxidation of SO2 in humic-like substance (HULIS) aerosol, where a 3-4 orders of magnitude increase in sulfate formation rate was speculated in cloud and aerosol relevant-sized particles relative to the conventional bulk-phase medium. The rapid formation of a battery of reactive oxygen species (ROS) comes from the accelerated electron transfer process at the AWI, where the excited triplet state of HULIS (3HULIS*) of the incomplete solvent cage can readily capture electrons from HSO3- in a way that is more efficient than that in the bulk medium fully blocked by water molecules. This phenomenon could be explained by the significantly reduced desolvation energy barrier required for reagents residing in the AWI region with an open solvent shell.

Exposure to Airborne PM2.5 Water-Soluble Inorganic Ions Induces a Wide Array of Reproductive Toxicity

Water-soluble inorganic ions (WSIIs, primarily NH4+, SO42-, and NO3-) are major components in ambient PM2.5, but their reproductive toxicity remains largely unknown. An animal study was conducted where parental mice were exposed to PM2.5 WSIIs or clean air during preconception and the gestational period. After delivery, all maternal and offspring mice lived in a clean air environment. We assessed reproductive organs, gestation outcome, birth weight, and growth trajectory of the offspring mice. In parallel, we collected birth weight and placenta transcriptome data from 150 mother-infant pairs from the Rhode Island Child Health Study. We found that PM2.5 WSIIs induced a broad range of adverse reproductive outcomes in mice. PM2.5 NH4+, SO42-, and NO3- exposure reduced ovary weight by 24.22% (p = 0.005), 14.45% (p = 0.048), and 16.64% (p = 0.022) relative to the clean air controls. PM2.5 SO42- exposure reduced the weight of testicle by 5.24% (p = 0.025); further, mice in the PM2.5 SO42- exposure group had 1.81 (p = 0.027) fewer offspring than the control group. PM2.5 NH4+, SO42-, and NO3- exposure all led to lower birth than controls. In mice, 557 placenta genes were perturbed by exposure. Integrative analysis of mouse and human data suggested hypoxia response in placenta as an etiological mechanism underlying PM2.5 WSII exposure's reproductive toxicity.

Molecular signatures and formation mechanisms of water-soluble chromophores in particulate matter from Karachi in Pakistan

Excitation-emission matrix (EEM) fluorescence spectroscopy is a widely-used method for characterizing the chemical components of brown carbon (BrC). However, the molecular basics and formation mechanisms of chromophores, which are decomposed by parallel factor (PARAFAC) analysis, are not yet fully understood. In this study, we characterized the water-soluble organic carbon (WSOC) in aerosols collected from Karachi, Pakistan, using EEM spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We identified three PARAFAC components, including two humic-like components (C1 and C2) and one phenolic-like species (C3). We determined the molecular families associated with each component by performing Spearman correlation analysis between FT-ICR MS peaks and PARAFAC component intensities. We found that the C1 and C2 components were associated with nitrogen-enriched compounds, where C2 with the longest emission wavelength exhibited a higher level of aromaticity, N content, and oxygenation than C1. The C3 associated formulas have fewer nitrogen-containing species, a lower unsaturation degree, and a lower oxidation state. An oxidation pathway was identified as an important process in the formation of C1 and C2 components at the molecular level, particularly for the assigned CHON compounds associated with the gas-phase oxidation process, despite their diverse precursor types. Numerous C2 formulas were found in the "potential BrC" region and overlapped with the BrC-associated formulas. It can be inferred that the compounds that fluoresce C2 contributed considerably to the light absorption of BrC. These findings are essential for future studies utilizing the EEM-PARAFAC method to explore the sources, processes, and compositions of atmospheric BrC.

Wintertime fine aerosol particles composition and its evolution in two megacities of southern and northern China

Study on fine aerosols composition can help understand the particles formation and is crucial for improving the accuracy of model simulations. Based on field data measured by a Q-ACSM (Quadrupole-Aerosol Chemical Speciation Monitor), we have comprehensively compared the characteristics, evolution, and potential formation mechanisms of the components in NR-PM2.5 during wintertime at two megacities (Beijing and Guangzhou) of southern and northern China. We show that as PM pollution intensifies, the mass fraction of the primary aerosols (e.g., COA, HOA) in PM2.5 in Guangzhou increased, along with a slight decline in proportion of both the secondary organic (SOA) and inorganic (SIA) aerosols; In contrast, in Beijing, the proportion of the SIA ramped up from 28 % to 53 % with the pollution evolution; and the fraction of SOA in total OA also increased due to a substantial increment in the proportion of MO-OOA (from 29 % to 48 %), suggesting a significance of the secondary processes in worsening aerosols pollution in Beijing. Our further analysis demonstrates a leading role of aqueous pathway in the secondary formation of aerosols at the Beijing site, presenting an exponential rising of SIA and SOA with the relative humidity (RH) increase. Compared to Beijing, however, we find that the photochemical oxidation other than aqueous process in Guangzhou plays a more critical role in those secondary aerosols formation. Combined with the Hysplit trajectory model, we identify the high humid conditions in Guangzhou are typically affected by clean marine air masses, explaining the slower response of secondary components to the RH changes. Moreover, the particles in Guangzhou were observed less hygroscopic that is adverse to the aerosol aqueous chemistry. The results provide basis for the precise control of PM pollution in different regions across China and would be helpful in improving model simulations.

Dominant physical and chemical processes impacting nitrate in Shandong of the North China Plain during winter haze events

Nitrate has been a dominant component of PM2.5 since the stringent emission control measures implemented in China in 2013. Clarifying key physical and chemical processes influencing nitrate concentrations is crucial for eradicating heavy air pollution in China. In this study, we explored dominant processes impacting nitrate concentrations in Shandong of the North China Plain during three haze events from 9 to 25 December 2021, named cases P1 (94.46 (30.85) μg m-3 for PM2.5 (nitrate)), P2 (148.95 (50.12) μg m-3) and P3 (88.03 (29.21) μg m-3), by using the Weather Research and Forecasting/Chemistry model with an integrated process rate analysis scheme and updated heterogeneous hydrolysis of dinitrogen pentoxide on the wet aerosol surface (HET-N2O5) and additional nitrous acid (HONO) sources (AS-HONO). The results showed that nitrate increases in the three cases were attributed to aerosol chemistry, whereas nitrate decreases were due mainly to the vertical mixing process in cases P1 and P2 and to the advection process in case P3. HET-N2O5 (the reaction of OH + NO2) contributed 45 % (51 %) of the HNO3 production rate during the study period. AS-HONO produced a nitrate enhancement of 24 % in case P1, 12 % in case P2 and 19 % in case P3, and a HNO3 production rate enhancement of 0.79- 0.97 (0.18- 0.60) μg m-3 h-1 through the reaction of OH + NO2 (HET-N2O5) in the three cases. This study implies that using suitable parameterization schemes for heterogeneous reactions on aerosol and ground surfaces and nitrate photolysis is vital in simulations of HONO and nitrate, and the MOSAIC module for aerosol water simulations needs to be improved.

Spatiotemporal empirical analysis of particulate matter PM2.5 pollution and air quality index (AQI) trends in Africa using MERRA-2 reanalysis datasets (1980-2021)

In this study, the spatial-temporal trends of PM2.5 pollution were analyzed for subregions in Africa and the entire continent from 1980 to 2021. The distributions and trends of PM2.5 were derived from the monthly concentrations of the aerosol species from MERRA-2 reanalysis datasets comprising of sulphates (SO4), organic carbon (OC), black carbon (BC), Dust2.5 and Sea Salt (SS2.5). The resulting PM2.5 trends were compared with the climate factors, socio-economic indicators, and terrain characteristics. Using the Mann-Kendall (M-K) test, the continent and its subregions showed positive trends in PM2.5 concentrations, except for western and central Africa which exhibited marginal negative trends. The M-K trends also determined Dust2.5 as the dominant contributing aerosol factor responsible for the high PM2.5 concentrations in the northern, western and central regions of Africa, while SO4 and OC were respectively the most significant contributors to PM2.5 in the eastern and southern Africa regions. For the climate factors, the PM2.5 trends were determined to be positively correlated with the wind speed trends, while precipitation and temperature trends exhibited low and sometimes negative correlations with PM2.5. Socio-economically, highly populated, and bare/sparse vegetated areas showed higher PM2.5 concentrations, while vegetated areas tended to have lower PM2.5 concentrations. Topographically, low laying regions were observed to retain the deposited PM2.5 especially in the northern and western regions of Africa. The Air Quality Index (AQI) results showed that 94 % of the continent had an average PM2.5 of 12-35 μg/m3 hence classified as "Moderate" AQI, and the rest of the continent's PM2.5 levels was between 35 and 55 μg/m3 implying AQI classification of "Unhealthy for Sensitive People". Northern and western Africa regions had the highest AQI, while southern Africa had the lowest AQI. The approach and findings in this study can be used to complement the evaluation and management of air quality in Africa.

Multiphase interactions between sulfur dioxide and secondary organic aerosol from the photooxidation of toluene: Reactivity and sulfate formation

There is growing evidence that the interactions between sulfur dioxide (SO2) and organic peroxides (POs) in aerosol and clouds play an important role in atmospheric sulfate formation and aerosol aging, yet the reactivity of POs arising from anthropogenic precursors toward SO2 remains unknown. In this study, we investigate the multiphase reactions of SO2 with secondary organic aerosol (SOA) formed from the photooxidation of toluene, a major type of anthropogenic SOA in the atmosphere. The reactive uptake coefficient of SO2 on toluene SOA was determined to be on the order of 10-4, depending strikingly on aerosol water content. POs contribute significantly to the multiphase reactivity of toluene SOA, but they can only explain a portion of the measured SO2 uptake, suggesting the presence of other reactive species in SOA that also contribute to the particle reactivity toward SO2. The second-order reaction rate constant (kII) between S(IV) and toluene-derived POs was estimated to be in the range of the kII values previously reported for commercially available POs (e.g., 2-butanone peroxide and 2-tert-butyl hydroperoxide) and the smallest (C1-C2) and biogenic POs. In addition, unlike commercial POs that can efficiently convert S(IV) into both inorganic sulfate and organosulfates, toluene-derived POs appear to mainly oxidize S(IV) to inorganic sulfate. Our study reveals the multiphase reactivity of typical anthropogenic SOA and POs toward SO2 and will help to develop a better understanding of the formation and evolution of atmospheric secondary aerosol.

Elucidating the Mechanism on the Transition-Metal Ion-Synergetic-Catalyzed Oxidation of SO2 with Implications for Sulfate Formation in Beijing Haze

Currently, atmospheric sulfate aerosols cannot be predicted reliably by numerical models because the pathways and kinetics of sulfate formation are unclear. Here, we systematically investigated the synergetic catalyzing role of transition-metal ions (TMIs, Fe3+/Mn2+) in the oxidation of SO2 by O2 on aerosols using chamber experiments. Our results showed that the synergetic effect of TMIs is critically dependent on aerosol pH due to the solubility of Fe(III) species sensitive to the aqueous phase acidity, which is effective only under pH < 3 conditions. The sulfate formation rate on aerosols is 2 orders of magnitude larger than that in bulk solution and increases significantly on smaller aerosols, suggesting that such a synergetic-catalyzed oxidation occurs on the aerosol surface. The kinetic reaction rate can be described as R = k*[H+]-2.95[Mn(II)][Fe(III)][S(IV)] (pH ≤ 3.0). We found that TMI-synergetic-catalyzed oxidation is the dominant pathway of sulfate formation in Beijing when haze particles are very acidic, while heterogeneous oxidation of SO2 by NO2 is the most important pathway when haze particles are weakly acidic. Our work for the first time clarified the role and kinetics of TMI-synergetic-catalyzed oxidation of SO2 by O2 in haze periods, which can be parameterized into models for future studies of sulfate formation.

Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective

Sulfate (SO42-) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO42- aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM2.5 samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO42- (nss-SO42-) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM2.5 in Haikou. The S(IV)+H2O2 pathway was the main formation pathway for secondary SO42- in wintertime in Haikou, contributing to 57% of secondary SO42- formation. By contrast, 54% of secondary SO42- was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H2O2, S(IV)+Fe×Mn, and S(IV)+NO2 pathways contributed equally to secondary SO42- formation. The ionic strength was the controlling parameter for the S(IV)+NO2 pathway, while pH was identified as a key factor that mediates the S(IV)+H2O2 and S(IV)+Fe×Mn pathways to produce secondary SO42-. This study contributes to our understanding of secondary SO42- production under low PM2.5 concentrations but high SO42- percentages.

The Variation in Chemical Composition and Source Apportionment of PM2.5 before, during, and after COVID-19 Restrictions in Zhengzhou, China

Despite significant improvements in air quality during and after COVID-19 restrictions, haze continued to occur in Zhengzhou afterwards. This paper compares ionic compositions and sources of PM2.5 before (2019), during (2020), and after (2021) the restrictions to explore the reasons for the haze. The average concentration of PM2.5 decreased by 28.5% in 2020 and 27.9% in 2021, respectively, from 102.49 μg m-3 in 2019. The concentration of secondary inorganic aerosols (SIAs) was 51.87 μg m-3 in 2019, which decreased by 3.1% in 2020 and 12.8% in 2021. In contrast, the contributions of SIAs to PM2.5 increased from 50.61% (2019) to 68.6% (2020) and 61.2% (2021). SIAs contributed significantly to PM2.5 levels in 2020-2021. Despite a 22~62% decline in NOx levels in 2020-2021, the increased O3 caused a similar NO3- concentration (20.69~23.00 μg m-3) in 2020-2021 to that (22.93 μg m-3) in 2019, hindering PM2.5 reduction in Zhengzhou. Six PM2.5 sources, including secondary inorganic aerosols, industrial emissions, coal combustion, biomass burning, soil dust, and traffic emissions, were identified by the positive matrix factorization model in 2019-2021. Compared to 2019, the reduction in PM2.5 from the secondary aerosol source in 2020 and 2021 was small, and the contribution of secondary aerosol to PM2.5 increased by 13.32% in 2020 and 12.94% in 2021. In comparison, the primary emissions, including biomass burning, traffic, and dust, were reduced by 29.71% in 2020 and 27.7% in 2021. The results indicated that the secondary production did not significantly contribute to the PM2.5 decrease during and after the COVID-19 restrictions. Therefore, it is essential to understand the formation of secondary aerosols under high O3 and low precursor gases to mitigate air pollution in the future.

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

In situ biomass burning enhanced the contribution of biogenic sources to sulfate aerosol in subtropical cities

Sulfurous gases released by biogenic sources play a key role in the global sulfur cycle. However, the contribution of biogenic sources to sulfate aerosol in the urban atmosphere has received little attention. Emission sources and formation process of sulfate in Guangzhou, a subtropical mega-city in China, were clarified using multiple methods, including isotope tracers and chemical markers. The δ18O of sulfate suggested that secondary sulfate was the dominant component (84 %) of sulfate aerosol, which mainly formed by transition metal ion (TMI) catalyzed oxidation (31 %) and OH radical oxidation (30 %). The factors driving secondary sulfate formation were revealed using a tree boosting model, which suggested that NH3, temperature, and oxidants were the most important factors. The δ34S of sulfate indicated that biogenic sources accounted for annual average of 26.0 % of the sulfate, which increased to 30.4 % in winter monsoon period. Rice straw burning enhanced sulfate formation by promoting the release of reduced sulfur from soil, which is rapidly converted into sulfate under a subtropical urban atmosphere with high concentration of NH3 and oxidants. This study revealed the important influence of rice straw burning on biogenic sulfur emission during the rice harvest, thereby providing insight into the sulfur cycle and regional air pollution.

Comparison of chemical composition and acidity of size-resolved inorganic aerosols at the top and foot of Mt. Hua, Northwest China: The role of the gas-particle distribution of ammonia

Aerosol pH is not only a diagnostic indicator of secondary aerosol formation, but also a key factor in the specific chemical reaction routes that produce sulfate and nitrate. To understand the characteristics of aerosol acidity in the Mt. Hua, the chemical fractions of water-soluble inorganic ions in the atmospheric PM2.5 and size-resolved particle at the top and foot of Mt. Hua in summer 2020 were studied. The results showed the mass concentrations of PM2.5 and water-soluble ions at the foot were 2.0-2.6 times higher than those at the top. The secondary inorganic ions, i.e., SO42-, NO3-, and NH4+ (SNA) were 56 %-61 % higher by day than by night. SO42- was mainly distributed in the fine particles (Dp < 2.1 μm). NO3- showed a unimodal size distribution (peaking at 0.7-1.1 μm) at the foot and a bimodal (0.7-1.1 μm and 4.7-5.8 μm) size distribution at the top. At the top site, the distribution of NO3- in coarse particles (> 2.1 μm) was mainly attributed to the gaseous HNO3 volatilized from fine particles reacting with cations in coarse particles to form non-volatile salts (such as Ca(NO3)2). The pH values of PM2.5 were 2.7 ± 1.3 and 3.3 ± 0.42 at the top and foot, respectively. NH4+/NH3(g) plays a decisive role in stabilizing aerosol acidity. In addition, the increase of the liquid water content (LWC) at the foot facilitates the gas-particle conversion of NH3, while the H+ concentration was diluted, resulting in a decrease in acidity at the foot. NH4+/NH3 had good linear correlations with SO42-, NO3-, and LWC during the daytime at both sites, indicating that SO42-, NO3-, and LWC together affect the gas-particle distribution of ammonia by day: however, the effect of LWC at night was not evident.

Impacts of local soil and vehicle NOx emissions on ground-level NO2 concentrations on a university campus in the city of Shenyang, China

Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant, and fossil fuel combustion is generally considered its predominant source in and around urban areas. As the total nitrogen deposition is high over here, soil NOx emissions from urban green space might also be an important local source of ground-level NO2. In this study, Willems badge samplers were employed to monitor the spatial and seasonal variations of 2-week mean atmospheric NO2 concentrations at a height of 1.7 m on an urban campus in Northeast China from November 2020 to December 2021. We found considerable small-scale spatial variations of ground-level NO2 concentrations on the campus during the growing season, with local soil NOx emissions as the main driver. According to its linear correlation with green space coverage, the increment in ground-level NO2 concentration was partitioned into two components, with one ascribed to the local soil source (referred to as NO2-Isoil) and the other the local vehicle source (NO2-Ivehicle). NO2-Isoil generally reached a maximum (as high as 25.6 μg/m3) during early spring, while its ratio to the background value generally reached a maximum (could be >1) during late spring and could reach 0.52 to 0.92 during summer. Therefore, soil NOx emissions were an important source of ground-level NO2 on the campus, with the contribution even higher than those of other anthropogenic sources during late spring. Even with light traffic on the campus, NO2-Ivehicle could reach 0.48 times the background value at a site with high frequencies of warm starts.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

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 SO2 oxidation at different levels of air pollution. The Δ33S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ33S signatures (0‰ < Δ33S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Δ33S signatures (-0.236‰ < Δ33S < ∼0‰) observed during winter haze events (PM2.5 > 200 μg/m3) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 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 SO2 can produce significant negative Δ33S values in sulfate aerosols.

Insight into the Mechanism and Kinetics of the Heterogeneous Reaction between SO2 and NO2 on Diesel Black Carbon under Light Irradiation

The heterogeneous oxidation of SO2 by NO2 has been extensively proposed as an important pathway of sulfate production during haze events in China. However, the kinetics and mechanism of oxidation of SO2 by NO2 on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO2 and NO2 on diesel black carbon (DBC) under light irradiation. The experimental results prove that DBC photochemistry can not only significantly promote the heterogeneous reduction of NO2 to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO2 as well as NO2 itself greatly promote the heterogeneous oxidation of SO2 on DBC. NO2 oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO2. The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO2 uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO2 (∼10%). This work highlights the significant synergistic roles of DBC, NO2, and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.

Characteristics of secondary inorganic aerosols and contributions to PM2.5 pollution based on machine learning approach in Shandong Province

Primary emissions of particulate matter and gaseous pollutants, such as SO2 and NOx have decreased in China following the implementation of a series of policies by the Chinese government to address air pollution. However, controlling secondary inorganic aerosol pollution requires attention. This study examined the characteristics of the secondary conversion of nitrate (NO3-) and sulfate (SO42-) in three coastal cities of Shandong Province, namely Binzhou (BZ), Dongying (DY), and Weifang (WF), and an inland city, Jinan (JN), during December 2021. Furthermore, the Shapley Additive Explanation (SHAP), an interpretable attribution technique, was adopted to accurately calculate the contributions of secondary formations to PM2.5. The nitrogen oxidation rate exhibited a significant dependence on the concentration of O3. High humidity facilitates sulfur oxidation. Compared to BZ, DY, and WF, the secondary conversion of NO3- and SO42- was more intense in JN. The light-gradient boosting model outperformed the random forest and extreme-gradient boosting models, achieving a mean R2 value of 0.92. PM2.5 pollution events in BZ, DY, and WF were primarily attributable to biomass burning, whereas pollution in Jinan was contributed by the secondary formation of NO3- and vehicle emissions. Machine learning and the SHAP interpretable attribution technique offer a precise analysis of the causes of air pollution, showing high potential for addressing environmental concerns.

Size-resolved ambient bioaerosols concentration, antibiotic resistance, and community composition during autumn and winter seasons in Qatar

This study investigates the size distribution, microbial composition, and antibiotic resistance (ABR) of airborne bioaerosols at a suburban location in Doha, Qatar between October 2021 and January 2022. Samples were collected using an Andersen six-stage viable cascade impactor and a liquid impinger. Findings showed that the mean bacteria concentration (464 CFU/m3) was significantly higher than that of fungi (242 CFU/m3) during the study period. Both bacteria and fungi were most abundant in the aerodynamic size fractions of 1.10-2.21 μm, with peak concentrations observed in the mornings and lowest concentrations in the afternoons across all size fractions. A total of 24 different culturable species were identified, with the most abundant ones being Pasteurella pneumotropica (9.71%), Pantoea spp. 1 (8.73%), and Proteus penneri (7.77%) spp. At the phylum level, the bacterial community configurations during the autumn and winter seasons were nearly identical as revealed by molecular genomics, with Proteobacteria being the most predominant, followed by Firmicutes, Bacteroidetes, Acidobacteriota, and Planctomycetota. However, there was a significant variation in dominant genera between autumn and winter. The most abundant genera included Sphingomonas, Paraburkholderia, Comamonas, Bacillus, and Lysinibacillus. Several bacterial genera identified in this study have important public health and ecological implications, including the risk of respiratory tract infections. Furthermore, the study found that ABR was highest in December, with bioaerosols exhibiting resistance to at least 5 out of 10 antibiotics, and 100% resistance to Metronidazole in all samples. Metagenomics analysis revealed the presence of various airborne bacteria that were not detected through culture-dependent methods. This study provides valuable insights into the airborne microbial composition, temporal variability and ABR in the Arabian Gulf region.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Efficient Adsorption of Ammonia by Surface-Modified Activated Carbon Fiber Mesh

In view of the characteristics and risks of ammonia, its removal is important for industrial production and environmental safety. In this study, viscose-based activated carbon fiber (ACF) was used as a substrate and chemically modified by nitric acid impregnation to enhance the adsorption capacity of the adsorbent for ammonia. A series of modified ACF-based adsorbents were prepared and characterized using BET, FTIR, XPS, and Boehm titration. Isotherm tests (293.15 K, 303.15 K, 313.15 K) and dynamic adsorption experiments were performed. The characterization results showed that impregnation with low concentrations of nitric acid not only increased the surface acidic functional group content but also increased the specific surface area, while impregnation with high concentrations of nitric acid could be able to decrease the specific surface area. ACF-N-6 significantly increased the surface functional group content without destroying the physical structure of the activated carbon fibers. The experimental results showed that the highest adsorption of ammonia by ACFs was 14.08 mmol-L-1 (ACF-N-6) at 293 K, and the adsorption capacity was increased by 165% compared with that of ACF-raw; by fitting the adsorption isotherm and calculating the equivalent heat of adsorption and thermodynamic parameters using the Langmuir-Freundlich model, the adsorption process could be found to exist simultaneously. Regarding physical adsorption and chemical adsorption, the results of the correlation analysis showed that the ammonia adsorption performance was strongly correlated with the carboxyl group content and positively correlated with the relative humidity (RH) of the inlet gas. This study contributes to the development of an efficient ammonia adsorption system with important applications in industrial production and environmental safety.

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.

Oxalate-Promoted SO2 Uptake and Oxidation on Iron Minerals: Implications for Secondary Sulfate Aerosol Formation

Mineral dust serves as a significant source of sulfate aerosols by mediating heterogeneous sulfur dioxide (SO2) oxidation in the atmosphere. Given that a considerable proportion of small organic acids are deposited onto mineral dust via long-range transportation, understanding their impact on atmospheric SO2 transformation and sulfate formation is of great importance. This study investigates the effect of oxalate on heterogeneous SO2 uptake and oxidation phenomenon by in situ FTIR, theoretical calculation, and continuous stream experiments, exploiting hematite (Fe2O3) as an environmental indicator. The results highlight the critical role of naturally deposited oxalate in mononuclear monodentate coordinating surface Fe atoms of Fe2O3 that enhances the activation of O2 for oxidizing SO2 into sulfate. Meanwhile, oxalate increases the hygroscopicity of Fe2O3, facilitating H2O dissociation into reactive hydroxyl groups and further augmenting the SO2 uptake capacity of Fe2O3. More importantly, other conventional iron minerals, such as goethite and magnetite, as well as authentic iron-containing mineral dust, exhibit similar oxalate-promoted sulfate accumulation behaviors. Our findings suggest that oxalate-assisted SO2 oxidation on iron minerals is one of the important contributors to secondary sulfate aerosols, especially during the nighttime with high relative humidity.

Diurnal Variations in High Time-Resolved Molecular Distributions and Formation Mechanisms of Biogenic Secondary Organic Aerosols at Mt. Huang, East China

The molecular characteristics and formation mechanism of biogenic secondary organic aerosols (BSOAs) in the forested atmosphere are poorly known. Here, we report the temporal variations in and formation processes of BSOA tracers derived from isoprene, monoterpenes, and β caryophyllene in PM2.5 samples collected at the foot of Mt. Huang (483 m a. s. l) in East China during the summer of 2019 with a 3 h time resolution. The concentrations of nearly all of the detected species, including organic carbon (OC), elemental carbon (EC), levoglucosan, and SIA (sum of SO42-, NO3-, and NH4+), were higher at night (19:00-7:00 of the next day) than in the daytime (7:00-19:00). In addition, air pollutants that accumulated by the dynamic transport of the mountain breeze at night were also a crucial reason for the higher BSOA tracers. Most of the BSOA tracers exhibited higher concentrations at night than in the daytime and peaked at 1:00 to 4:00 or 4:00 to 7:00. Those BSOA tracers presented strong correlations with O3 in the daytime rather than at night, indicating that BSOAs in the daytime were primarily derived from the photo-oxidation of BVOCs with O3. The close correlations of BSOA tracers with SO42- and particle acidity (pHis) suggest that BSOAs were primarily derived from the acid-catalyzed aqueous-phase oxidation. Considering the higher relative humidity and LWC concentration at night, the promoted aqueous oxidation was the essential reason for the higher concentrations of BSOA tracers at night. Moreover, levoglucosan exhibited a robust correlation with BSOA tracers, especially β-caryophyllinic acid, suggesting that biomass burning from long-distance transport exerted a significant impact on BSOA formation. Based on a tracer-based method, the estimated concentrations of secondary organic carbon (SOC) derived from isoprene, monoterpenes, and β caryophyllene at night (0.90 ± 0.57 µgC m-3) were higher than those (0.53 ± 0.34 µgC m-3) in the daytime, accounting for 14.5 ± 8.5% and 12.2 ± 5.0% of OC, respectively. Our results reveal that the BSOA formation at the foot of Mt. Huang was promoted by the mountain-valley breezes and anthropogenic pollutants from long-range transport.

Secondary Formation of Atmospheric Brown Carbon in China Haze: Implication for an Enhancing Role of Ammonia

Optical characteristics and molecular compositions of brown carbon (BrC) were investigated during winter 2019 at a rural site of China with a focus on nitro-aromatic compounds (NACs) and imidazoles (IMs). The abundance of gaseous nitrophenols relative to CO during the campaign maximized at noontime, being similar to O3, while the particulate NACs during the haze periods strongly correlated with toluene and NO2, suggesting that NACs in the region are largely formed from the gas-phase photooxidation. Strong correlations of particulate IMs in the dry haze periods with the mass ratio of EC/PM2.5 and the concentration of levoglucosan were observed, indicating that IMs during the dry events are largely derived from biomass burning emissions. However, an increase in IMs with the increasing aerosol liquid water content and pH was observed in the humid haze events, along with much lower abundances of levoglucosan and K+ relative to PM2.5, suggesting that IMs were mostly formed from aqueous reactions in the humid haze periods. These IMs exponentially increased with an increasing NH3 owing to an aqueous reaction of carbonyls with free ammonia. Our findings for the first time revealed an enhancing effect of ammonia on BrC formation in China, especially in humid haze periods.

Sulfate Formation by Photosensitization in Mixed Incense Burning-Sodium Chloride Particles: Effects of RH, Light Intensity, and Aerosol Aging

Elevated particulate sulfate concentrations have been frequently observed in coastal areas when air masses are influenced by continental emissions, especially combustion sources like biomass burning. We studied the SO2 uptake by laboratory-generated droplets containing incense smoke extracts and sodium chloride (IS-NaCl) under irradiation and found enhanced sulfate production over pure NaCl droplets, attributable to photosensitization induced by constituents in IS. Low relative humidity and high light intensity facilitated sulfate formation and increased the SO2 uptake coefficient by IS-NaCl particles. Aging of the IS particles further enhanced sulfate production, attributable to the enhanced secondary oxidant production promoted by increased proportions of nitrogen-containing CHN and oxygen- and nitrogen-containing CHON species under light and air. Experiments using model compounds of syringaldehyde, pyrazine, and 4-nitroguaiacol verified the enhancements of CHN and CHON species in sulfate formation. This work provides experimental evidence of enhanced sulfate production in laboratory-generated IS-NaCl droplets via enhanced secondary oxidant production triggered by photosensitization in multiphase oxidation processes under light and air. Our results can shed light on the possible interactions between sea salt and biomass burning aerosols in enhancing sulfate production.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.