Elucidating severe urban haze formation in China

Multiphase sulfur chemistry facilitates particle growth in a cold and dark urban environment

Sulfate comprises an average of 20% of the ambient PM2.5 mass during the winter months in Fairbanks, based on 24-hour filter measurements. During the ALPACA 2022 field campaign (Jan 15th-Feb 28th of 2022), we deployed two aerosol mass spectrometers (AMS) and one aerosol chemical speciation monitor (ACSM) at three urban sites, combined with Scanning Mobility Particle Sizers (SMPS), to examine the evolution of aerosol composition and size distribution at a sub-hourly time scale. During an intense pollution episode with ambient temperature between -25 and -35 °C, all three instruments (two AMS and one ACSM) recorded a sharp increase in sulfate mass, ranging from 5 to 40 μg m-3 within a few hours. This increase contributed up to half of the observed rise in ambient PM2.5 mass concentration and coincided with a substantial shift in the number distribution from particle sizes less than 100 nm diameter (Dp < 100 nm) to larger particles (Dp > 100 nm) with little increase in number concentration. The corresponding increase in the volume concentration and distribution shift to larger particle size suggests the secondary formation of sulfate and organic aerosol onto pre-existing aerosols. Comparing AMS-sulfate (all sulfur species) to inorganic sulfate measured by online particle-into-liquid sampler-ion chromatography (PILS-IC), we find roughly 80% of sulfate increase was due to organic sulfur, consistent with the observation of mass spectral signatures in the AMS of organosulfur compounds. The rapid formation of sulfate appears to coincide with spikes in ambient aldehyde concentrations (formaldehyde and acetaldehyde) and an increase in S(IV) in ambient PM2.5. This likely results from multiphase chemistry, where hydroxymethanesulfonate (HMS) and other aldehyde-S(IV) adducts are formed through reactions between aldehydes and SO2 in deliquesced aerosols. We estimate that all S(IV) species, including HMS, contribute an average of 30% to aerosol sulfur, with a dominant fraction occurring during rapid sulfate increase events. Our work highlights the crucial role of controlling aldehydes to mitigate severe air pollution events in Fairbanks and may apply to other urban areas. It also emphasizes the significance of multiphase chemistry in driving particle growth from Aitken mode to accumulation mode, a key step for aerosol-cloud interactions.

Anthropogenic organic aerosol in Europe produced mainly through second-generation oxidation

Exposure to anthropogenic atmospheric aerosol is a major health issue, causing several million deaths per year worldwide. The oxidation of aromatic hydrocarbons from traffic and wood combustion is an important anthropogenic source of low-volatility species in secondary organic aerosol, especially in heavily polluted environments. It is not yet established whether the formation of anthropogenic secondary organic aerosol involves mainly rapid autoxidation, slower sequential oxidation steps or a combination of the two. Here we reproduced a typical urban haze in the 'Cosmics Leaving Outdoor Droplets' chamber at the European Organization for Nuclear Research and observed the dynamics of aromatic oxidation products during secondary organic aerosol growth on a molecular level to determine mechanisms underlying their production and removal. We demonstrate that sequential oxidation is required for substantial secondary organic aerosol formation. Second-generation oxidation decreases the products' saturation vapour pressure by several orders of magnitude and increases the aromatic secondary organic aerosol yields from a few percent to a few tens of percent at typical atmospheric concentrations. Through regional modelling, we show that more than 70% of the exposure to anthropogenic organic aerosol in Europe arises from second-generation oxidation.

Synergistic effects of climate change and nitrogen use on future nitric oxide emissions from China's croplands

Accurate quantification of soil nitric oxide (NO) emissions can establish a scientific foundation for developing targeted strategies to mitigate emissions, thereby reducing their environmental impact. Using a database with 476 field measurements across China, a NO emission model was constructed by employing four machine learning algorithms including Extreme Gradient Boosting (XGBoost), Random Forest (RF), Support Vector Machine (SVM), and Artificial Neural Network (ANN). Our validation with independent observational data revealed that the XGBoost model performed the best, achieving a R2 of 0.67. The most important management, soil, and meteorological variables affecting the NO model were mineral nitrogen input, soil organic carbon content, and air temperature, respectively. This study also found that NO emissions exhibited nonlinear responses to different variables. NO emissions from China's farmland were estimated to be approximately 204.48 kt NO-N in 2020. By 2050, we predicted that NO emissions could increase by 2.9%-9.9% under various climate change scenarios, with the highest increment of 9.9% occurring under the RCP8.5 scenario. The southern agricultural region, which was particularly vulnerable to climate change, experienced the largest increase, ranging from approximately 15%-31%. The implementation of nitrogen management strategies that are adapted to future climate conditions could potentially reduce NO emissions by 15%-16.2%.

Molecular dynamics-guided reaction discovery reveals endoperoxide-to-alkoxy radical isomerization as key branching point in α-pinene ozonolysis

Secondary organic aerosols (SOAs) significantly impact Earth's climate and human health. Although the oxidation of volatile organic compounds (VOCs) has been recognized as the major contributor to the atmospheric SOA budget, the mechanisms by which this process produces SOA-forming highly oxygenated organic molecules (HOMs) remain unclear. A major challenge is navigating the complex chemical landscape of these transformations, which traditional hypothesis-driven methods fail to thoroughly investigate. Here, we explore the oxidation of α-pinene, a critical atmospheric biogenic VOC, using a novel reaction discovery approach based on molecular dynamics and state-of-the-art enhanced sampling techniques. Our approach successfully identifies all established reaction pathways of α-pinene ozonolysis, as well as discovers multiple novel species and pathways without relying on a priori chemical knowledge. In particular, we unveil a key branching point that leads to the rapid formation of alkoxy radicals, whose high and diverse reactivity help to explain hitherto unexplained oxidation pathways suggested by mass spectral peaks observed in α-pinene ozonolysis experiments. This branching point is likely prevalent across a variety of atmospheric VOCs and could be crucial in establishing the missing link to SOA-forming HOMs.

Profiles of ambient semi- and intermediate- volatility organic compounds (S/IVOCs) in winter of urban Chengdu

Semi-volatile/intermediate-volatility organic compounds (S/IVOCs) are considered to be missing precursors of secondary organic aerosol (SOA). In this study, ambient S/IVOCs were collected in the winter of Chengdu and were analyzed by a thermal desorption-comprehensive two-dimensional gas chromatography-quadrupole mass spectrometer (TD-GC×GC-qMS). Molecular characteristics and volatility-polarity distributions of the S/IVOCs were specified. A total of 351 compounds were quantified with S/IVOCs concentration of 2.63 ± 1.25 μg m-3 in gas phase and 1.83 ± 1.15 μg m-3 in particle phase. The volatility-polarity distributions are generally similar during polluted days and non-polluted days. Most of the gas-phase compounds fell in the high volatility and intermediate polarity bins, with IVOCs and SVOCs accounting for 16.9 % and 0.3 %, respectively, while IVOCs and SVOCs took up 73.0 % and 9.6 % of the total particle-phase concentration. Among gaseous S/IVOCs, acids and ketones were the main compounds, and benzoic acid was the most abundant species (0.86 ± 0.47 μg m-3), followed by 1,2-diphenyl-ethanone (0.20 ± 0.17 μg m-3), 2-hydroxy-1-phenyl-ethanone (0.20 ± 0.17 μg m-3) and C12-C15 alkenes. Alcohols and esters were the most abundant compounds in particle phase (62.8 %, including volatile chemical product compounds, VCPs), with tracers like Texanol detected (0.11 ± 0.07 μg m-3). Partial least squares-discriminant analysis (PLS-DA) indicated that combustion sources, VCPs and other industrial productions were closely related to pollution levels. The SOA yield method was used to estimate contributions of gas-phase S/IVOCs oxidation to SOA. Despite of their lower concentrations (2.63 ± 1.25 μg m-3 in gas phase, 17.2 %), S/IVOCs could contributed 27.4 % of the total estimated gas-phase SOA production. Emissions with more reactive S/IVOCs during non-polluted days may result in relatively higher SOA contribution fraction of S/IVOCs. Our result provided a deep insight into the molecular composition of ambient organics and elucidated the importance of S/IVOCs to SOA formation in the ambient air of Chengdu.

Modeling and Simulation of Micron Particle Agglomeration in a Turbulent Flow: Impact of Cylindrical Disturbance and Particle Properties

The fly ash generated by coal combustion is one of the main sources of PM2.5, so the particulate matter removal technology of coal-fired boilers is receiving increasing attention. Turbulent agglomeration has emerged as a powerful tool for improving the efficiency of removing fine particulates from environments, sparking interest in its study. Our research meticulously investigated the influence of cylindrical vortex wakes on particle flow, agglomeration patterns, and the dynamics between fluids and particles. By employing a novel hybrid computational approach that integrates the discrete element method (DEM) with large Eddy simulation (LES), we were able to accurately simulate particle-particle interactions. The study focused on understanding how particles with different diameters (2, 5, 10, and 20 μm), densities (2,500, 5,000, 7,500, and 10,000 kg·m-3), and surface energies (0.01, 0.1, and 1 J·m-2) behaved within transitioning shear layer flow conditions. Our findings revealed that particles tended to congregate in areas of lower vorticity, with larger and denser particles demonstrating greater agglomeration efficiency due to their resilience against turbulent forces. Conversely, particles of lower density formed smaller agglomerates as their susceptibility to shear forces increased. Additionally, the study discovered that higher surface energies enhance adhesion, leading to the formation of larger agglomerates.

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.

Seasonal variation and size distribution of aromatic acids in urban aerosols in Beijing, China

Aromatic acids are an integral component of organic acids in the atmosphere, contributing to the formation of climate-altering brown carbon (BrC). To better understand the sources and formation processes of aromatic acids, we collected size-segregated particulate matter samples in urban Beijing from April 2017 to January 2018, which were analyzed using solvent-extraction followed by gas chromatography/mass spectrometry. Phthalic acid (o-PhA) had the greatest average annual concentration, followed by terephthalic acid (p-PhA), 4-hydroxybenzoic acid (4-OHBA), dehydroabietic acid (DA), syringic acid (SA), 3-hydroxybenzoic acid (3-OHBA), isophthalic acid (m-PhA), and vanillic acid (VA). We identified distinct seasonal variations in aromatic acids, with o-PhA peaking in summer due to photochemical activity, while p-PhA and 4-OHBA elevated in autumn and summer, respectively, influenced by open waste and biomass burning. Wintertime variations in all aromatic acids were driven by complex meteorology and increased anthropogenic emissions, including rural biomass burning for cooking and heating. Particle size distribution of aromatic acids was affected by seasonal agricultural activities and dust storms, multiple emission sources, and formation mechanisms. The o-PhA has predominantly bimodal distribution, with diverse sources and complex formation mechanisms including gas- and aqueous-phase chemistry. The applicability of o-PhA as a tracer for specific secondary organic aerosols has been questioned due to its potential primary sources. The 3-OHBA, 4-OHBA, VA, SA, and DA exhibited bimodal or trimodal patterns during haze and non-haze periods across different particle size ranges. The seasonal variation in VA/SA and VA/4-OHBA ratios demonstrated the complexity of biomass burning types, influenced by season, particle size, meteorological conditions, and combustion sources.

Chemical composition, mutagenicity, and cytotoxicity of urban submicron particulate matter (PM1) in Agra, India

This study, conducted in Agra, India, examined the mass concentrations, chemical compositions, and seasonal variations of submicron particles (PM1). The concentrations of metals, water-soluble inorganic ions (WSIs) including anions (F-, Cl-, NO₃-, SO₄2-) and cations (Ca2+, K+, Mg2+, NH₄+, Na+), organic carbon (OC) and elemental carbon (EC) and polycyclic aromatic hydrocarbons (PAHs) in PM1 extract were determined using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), Ion Chromatography, Thermogravimetric Analysis and Gas Chromatography-Mass Spectrometry (GC-MS) respectively. For morphological observation of PM1 particles, Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray spectrometry (FESEM-EDS) was used. The annual average concentration of PM1 was 82.9 ± 33.4 μg/m3, which exceeds the World Health Organisation's (WHO) safe limit for PM2.5 of 5 μg/m3 by a factor of 17. The PM1 mass composition included metals (31 %), WSIs (28 %), OC and EC (9.8 %), and PAHs (0.4 %). Winter recorded the highest PM1 concentration (96.1 ± 25.8 μg/m3), followed by post-monsoon, summer, and monsoon seasons. The average concentration of PAHs was 364.6 ± 226.6 ng/m3. Positive Matrix Factorization (PMF) identified traffic, emissions from biomass/coal and wood combustion, industrial/stationary sources, and secondary aerosols as potential contributors. The Ames test revealed the presence of frameshift mutations and base pair substitutions, especially in winter and post-monsoon. Additionally, PM1 exhibited cytotoxic effects on V-79 cells, with heightened toxicity during winter and prolonged exposure in other seasons. This study underscores the urgent need to address local emission sources and establish regulatory standards for PM1 in urban areas.

Strong upwards transport of HONO in daytime over urban area of Beijing, China

Strong upwards transport of Nitrous acid (HONO) in daytime over urban area of Beijing was observed based on combined observations of HONO, NOx (NO and NO2), nitrate, and PM2.5 at two heights (90 m and 528 m) on the highest building of Beijing (528 m above ground). The mean HONO at the 528 m (0.26 ppb) was lower than that at the 90 m (0.54 ppb), and a clear difference in diurnal variation of HONO between the two heights was observed. HONO at the 90 m showed two peaks in the morning rush hour and mid-night, but decreased sharply in daytime (e.g., from 0.62 ppb at 08:00 to 0.34 at 14:00); while the decreasing trend of HONO in daytime significantly weakened at the 528 m (e.g., from 0.26 ppb at 08:00 to 0.27 at 14:00).With PBL development in the morning, HONO in low layer was upwards transported to the 528 m, which compensated partly HONO loss via photolysis and resulted in a relatively stable concentration at the 528 m in daytime. A positive relationship of the bulk Richardson number (Ri) in 0-500 m with the difference of HONO between the two heights during daytime (08:00-18:00) confirmed the above analyses. HONO budget analysis indicated that a strong unknown HONO source existed at the 528 m in daytime, which was negative correlated to the Ri. These results further confirmed that vertical transport of HONO from low layer was a potential HONO source at the 528 m. Moreover, the contribution of photolysis of particulate nitrate significantly increased at the 528 m. Its contribution in total HONO sources increased from 11.9 % at the 90 m to 16.0 % at the 528 m.

Formation of atmospheric molecular clusters containing nitric acid with ammonia, methylamine, and dimethylamine

This study investigates the formation of atmospheric molecular clusters containing ammonia (NH3, A), methylamine (CH3NH2, MA), or dimethylamine (CH3NHCH3, DMA) with nitric acid (HNO3, NA) using quantum mechanics. The Atmospheric Cluster Dynamic Code (ACDC) was employed to simulate the total evaporation rate, formation rate, and growth pathways of three types of clusters under dry and hydrated conditions. This study evaluates the enhancing potential of A/MA/DMA for NA-based new particle formation (NPF) at parts per trillion (ppt) levels. The results indicate that A/MA/DMA can enhance NA-based NPF at high nitric acid concentrations and low temperatures in the atmosphere. The enhancing potential of MA is weaker than that of DMA but stronger than that of A. Cluster growth predominantly follows the lowest free energy pathways on the acid-base grid, with the formation of initial acid-base dimers (NA)(A), (NA)(MA), and (NA)(DMA) being crucial. Hydration influences the evaporation rate and formation rate of clusters, especially for initial clusters. When the humidity is at 100%, the formation rate for NA-A, NA-MA, and NA-DMA clusters can increase by approximately 109, 107, and 104-fold compared to the corresponding unhydrated clusters, respectively. These results highlight the significance of nitric acid nucleation in NPF events in low-temperature, high-humidity atmospheres, particularly in regions like China with significant automobile exhaust pollution.

Tower-based profiles of wintertime secondary organic aerosols in the urban boundary layer over Guangzhou

Secondary organic aerosol (SOA) accounts for a large fraction of fine particulate matter (PM2.5), but the lack of vertical observations of SOA in the urban boundary layer (UBL) limits a comprehensive understanding of its sources and formation mechanisms. In this study, PM2.5 samples were simultaneously collected at 3 m, 118 m, and 488 m on the Canton Tower in Guangzhou during winter. Typical SOA tracers, including oxidation products of isoprene (SOAI), monoterpene (SOAM), sesquiterpene (SOAS), and toluene (ASOA), were investigated alongside meteorological parameters and gaseous/particulate pollutants. Total concentrations of SOA tracers showed an increasing trend with height, with daytime levels exceeding nighttime levels. C5-alkene triols and 2-methylglyceric acid displayed a significant increase with height, potentially affected by nighttime chemistry in the residual layer, determining the overall vertical trend of SOAI tracers. Concentrations of later-generation SOAM (SOAM_S) tracers also increased with height, while those of first-generation SOAM (SOAM_F) tracers decreased, indicating relatively aged SOAM in the upper layers. SOAS and ASOA tracers exhibited higher enhancement under polluted conditions, likely impacted by biomass burning and anthropogenic emissions. The yields of SOAI tracers varied with temperature in the vertical profile. The formation of SOAM_F tracers was negatively correlated with relative humidity, liquid water content, and pH, affecting their vertical distributions. The effect of O3 on SOA formation enhanced significantly with height, influenced by air mass transport, and likely contributed to the higher yields of SOA in the upper layer. However, at ground level, SOA formation was primarily driven by high local emissions of both NOx and volatile organic compounds. We also observed the roles of SO2 in SOA generation, particularly at 118 m. This study demonstrates the vertical diurnal characteristics of SOA tracers in the UBL, highlighting the varying effects of meteorological conditions and anthropogenic pollutants on SOA formation at different heights.

ATF3 mediates PM2.5-induced apoptosis and inflammation in ovarian granulosa cells

Particulate matter 2.5 (PM2.5) pollution has emerged as a major global public health concern because of its adverse effects on human health. Our group has previously demonstrated that PM2.5 exposure can seriously impair ovarian function. However, the underlying mechanisms remain a mystery. This study verifies ovarian damage in mice, evidenced by inflammatory cell infiltration and follicular atresia, following 5 months of PM2.5 exposure via tracheal drip (35 µg/m³ for low dose and 150 µg/m³ for high dose). In addition, PM2.5 exposure inhibited the cell viability of human granulosa cells (KGN) and induced apoptosis at the concentrations of 50, 100, and 150 µg/mL for 24 h. The apoptosis of KGN cells induced by inflammation contributes to follicular atresia. Furthermore, we conducted RNA-sequencing analysis to identify the genes and pathways triggered by PM2.5 (100 µg/mL) exposure, which decreases the KGN cell viability. We found a significant increase in Activating Transcription Factor 3 (ATF3). Further mechanistic studies reveal a strong association between PM2.5-induced apoptosis, inflammation, and ATF3 with its downstream oxidative stress signals. In summary, the ATF3 pathway serves a vital role in the ovarian injury caused by PM2.5 exposures.

Enhancement of atmospheric nucleation precursors on formic sulfuric anhydride induced nucleation: Theoretical mechanism

As an intermediate formed by H2SO4 (SA), formic sulfate anhydride (FSA) has been hypothesized to play a role in the nucleation of atmospheric aerosols. It is the first time that the clusters (SA)x(A)y(W)n and (FSA)x(A)y(W)n (x = 1-2; y = 1-2; n = 0-4) were systematically studied in theory on the structures, thermodynamics, intermolecular interactions, humidity dependence, atmospheric dependence and optical properties. FSA is predicted to be more stronger to promote the clustering with ammonia (A) than SA, suggesting that substituent group enhances nucleation capability of FSA. Whereas, the substituent group does not influence the humidity sensitivity of hydrated clusters. The clusters trend to form small hydrated clusters (nwater≦3). The study on atmospheric dependence indicates that the stability of the clusters depends more on temperature other than pressure. Moreover, FSA shows a stronger ability on reducing atmospheric visibility than A, SA and water molecules. This finding aims to draw attention to FSA about atmospheric nucleation.

Impact of visibility limiting conditions on satellite and high-altitude platform quantum key distribution links

Satellite and aerial platforms are critical in the deployment of global quantum communications networks. Currently, there remain significant challenges including operation during daytime and robustness to visibility limiting conditions. In this work we investigate, through simulation, the impact of visibility limiting conditions on low-Earth orbit CubeSat dimensioned satellites, small satellites and high-altitude platform implementations. Three different operational wavelengths were considered: currently used near-infrared (at 850 nm); next-generation short-wave infrared (at 1550 nm); and a candidate longer wavelength (at 2133 nm). We present channel attenuation and consider quantum key distribution (QKD) system performance parameters. Results indicate that the "best wavelength" for an implementation depends on the minimum visibility rated and the single-photon detector technology utilized. In the cases where tolerated meteorological visibility is short, 1550 nm and 2133 nm wavelengths provide better performance. In cases when the visibility is long, the operational wavelength of 850 nm provides better QKD system performance.

Distinct Photochemistry of Odd-Carbon PAHs from the Even-Carbon Ones During the Photoaging and Analysis of Soot

Polycyclic aromatic hydrocarbons (PAHs) are the primary organic carbons in soot. In addition to PAHs with even carbon numbers (PAHeven), substantial odd-carbon PAHs (PAHodd) have been widely observed in soot and ambient particles. Analyzing and understanding the photoaging of these compounds are essential for assessing their environmental effects. Here, using laser desorption ionization mass spectrometry (LDI-MS), we reveal the substantially different photoreactivity of PAHodd from PAHeven in the aging process and their MS detection through their distinct behaviors in the presence and absence of elemental carbon (EC) in soot. During direct photooxidation of organic carbon (OC) alone, the PAHeven are oxidized more rapidly than the PAHodd. However, the degradation of PAHodd becomes preponderant over PAHeven in the presence of EC during photoaging of the whole soot. All of these observations are proposed to originate from the more rapid hydrogen abstraction reaction from PAHodd in the EC-photosensitized reaction, owing to its unique structure of a single sp3-hybridized carbon site. Our findings reveal the photoreactivity and reaction mechanism of PAHodd for the first time, providing a comprehensive understanding of the oxidation of PAHs at a molecular level during soot aging and highlight the enhanced effect of EC on PAHodd ionization in LDI-MS analysis.

Elevated aerosol enhances plant water-use efficiency by increasing carbon uptake while reducing water loss

Aerosols could significantly influence ecosystem carbon and water fluxes, potentially altering their interconnected dynamics, typically characterized by water-use efficiency (WUE). However, our understanding of the underlying ecophysiological mechanisms remains limited due to insufficient field observations. We conducted 4-yr measurements of leaf photosynthesis and transpiration, as well as 3-yr measurements of stem growth (SG) and sap flow of poplar trees exposed to natural aerosol fluctuation, to elucidate aerosol's impact on plant WUE. We found that aerosol improved sun leaf WUE mainly because a sharp decline in photosynthetically active radiation (PAR) inhibited its transpiration, while photosynthesis was less affected, as the negative effect induced by declined PAR was offset by the positive effect induced by low leaf vapor pressure deficit (VPDleaf). Conversely, diffuse radiation fertilization (DRF) effect stimulated shade leaf photosynthesis with minimal impact on transpiration, leading to an improved WUE. The responses were further verified by a strong DRF on SG and a decrease in sap flow due to the suppresses in total radiation and VPD. Our field observations indicate that, contrary to the commonly assumed coupling response, carbon uptake and water use exhibited dissimilar reactions to aerosol pollution, ultimately enhancing WUE at the leaf and canopy level.

The effect of cross-regional transport on ozone and particulate matter pollution in China: A review of methodology and current knowledge

China is currently one of the countries impacted by severe atmospheric ozone (O3) and particulate matter (PM) pollution. Due to their moderately long lifetimes, O3 and PM can be transported over long distances, cross the boundaries of source regions and contribute to air pollution in other regions. The reported contributions of cross-regional transport (CRT) to O3 and fine PM (PM2.5) concentrations often exceed those of local emissions in the major regions of China, highlighting the important role of CRT in regional air pollution. Therefore, further improvement of air quality in China requires more joint efforts among regions to ensure a proper reduction in emissions while accounting for the influence of CRT. This review summarizes the methodologies employed to assess the influence of CRT on O3 and PM pollution as well as current knowledge of CRT influence in China. Quantifying CRT contributions in proportion to O3 and PM levels and studying detailed CRT processes of O3, PM and precursors can be both based on targeted observations and/or model simulations. Reported publications indicate that CRT contributes by 40-80 % to O3 and by 10-70 % to PM2.5 in various regions of China. These contributions exhibit notable spatiotemporal variations, with differences in meteorological conditions and/or emissions often serving as main drivers of such variations. Based on trajectory-based methods, transport pathways contributing to O3 and PM pollution in major regions of China have been revealed. Recent studies also highlighted the important role of horizontal transport in the middle/high atmospheric boundary layer or low free troposphere, of vertical exchange and mixing as well as of interactions between CRT, local meteorology and chemistry in the detailed CRT processes. Drawing on the current knowledge on the influence of CRT, this paper provides recommendations for future studies that aim at supporting ongoing air pollution mitigation strategies in China.

Characterization of PM2.5 Carbonaceous Components in a Typical Industrial City in China under Continuous Mitigation Measures

The goals of the "dual carbon" program in China are to implement a series of air pollution policies to reduce the emission of carbon-bearing particulate matter (PM). Following improvements in the reduction in carbon emissions in Handan City, China, fine particulate matter (PM2.5) was collected in the winters from 2016 to 2020 to characterize the concentrations and sources of carbonaceous components in PM2.5. Trend analysis revealed that both organic carbon (OC) and elemental carbon (EC) concentrations significantly decreased. The proportion of total carbon aerosol (TCA) in PM2.5 decreased by 47.0%, highlighting the effective reduction in carbon emissions. Secondary organic carbon (SOC) concentrations increased from 2016 (12.86 ± 14.10 μg·m-3) to 2018 (36.76 ± 21.59 μg·m-3) and then declined gradually. SOC/OC was larger than 67.0% from 2018 to 2020, implying that more effective synergistic emission reduction measures for carbonaceous aerosol and volatile organic compounds (VOCs) were needed. In the winters from 2016 to 2020, primary organic carbon (POC) concentrations reduced by 76.1% and 87.6% under a light/moderate pollution period (LP) and heavy/severe pollution periods (HPs), respectively. The TCA/PM2.5 showed a decreasing trend under LP and HP conditions, decreasing by 42.1% and 54.7%, respectively. Source analysis revealed that carbonaceous components were mainly from biomass burning, coal combustion and automotive exhaust emissions in the winters of 2016 and 2020. OC/EC and K+/EC analysis pointed out that air pollutant reduction measurements should focus on rectification biomass fuels in the next stage. Compared with 2016, the contributions of automotive exhaust emissions decreased in 2020. OC and EC concentrations decreased due to control measures on automotive exhaust emissions.

Using the Multicomponent Aerosol FORmation Model (MAFOR) to Determine Improved VOC Emission Factors in Ship Plumes

International shipping's particulate matter primary emissions have a share in global anthropogenic emissions of between 3% and 4%. Ship emissions of volatile organic compounds (VOCs) can play an important role in the formation of fine particulate matter. Using an aerosol box model for the near-plume scale, this study investigated how the changing VOC emission factor (EF) for ship engines impacts the formation of secondary PM2.5 in ship exhaust plumes that were detected during a measurement campaign. The agreement between measured and modeled particle number size distribution was improved by adjusting VOC emissions, in particular of intermediate-, low-, and extremely low-volatility compounds. The scaling of the VOC emission factor showed that the initial emission factor, based on literature data, had to be multiplied by 3.6 for all VOCs. Information obtained from the box model was integrated into a regional-scale chemistry transport model (CTM) to study the influence of changed VOC ship emissions over the Mediterranean Sea. The regional-scale CTM run with adjusted ship emissions indicated a change in PM2.5 of up to 5% at the main shipping routes and harbor cities in summer. Nevertheless, overall changes due to a change in the VOC EF were rather small, indicating that the size of grid cells in CTMs leads to a fast dilution.

Major source categories of PM2.5 oxidative potential in wintertime Beijing and surroundings based on online dithiothreitol-based field measurements

Fine particulate matter (PM2.5) causes millions of premature deaths each year worldwide. Oxidative potential (OP) has been proposed as a better metric for aerosol health effects than PM2.5 mass concentration alone. In this study, we report for the first time online measurements of PM2.5 OP in wintertime Beijing and surroundings based on a dithiothreitol (DTT) assay. These measurements were combined with co-located PM chemical composition measurements to identify the main source categories of aerosol OP. In addition, we highlight the influence of two distinct pollution events on aerosol OP (spring festival celebrations including fireworks and a severe regional dust storm). Source apportionment coupled with multilinear regression revealed that primary PM and oxygenated organic aerosol (OOA) were both important sources of OP, accounting for 41 ± 12 % and 39 ± 10 % of the OPvDTT (OP normalized by the sampled air volume), respectively. The small remainder was attributed to fireworks and dust, mainly resulting from the two distinct pollution events. During the 3.5-day spring festival period, OPvDTT spiked to 4.9 nmol min-1 m-3 with slightly more contribution from OOA (42 ± 11 %) and less from primary PM (31 ± 15 %). During the dust storm, hourly-averaged PM2.5 peaked at a very high value of 548 μg m-3 due to the dominant presence of dust-laden particles (88 % of total PM2.5). In contrast, only mildly elevated OPvDTT values (up to 1.5 nmol min-1 m-3) were observed during this dust event. This observation indicates that variations in OPvDTT cannot be fully explained using PM2.5 alone; one must also consider the chemical composition of PM2.5 when studying aerosol health effects. Our study highlights the need for continued pollution control strategies to reduce primary PM emissions, and more in-depth investigations into the source origins of OOA, to minimize the health risks associated with PM exposure in Beijing.

Evaluation of a new real-time source apportionment system of PM2.5 and its implication on rapid aging of vehicle exhaust

Accurate identification and rapid analysis of PM2.5 sources and formation mechanisms are essential to mitigate PM2.5 pollution. However, studies were limited in developing a method to apportion sources to the total PM2.5 mass in real-time. In this study, we developed a real-time source apportionment method based on chemical mass balance (CMB) modeling and a mass-closure PM2.5 composition online monitoring system in Shenzhen, China. Results showed that secondary sulfate, secondary organic aerosol (SOA), vehicle emissions and secondary nitrate were the four major PM2.5 sources during autumn 2019 in Shenzhen, together contributed 76 % of PM2.5 mass. The novel method was verified by comparing with other source apportionment methods, including offline filter analysis, aerosol mass spectrometry, and carbon isotopic analysis. The comparison of these methods showed that the new real-time method obtained results generally consistent with the others, and the differences were interpretable and implicative. SOA and vehicle emissions were the major PM2.5 and OA contributors by all methods. Further investigation on the OA sources indicated that vehicle emissions were not only the main source of primary organic aerosol (POA), but also the main contributor to SOA by rapid aging of the exhaust in the atmosphere. Our results demonstrated the great potential of the new real-time source apportionment method for aerosol pollution control and deep understandings on emission sources.

The Long Hazy Tail: Analysis of the Impacts and Trends of Severe Outdoor and Indoor Air Pollution in North China

China, especially the densely populated North China region, experienced severe haze events in the past decade that concerned the public. Although the most extreme cases have been largely eliminated through recent mitigation measures, severe outdoor air pollution persists and its environmental impact needs to be understood. Severe indoor pollution draws less public attention due to the short visible distance indoors, but its public health impacts cannot be ignored. Herein, we assess the trends and impacts of severe outdoor and indoor air pollution in North China from 2014 to 2021. Our results demonstrate the uneven contribution of severe hazy days to ambient and exposure concentrations of particulate matter with an aerodynamic diameter <2.5 (PM2.5). Although severe indoor pollution contributes to indoor PM2.5 concentrations (23%) to a similar extent as severe haze contributes to ambient PM2.5 concentrations (21%), the former's contribution to premature deaths was significantly higher. Furthermore, residential emissions contributed more in the higher PM2.5 concentration range both indoors and outdoors. Notably, severe haze had greater health impacts on urban residents, while severe indoor pollution was more impactful in rural areas. Our findings suggest that, besides reducing severe haze, mitigating severe indoor pollution is an important aspect of combating air pollution, especially toward improving public health.

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.

Knowledge-guided machine learning reveals pivotal drivers for gas-to-particle conversion of atmospheric nitrate

Particulate nitrate, a key component of fine particles, forms through the intricate gas-to-particle conversion process. This process is regulated by the gas-to-particle conversion coefficient of nitrate (ε(NO3-)). The mechanism between ε(NO3-) and its drivers is highly complex and nonlinear, and can be characterized by machine learning methods. However, conventional machine learning often yields results that lack clear physical meaning and may even contradict established physical/chemical mechanisms due to the influence of ambient factors. It urgently needs an alternative approach that possesses transparent physical interpretations and provides deeper insights into the impact of ε(NO3-). Here we introduce a supervised machine learning approach-the multilevel nested random forest guided by theory approaches. Our approach robustly identifies NH4+, SO42-, and temperature as pivotal drivers for ε(NO3-). Notably, substantial disparities exist between the outcomes of traditional random forest analysis and the anticipated actual results. Furthermore, our approach underscores the significance of NH4+ during both daytime (30%) and nighttime (40%) periods, while appropriately downplaying the influence of some less relevant drivers in comparison to conventional random forest analysis. This research underscores the transformative potential of integrating domain knowledge with machine learning in atmospheric studies.

The role of aldehydes on sulfur based-new particle formation: a theoretical study

Aldehydes play a crucial role in the formation of atmospheric particles, attracting significant attention due to their environmental impact. However, the microscopic mechanisms underlying the formation of aldehyde-involved particles remain uncertain. In this study, through quantum chemical calculations and molecular dynamics (MD) simulations, we investigate the microscopic formation mechanisms of binary and ternary systems composed of three representative aldehydes, two sulfur-based acids, water, and two bases. Our research findings reveal that the most stable structures of acid-aldehyde clusters involve the connection of acids and aldehyde compounds through hydrogen bonds without involving proton transfer reactions, indicating relatively poor cluster stability. However, with the introduction of a third component, the stability of 18 clusters significantly increase. Among these, in ten systems, acids act as catalysts, facilitating reactions between aldehyde compounds and water or alkaline substances to generate glycols and amino alcohols. However, according to MD simulations conducted at 300 K, these acids readily dissociate from the resulting products. In the remaining eight systems, the most stable structural feature involves ion pairs formed by proton transfer reactions between acids and aldehyde compounds. These clusters exhibit remarkable thermodynamic stability. Furthermore, the acidity of the acid, the nature of nucleophilic agents, and the type of aldehyde all play significant roles in cluster stability and reactivity, and they have synergistic effects on the nucleation process. This study offers microscopic insights into the processes of new particle formation involving aldehydes, contributing to a deeper understanding of atmospheric chemistry at the molecular level.

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.

The Significant Role of New Particle Composition and Morphology on the HNO3-Driven Growth of Particles down to Sub-10 nm

New particle formation and growth greatly influence air quality and the global climate. Recent CERN Cosmics Leaving OUtdoor Droplets (CLOUD) chamber experiments proposed that in cold urban atmospheres with highly supersaturated HNO3 and NH3, newly formed sub-10 nm nanoparticles can grow rapidly (up to 1000 nm h-1). Here, we present direct observational evidence that in winter Beijing with persistent highly supersaturated HNO3 and NH3, nitrate contributed less than ∼14% of the 8-40 nm nanoparticle composition, and overall growth rates were only ∼0.8-5 nm h-1. To explain the observed growth rates and particulate nitrate fraction, the effective mass accommodation coefficient of HNO3 (αHNO3) on the nanoparticles in urban Beijing needs to be 2-4 orders of magnitude lower than those in the CLOUD chamber. We propose that the inefficient uptake of HNO3 on nanoparticles is mainly due to the much higher particulate organic fraction and lower relative humidity in urban Beijing. To quantitatively reproduce the observed growth, we show that an inhomogeneous "inorganic core-organic shell" nanoparticle morphology might exist for nanoparticles in Beijing. This study emphasized that growth for nanoparticles down to sub-10 nm was largely influenced by their composition, which was previously ignored and should be considered in future studies on nanoparticle growth.

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.

Assessment of seasonal variability of PM, BC and UFP levels at a highway toll stations and their associated health risks

As a part of their occupation, workers at toll stations are exposed to traffic emissions during the working shift, which sometimes stretches to 12 h. To assess the exposure and subsequent health risk of these workers, a study was performed on a highway toll station in India. PM1, PM2.5, PM10, BC and UFP concentration were determined inside a toll collectors' cabin and outside in a free-flowing traffic section (125 m from the toll cabin). The concentrations varied in the following range: PM1 (40.69-226.13 μg m-3), PM2.5 (49.71-247.36 μg m-3), PM10 (83.15-458.14 μg m-3) and BC (2.1-87.5 μg m-3) and UFP: 101-53705 pt cm-3. The mean concentration inside the cabin was 1.34 (PM1), 1.35 (PM2.5), 1.16 (PM10) and 2.91 (BC) times the concentration outside for the summer season. The corresponding levels in the winter season were 1.14 (PM1), 1.11 (PM2.5), 1.11 (PM10), 2.50 (BC) and 1.82 (UFP). In addition to the exhaust emission, the non-exhaust emissions such as resuspension of crustal particles, fly ash and bioaerosols were identified. Using the Multiple Path Particle Dosimetry model for two groups - adults (18-21 years) and adults (21+ years), it was estimated that the pulmonary deposition of in-cabin workers were 50% (PM2.5) -75% (PM1) higher than the workers outside the cabin. Particle mass deposition was found to be higher for adults (21+ years) than adults (18-21 years) for both the seasons. The study quantitatively assessed the health risk faced by the workers in terms of exposure concentration and deposition in respiratory tract. More such studies at different traffic mix and climate can provide better estimates of health risk of toll workers that can be used to devise appropriate strategies for control of it.

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.

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.

Contradictory response of ozone and particulate matter concentrations to boundary layer meteorology

At the present stage, collaborative control of particulate matter and ozone pollution has become a modern challenge. The atmospheric boundary layer height (ABLH) is an important meteorological parameter for the sources and sinks of air pollutants. It is generally recognized that the reduction of boundary layer is conducive to the accumulation of pollutants. However, in recent years, some studies have shown that the relationship between ABLH and ozone is not negatively correlated. Here, we analyzed the spatial distribution characteristics of PM2.5 and ozone exceedance in China from 2015 to 2022. The relationships between particulate pollution and ozone pollution and boundary layer meteorology were discussed. The key to coordinated control is to control the PM2.5 concentration in the winter and ozone in summer. Moreover, the two have different responses to meteorological factors, especially to the ABLH. Low temperature and low ABLH are conducive to the deterioration of particulate pollution, but high temperature and high ABLH are conducive to the occurrence and development of ozone pollution. The response of ozone to ABLH is contrary to previous studies in Europe and the United States. Moreover, an abnormal positive correlation was observed for PM2.5 and ABLH in Southwest China, which was mainly due to the impact of biomass combustion in Southeast Asia.

Large contributions of emission reductions and meteorological conditions to the abatement of PM2.5 in Beijing during the 24th Winter Olympic Games in 2022

To guarantee the blue skies for the 2022 Winter Olympics held in Beijing and Zhangjiakou from February 4 to 20, Beijing and its surrounding areas adopted a series of emission control measures. This provides an opportunity to determine the impacts of large-scale temporary control measures on the air quality in Beijing during this special period. Here, we applied the WRF-CMAQ model to quantify the contributions of emission reduction measures and meteorological conditions. Results show that meteorological conditions in 2022 decreased PM2.5 in Beijing by 6.9 and 11.8 µg/m3 relative to 2021 under the scenarios with and without emission reductions, respectively. Strict emission reduction measures implemented in Beijing and seven neighboring provinces resulted in an average decrease of 13.0 µg/m3 (-41.2%) in PM2.5 in Beijing. Over the entire period, local emission reductions contributed more to good air quality in Beijing than nonlocal emission reductions. Under the emission reduction scenario, local, controlled regions, other regions, and boundary conditions contributed 47.7%, 42.0%, 5.3%, and 5.0% to the PM2.5 concentrations in Beijing, respectively. The results indicate that during the cleaning period with the air masses from the northwest, the abatements of PM2.5 were mainly caused by local emission reductions. However, during the potential pollution period with the air masses from the east-northeast and west-southwest, the abatements of PM2.5 were caused by both local and nonlocal emission reductions almost equally. This implies that regional coordinated prevention and control strategies need to be arranged scientifically and rationally when heavy pollution events are forecasted.

Distinct Temperature Trends in the Uptake of Gaseous n-Butylamine on Two Solid Diacids

Uptake coefficients of n-butylamine (BA) on solid succinic (SA) and glutaric acids (GA) from 298 to 177 K were measured using a newly combined Knudsen cell temperature-programmed desorption apparatus. The uptake coefficients on SA increase monotonically from (1.9 ± 0.5) × 10-4 at 298 K to 0.14 ± 0.05 at 177 K (errors represent 2σ statistical errors, overall errors are estimated to be ±60%). This is consistent with a surface reaction mechanism to form solid aminium carboxylate. In contrast, the uptake coefficients on GA increase from 0.11 ± 0.04 at 298 K to 0.25 ± 0.04 at 248 K but then decrease to 0.030 ± 0.010 at 177 K. This unusual trend in temperature dependence of the uptake coefficient is due to formation of an ionic liquid (IL) layer upon the surface reaction of BA with GA, leading to a competition between the rate of desorption of BA and the rates of diffusion and reaction within the IL. Overall, the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB) satisfactorily reproduces these unique trends. This work provides mechanistic insight and predictive capability for the temperature-dependence of reactive uptake processes involving multiple phase changes upon surface reaction.

Sources and composition of elemental carbon during haze events in North China by a high time-resolved study

Despite the implementation of stringent emission reduction measures in the past year, severe winter haze events still occurred frequently in North China, with only marginal decreases observed in elemental carbon (EC) concentrations. EC not only constitutes a fraction of particle mass but also interacts with boundary layer and influences haze formation. Given the complex composition of EC, characterizing its sources and composition during haze processes is challenging yet crucial for understanding haze formation and evolution. Here, hourly-resolution PM2.5 samples were collected during three different haze event (P1-P3) in the North China Plain to investigate dynamic changes in EC source across different haze processes. The average EC concentrations and char/soot ratios were 9.94 ± 4.80 μgC·m-3, 14.5 ± 6.93 μgC·m-3, 15.9 ± 5.54 μgC·m-3, and 2.42 ± 0.98, 2.70 ± 0.88, 2.61 ± 0.95 for P1, P2 and P3, respectively. Backward trajectory analysis showed distinct variations in EC concentration and composition under the influence of different air masses during the three haze events, with local air masses influenced days exhibiting higher EC concentrations and char/soot ratios. The char/soot ratio based diagnostic method suggested that EC was predominantly influenced by coal combustion and vehicle emissions. Further positive matrix factorization analysis suggested that biomass burning and residential coal combustion were the main contributors of EC (58 %) and played a dominant role in driving variations in EC concentrations during haze episodes. Potential source contribution function analysis results highlight that local biomass burning and residential coal combustion could be major reason for the EC elevation in different haze events. Our results provide valuable insights into the sources and composition of EC during haze events, facilitating the implementation of effective measures for mitigating both EC and PM pollution.

VOC emitted by biopharmaceutical industries: Source profiles, health risks, and secondary pollution

The biopharmaceutical industry contributes substantially to volatile organic compounds (VOCs) emissions, causing growing concerns and social developmental conflicts. This study conducted an on-site investigation of the process-based emission of VOCs from three biopharmaceutical enterprises. In the workshops of the three enterprises, 26 VOCs were detected, which could be sorted into 4 classes: hydrocarbons, aromatic hydrocarbons, oxygen-containing compounds, and nitrogen-containing compounds. Ketones were the main components of waste gases, accounting for 44.13%-77.85% of the overall VOCs. Process-based source profiles were compiled for each process unit, with the fermentation and extraction units of tiamulin fumarate being the main source of VOC emissions. Dimethyl heptanone, vinyl acetate, diethylamine, propylene glycol methyl ether (PGME), and benzene were screened as priority pollutants through a fuzzy comprehensive evaluation system. Ground level concentration simulation results of the Gauss plume diffusion model demonstrated that the diffusivity of VOCs in the atmosphere was relatively high, indicating potential non-carcinogenic and carcinogenic risks 1.5-2 km downwind. Furthermore, the process-based formation potentials of ozone and secondary organic aerosols (SOAs) were determined and indicated that N-methyl-2-pyrrolidone, dimethyl heptanone, and PGME should be preferentially controlled to reduce the ozone formation potential, whereas the control of benzene and chlorobenzene should be prioritized to reduce the generation of SOAs. Our results provide a basis for understanding the characteristics of VOC emission by biopharmaceutical industries and their diffusion, potentially allowing the development of measures to reduce health risks and secondary pollution.

Surface Hydrogen Bond-Induced Oxygen Vacancies of TiO2 for Two-Electron Molecular Oxygen Activation and Efficient NO Oxidation

Defect engineering can provide a feasible approach to achieving ambient molecular oxygen activation. However, conventional surface defects (e.g., oxygen vacancies, OVs), featured with the coordinatively unsaturated metal sites, often favor the reduction of O2 to •O2- rather than O22- via two-electron transfer, hindering the efficient pollutant removal with high electron utilization. Herein, we demonstrate that this bottleneck can be well discharged by modulating the electronic structure of OVs via phosphorization. As a proof of concept, TiO2 nanoparticles are adopted as a model material for NaH2PO2 (HP) modification, in which HP induces the formation of OVs via weakening the Ti-O bonds through the hydrogen bond interactions. Additionally, the formed Ti-O-P covalent bond refines the electronic structure of OVs, which enables rapid electron transfer for two-electron molecular oxygen activation. As exemplified by NO oxidation, HP-modified TiO2 with abundant OVs achieved complete NO removal with high selectivity for benign nitrate, superior to that of pristine TiO2. This study highlights a promising approach to regulate the O2 activation via an electronic structure modulation and provides fresh insights into the rational design of a photocatalyst for environmental remediation.

Comparing sources of carbonaceous aerosols during haze and nonhaze periods in two northern Chinese cities

Radiocarbon (14C), stable carbon isotope (13C), and levoglucosan in PM2.5 were measured in two northern Chinese cities during haze events and nonhaze periods in January 2019, to ascertain the sources and their differences in carbonaceous aerosols between the two periods. The contribution of primary vehicle emissions (17.8 ± 3.7%) to total carbon in Beijing during that haze event was higher than that of primary coal combustion (7.3 ± 4.2%), and it increased significantly (7.1%) compared to the nonhaze period. The contribution of primary vehicle emissions (4.1 ± 2.8%) was close to that of primary coal combustion (4.3 ± 3.3%) during the haze event in Xi'an, and the contribution of primary vehicle emissions decreased by 5.8% compared to the nonhaze period. Primary biomass burning contributed 21.1 ± 10.5% during the haze event in Beijing and 40.9 ± 6.6% in Xi'an (with an increase of 3.3% compared with the nonhaze period). The contribution of secondary fossil fuel sources to total secondary organic carbon increased by 29.2% during the haze event in Beijing and by 18.4% in Xi'an compared to the nonhaze period. These results indicate that specific management measures for air pollution need to be strengthened in different Chinese cities in the future, that is, controlling vehicle emissions in Beijing and restricting the use of coal and biomass fuels in winter in Xi'an.

Environmental factors and stroke: Risk and prevention

Stroke is a leading cause of death and adult disability globally. In addition to traditional risk factors, environmental risk factors have emerged over the recent past and are becoming increasingly important. The disproportionate rise of stroke incidence in low- and middle-income countries has been attributed, at least in part, to environmental factors. This narrative review provides details on the interplay between the environment and health generally and stroke specifically, covering topics including air pollution, atmospheric brown clouds, desert dust storms, giant wildfires, chemical contamination, biological aggressors, urbanization, and climate change. It also covers some beneficial environmental effects such as can be harnessed from the exposure to green spaces. It concludes with a summary of pragmatic actions that can be taken to help address some of these challenges at individual, community, and political advocacy levels.

Significantly mitigating PM2.5 pollution level via reduction of NOx emission during wintertime

Despite considerable decreases in fine particulate matter (PM2.5) in Chinese megacities over the past decade, many second- and third-tier cities that distribute abundant industrial enterprises are still facing great challenges for PM2.5 further reduction under the recent policy background of eliminating heavily-polluted weather. In view of core effects of NOx on PM2.5, the deeper reductions of NOx in these cities are expected to break the plateau of PM2.5 decline, however, the link between NOx emission and PM2.5 mass loading is currently lacking. Herein, we progressively construct an evaluation system for PM2.5 productions based on daily NOx emissions in a typical industrial city (Jiyuan), considering a sequence of nested parameters involving evolutions of NO2 into nitric acid and then nitrate, and contributions of nitrate to PM2.5. The evaluation system was subsequently validated to better reproduce real increasing processes for PM2.5 pollution based on 19 pollution cases, with root mean square errors of 19.2 ± 16.4 %, suggesting the feasibility of developing NOx emission indicators linked to goals of mitigating atmospheric PM2.5. Additionally, further comparative results reveal that currently high NOx emissions in this industrial city severely hinder the achievement of atmospheric PM2.5 environmental capacity targets, especially in the scenarios of high initial PM2.5 level, low planetary boundary layer height and long pollution duration. It is anticipated that these methodologies and findings would supply guidelines for further regional PM2.5 mitigation, in which source-oriented NOx indicators could also provide some orientations for industrial cleaner production such as denitrification and low nitrogen combustion.

Characterizing nighttime vertical profiles of atmospheric particulate matter and ozone in a megacity of south China using unmanned aerial vehicle measurements

Daytime atmospheric pollution has received wide attention, while the vertical structures of atmospheric pollutants at night play a crucial role in the photochemical process on the following day, which is still less reported. Focusing on Guangzhou, a megacity of South China, we established an unmanned aerial vehicle (UAV) equipped with micro detectors to collect consecutive high-resolution samples of fine particle (PM2.5), submicron particle (PM1.0), black carbon (BC) and ozone (O3) concentrations in the atmosphere, as well as the air temperature (AT) and relative humidity (RH) within a 500 m altitude during nighttime from Oct. 24th to Nov. 6th, 2018. The measurements showed that PM2.5, PM1.0, and BC decreased with altitude and were influenced by the nighttime shallow planetary boundary layer (PBL) where BC was more accumulated and fluctuated. In contrast, O3 was positively correlated with altitude. Backward trajectory clustering and Pasquill stability classification showed that advection and convection significantly influenced the vertical distribution of all pollutants, particularly particulate matter. External air masses carrying high concentrations of pollutants increased PM1.0 and PM2.5 levels by 145% and 455%, respectively, compared to unaffected periods. The ratio of BC to PM2.5 indicated that local emissions had a minor role in nighttime particulate matter. Vertical transport caused by atmospheric instability reduced the differences in pollutant concentrations at various heights. Geodetector and generalized additive model showed that RH and BC accumulation in the PBL were significant factors influencing vertical changes of the secondary aerosol intensity as indicated by the ratio of PM1.0 to PM2.5. The joint explanation of RH and atmospheric stability with other variables such as BC is essential to understand the generation of secondary aerosols. These findings provide insights into regional and local measures to prevent and control night-time particulate matter pollution.

Aqueous-Phase Reactions of Anthropogenic Emissions Lead to the High Chemodiversity of Atmospheric Nitrogen-Containing Compounds during the Haze Event

Nitrogen-containing organic compounds (NOCs), a type of important reactive-nitrogen species, are abundant in organic aerosols in haze events observed in Northern China. However, due to the complex nature of NOCs, the sources, formation, and influencing factors are still ambiguous. Here, the molecular composition of organic matters (OMs) in hourly PM2.5 samples collected during a haze event in Northern China was characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We found that CHON compounds (formulas containing C, H, O, and N atoms) dominated the OM fractions during the haze and showed high chemodiversity and transformability. Relying on the newly developed revised-workflow and oxidation-hydrolyzation knowledge for CHON compounds, 64% of the major aromatic CHON compounds (>80%) could be derived from the oxidization or hydrolyzation processes. Results from FT-ICR MS data analysis further showed that the aerosol liquid water (ALW)-involved aqueous-phase reactions are important for the molecular distribution of aromatic-CHON compounds besides the coal combustion, and the ALW-involved aromatic-CHON compound formation during daytime and nighttime was different. Our results improve the understanding of molecular composition, sources, and potential formation of CHON compounds, which can help to advance the understanding for the formation, evolution, and control of haze.

Changes of water-soluble inorganic sulfate and nitrate during severe dust storm episodes in a coastal city of North China

Dust storms are one of the largest sources of non-exhaust emissions in China, which can adversely affect air quality and human health during long-distance transportation. To study the influence of dust storms on aerosol particle composition, samples of fine aerosol (PM2.5) were collected before, during, and after the severe dust storm episodes in a coastal city of North China. Then the water-soluble inorganic ions in the filters were analyzed. The results showed that the chemical composition varied significantly in different sampling periods. Before the dust storm periods (Phase 1), the weather was characterized by high relative humidity. NO3- was the main water-soluble inorganic ion, accounting for about 1/3 of the total mass of PM2.5, which is very different from the situation a few years ago when sulfate was the dominant. The results indicated that the chemical composition of the atmosphere in China has changed significantly after the implementation of strict air pollution control measures. During the severe dust storm periods (within a few hours after the dust invasion, Phase 2), the proportion of Ca2+ in PM2.5 was high; the sulfate formation was limited due to adiabatic air mass affected by the cold front, and the sulfate content might be mainly from desert soil. However, a small amount of nitrate can be formed during their long-distance transportation. After the dust storm periods (Phase 3), dust plums and local polluted air mass mixed well. The proportion of secondary inorganic ions increased, and nitrate formation was still the main. The changes in the chemical composition from a few years ago during Phase 1 and the sharp changes in different water-soluble inorganic ions during different Phases should be carefully considered to evaluate their implications for air quality and human health.

Hindered visibility improvement despite marked reduction in anthropogenic emissions in a megacity of southwestern China: An interplay between enhanced secondary inorganics formation and hygroscopic growth at prevailing high RH conditions

The PM2.5-bound visibility improvement remains challenging in China despite vigorous control on anthropogenic emissions in recent years. One critical issue could exist in the distinct physicochemical properties especially of secondary aerosol components. Taken the COVID-19 lockdown as an extreme case, we focus on the relationship between visibility, emission cuts, and secondary formation of inorganics with changing optical and hygroscopic behaviors in Chongqing, a representative city characterized with humid weather and poor diffusion conditions in Sichuan Basin, southwest of China. It is found that the increased secondary aerosol abundance (e.g., PM2.5/CO and PM2.5/PM10 as a proxy) with enhanced atmospheric oxidative capacity (e.g., O3/Ox, Ox = O3 + NO2), combined with insignificant meteorological dilution effect, might partly offset the benefit on the improved visibility from substantial reduction in anthropogenic emissions during the COVID-19 lockdown. This is in line with the efficient oxidation rates of sulfur and nitrogen (i.e., SOR, NOR), increasing more significantly with PM2.5 and relative humidity (RH) in comparison to O3/Ox. The resulted larger fraction of nitrate and sulfate (i.e., fSNA) would promote the optical enhancement (i.e., f(RH)) and mass extinction efficiency (MEE) of PM2.5, especially under highly humid conditions (e.g., RH > 80 %, with approximately half of the occurrence frequency). This could further facilitate secondary aerosol formation via aqueous-phase reaction and heterogeneous oxidation, likely due to enhanced water uptake and enlarged size/surface area upon hydration. In combination of gradually increased atmospheric oxidative capacity, this positive feedback would in turn inhibit the visibility improvement particularly at high RH environment. Considering the current air pollution complex status over China, further work on the formation mechanisms of major secondary species (e.g., sulfate, nitrate, and secondary organics), size-resolved chemical and hygroscopic properties, together with their interactions are highly recommended. Our results are hoping to assist in the atmospheric pollution complex mitigation and prevention in China.

Combined Smog Chamber/Oxidation Flow Reactor Study on Aging of Secondary Organic Aerosol from Photooxidation of Aromatic Hydrocarbons

Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5), and their physicochemical properties can be significantly changed in the aging process. In this study, we used a combination consisting of a smog chamber (SC) and oxidation flow reactor (OFR) to investigate the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, a typical aromatic hydrocarbon. Our results showed that as the OH exposure increased from 2.6 × 1011 to 6.3 × 1011 molecules cm-3 s (equivalent aging time of 2.01-4.85 days), the SOA mass concentration (2.9 ± 0.05-28.7 ± 0.6 μg cm-3) and corrected SOA yield (0.073-0.26) were significantly enhanced. As the aging process proceeds, organic acids and multiple oxygen-containing oxidation products are continuously produced from the photochemical aging process of gas-phase organic intermediates (mainly semi-volatile and intermediate volatility species, S/IVOCs). The multigeneration oxidation products then partition to the aerosol phase, while functionalization of SOA rather than fragmentation dominated in the photochemical aging process, resulting in much higher SOA yield after aging compared to that in the SC. Our study indicates that SOA yields as a function of OH exposure should be considered in air quality models to improve SOA simulation, and thus accurately assess the impact on SOA properties and regional air quality.

High contribution of new particle formation to ultrafine particles in four seasons in an urban atmosphere in south China

Ultra fine particles (UFP) cover the size range of both nucleation mode particles (NUC, Dp < 25 nm) and Aitken mode particles (AIT, 25 nm < Dp < 100 nm), and play important roles in radiative forcing and human health. In this study, we identified new particle formation (NPF) events and undefined events, explored their potential formation mechanism, and quantified their contributions to UFP number concentration (NUFP) in urban Dongguan of the Pearl River Delta (PRD) region. Field campaigns were carried out in four seasons in 2019 to measure particle number concentration in the size range of 4.7-673.2 nm, volatile organic compounds (VOCs), gaseous pollutants, chemical compositions in PM2.5, and meteorological parameters. The frequency of the occurrence of NPF, as indicated by a significant increase in NUC number concentration (NNUC), was 26 %, and that of the undefined event, as indicated by substantial increases in NNUC or AIT number concentration (NAIT), was 32 % during the whole campaign period. The NPF events mainly occurred in autumn (with a frequency of 59 %) and winter (33 %) and only occasionally in spring (4 %) and summer (4 %). On the contrary, the frequencies of the undefined events were higher in spring (52 %) and summer (38 %) than in autumn (19 %) and winter (22 %). The burst periods of the NPF events mainly occurred before 11:00 Local Time (LT), while those of the undefined events mainly occurred after 11:00 LT. Accompanied to NPF events were low concentrations of VOCs and high concentrations of O3. The undefined events by NUC or AIT were associated with the upwind transport of newly formed particles. Source apportionment analysis suggested that NPF and undefined events were the largest contributor to NNUC (51 ± 28 %), NAIT (41 ± 26 %), and NUFP (45 ± 27 %), while coal combustion and biomass burning, and traffic emission were the second largest contributor to NNUC (22 ± 20 %) and NAIT (39 ± 28 %), respectively.

Time-Resolved Molecular Characterization of Secondary Organic Aerosol Formed from OH and NO3 Radical Initiated Oxidation of a Mixture of Aromatic Precursors

Aromatic hydrocarbons (ArHCs) and oxygenated aromatic hydrocarbons (ArHC-OHs) are emitted from a variety of anthropogenic activities and are important precursors of secondary organic aerosol (SOA) in urban areas. Here, we analyzed and compared the composition of SOA formed from the oxidation of a mixture of aromatic VOCs by OH and NO3 radicals. The VOC mixture was composed of toluene (C7H8), p-xylene + ethylbenzene (C8H10), 1,3,5-trimethylbenzene (C9H12), phenol (C6H6O), cresol (C7H8O), 2,6-dimethylphenol (C8H10O), and 2,4,6-trimethylphenol (C9H12O) in a proportion where the aromatic VOCs were chosen to approximate day-time traffic-related emissions in Delhi, and the aromatic alcohols make up 20% of the mixture. These VOCs are prominent in other cities as well, including those influenced by biomass combustion. In the NO3 experiments, large contributions from CxHyOzN dimers (C15-C18) were observed, corresponding to fast SOA formation within 15-20 min after the start of chemistry. Additionally, the dimers were a mixture of different combinations of the initial VOCs, highlighting the importance of exploring SOAs from mixed VOC systems. In contrast, the experiments with OH radicals yielded gradual SOA mass formation, with CxHyOz monomers (C6-C9) being the dominant constituents. The evolution of SOA composition with time was tracked and a fast degradation of dimers was observed in the NO3 experiments, with concurrent formation of monomer species. The rates of dimer decomposition in NO3 SOA were ∼2-3 times higher compared to those previously determined for α-pinene + O3 SOA, highlighting the dependence of particle-phase reactions on VOC precursors and oxidants. In contrast, the SOA produced in the OH experiments did not dramatically change over the same time frame. No measurable effects of humidity were observed on the composition and evolution of SOA.

Characteristics and sources of PM2.5-bound elements in Shanghai during autumn and winter of 2019: Insight into the development of pollution episodes

Elemental composition of PM_2.5 dispersed in the atmosphere has received increasing attention due to its health effect and catalytic activities. In this study, the characteristics and source apportionment of PM_2.5-bound elements were investigated using hourly measurements. K is the most abundant metal element, followed by Fe > Ca > Zn > Mn > Ba > Pb > Cu > Cd. With an average of 8.8 ± 4.1 ng m^-3, Cd was the only element whose pollution level exceeded the limits of Chinese standards and WHO guidelines. The concentrations of As, Se, and Pb doubled in December compared to November, indicating a large increase in coal consumption in winter. The enrichment factors of As, Se, Hg, Zn, Cu, Cd, and Ag were larger than 100, indicating that anthropogenic activities greatly affected them. Ship emissions, coal combustion, soil dust, vehicle emissions, and industrial emissions were identified as major sources of trace elements. In November, the pollution from coal burning and industrial activities was significantly reduced, demonstrating the remarkable achievement of coordinated control measures. For the first time, hourly measurements of PM_2.5-bound elements and secondary sulfate and nitrate were used to investigate the development of dust and PM_2.5 events. During a dust storm event, secondary inorganic salts, potentially toxic elements, and crustal elements sequentially reached peak concentrations, indicating different source origins and formation mechanisms. During the winter PM_2.5 event, the sustained increase of trace elements was attributed to the accumulation of local emissions, while regional transport was responsible for the explosive growth before the end of the event. This study highlights the important role of hourly measurement data in distinguishing local accumulation from regional and long-range transport.