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There are thousands of volatile organic compound (VOC) species in ambient air, while existing techniques can only detect a small part of them (approximately several hundred). The large number of unmeasured VOCs prevents us from understanding the photochemistry of ozone and aerosols in the atmosphere. The major sources and photochemical effects of these unmeasured VOCs in urban areas remain unclear. The missing VOC reactivity, which is defined as the total OH reactivity of the unmeasured VOCs, is a good indicator for constraining the photochemical effect of unmeasured VOCs. Here, we identified the dominant role of anthropogenic emission sources in the missing VOC reactivity (accounting for up to 70 %) by measuring missing VOC reactivity and tracer-based source analysis in a typical megacity in China. Omitting the missing VOC reactivity from anthropogenic emissions in model simulations will remarkably affect the diagnosis of sensitivity regimes for ozone formation, overestimating the degree of VOC-limited regimes by up to 46 %. Therefore, a thorough quantification of missing VOC reactivity from various anthropogenic emission sources is urgently needed for constraints of air quality models and the development of effective ozone control strategies. [ABSTRACT FROM AUTHOR]

PM 2.5 pollution is a major air quality issue that deteriorates human health, and numerous studies have focused on PM 2.5 pollution in major regions such as the North China Plain (NCP) and Yangtze River Delta (YRD). However, the characteristics of PM 2.5 concentrations and the associated formation mechanism in the transport corridor (referred to as SWLY) between the NCP and YRD are largely ignored. Based on observational data, we find that the number of PM 2.5 pollution events in SWLY is comparable to that in the NCP, far exceeding that in the YRD, which is indicative of the severity of air pollution in this area. Utilizing a regional climate and air quality model, we isolate the effect of seesaw transport events, e.g., transport between the NCP and YRD, and atmospheric stagnation on the accumulation of PM 2.5 over SWLY. Specifically, seesaw events and stagnation, comparable to each other, collectively account for an average of 67 % of pollution days, with PM 2.5 exceeding 75 µgm-3 , and this fraction (85 %) is even larger for severe haze events, with PM 2.5 exceeding 150 µgm-3. Furthermore, the connection between seesaw transport and large-scale circulation is examined. The transregional transport of pollutants from the NCP to the YRD (YRD to NCP) is likely stimulated by positive (negative) to negative (positive) geopotential height anomalies at 500 hPa located in northern China. The health effect due to short-term PM 2.5 exposure induced by the transregional transport and stagnation is investigated, yielding a total of 8634 (95 % CI: 6023–11 223) and 9496 (95 % CI: 6552–12 413) premature deaths, respectively, in SWLY during winter 2014–2019, which is as high as 9 % of the total premature deaths in China, even though SWLY takes up less than 1 % of China's area. While atmospheric stagnation is in general projected to occur more frequently under a warming climate, this study indicates the importance of regional emission control to alleviate PM 2.5 pollution from seesaw transport and stagnation. [ABSTRACT FROM AUTHOR]

There are thousands of VOC species in ambient air, while existing techniques can only detect a small part of them (~ several hundred). The large number of unmeasured VOCs prevents us from understanding the photochemistry of ozone and aerosols in the atmosphere. The major sources and photochemical effects of these unmeasured VOCs in urban areas remain unclear. The missing VOC reactivity, which is defined as the total OH reactivity of the unmeasured VOCs, is a good indicator to constrain the photochemical effect of unmeasured VOCs. Here, we identified the dominant role of anthropogenic emission sources in the missing VOC reactivity (accounting for up to 70 %) by measuring missing VOC reactivity and tracer-based source analysis in a typical megacity in China. VOC reactivity from anthropogenic emissions will remarkably affect the diagnosis of sensitivity regimes for ozone formation, overestimating the degree of VOC-limited regime by up to 46 %. Therefore, a thorough quantification of missing VOC reactivity from various anthropogenic emission sources is urgently needed for constraints of air quality models and the development of effective ozone control strategies. [ABSTRACT FROM AUTHOR]

PM2.5 pollution is a major air quality issue deteriorating human health, and numerous studies focus on PM2.5 pollution in major regions such as North China Plain (NCP) and Yangtze River Delta (YRD). However, the characteristics of PM2.5 concentrations and the associated formation mechanism in the transport corridor (referred to as SWLY) between NCP and YRD are largely ignored. Based on observational data, we find the number of PM2.5 pollution events in SWLY is comparable to that in NCP, far exceeding those in YRD, indicative of the severity of air pollution over this area. Utilizing a regional climate and air quality model, we isolate the effect of seesaw transport events, e.g., transport between NCP and YRD, as well as the atmospheric stagnation on the accumulation of PM2.5 over SWLY. Specifically, seesaw events and stagnation, comparable to each other, collectively account for an average of 67 % pollution days with PM2.5 exceeding 75 µg/m3, and this fraction (85 %) is even larger for severe haze events with PM2.5 exceeding 150 µg/m3. Furthermore, the connection between seesaw transport and large-scale circulation is examined. The trans-regional transport of pollutants from NCP to YRD (YRD to NCP) is likely stimulated by positive (negative) to negative (positive) geopotential height anomaly at 500 hPa located in northern China. The health effect due to short-term PM2.5 exposure induced by the trans-regional transport and stagnation is investigated, yielding a total of 8,634 (95 % CI: 6,023–11,223) and 9,496 (95 % CI: 6,552–12,413) premature deaths respectively in SWLY during winter 2014–2019, as high as 9 % of the total premature deaths in China although the area coverage of SWLY is within 1 %. While atmospheric stagnation is in general projected to occur more frequently under a warming climate, this study indicates the importance of regional emission control to alleviate PM2.5 pollution from seesaw transport and stagnation. [ABSTRACT FROM AUTHOR]

Aerosol acidity plays a key role in regulating the chemistry and toxicity of atmospheric aerosol particles. The trend of aerosol pH and its drivers is crucial in understanding the multiphase formation pathways of aerosols. Here, we reported the first trend analysis of aerosol pH from 2011 to 2019 in eastern China, calculated with the ISORROPIA model based on observed gas and aerosol compositions. The implementation of the Air Pollution Prevention and Control Action Plan led to -35.8 %, -37.6 %, -9.6 %, -81.0 % and 1.2 % changes of PM 2.5 , SO42- , NHx , non-volatile cations (NVCs) and NO3- in the Yangtze River Delta (YRD) region during this period. Different from the drastic changes of aerosol compositions due to the implementation of the Air Pollution Prevention and Control Action Plan, aerosol pH showed a minor change of -0.24 over the 9 years. Besides the multiphase buffer effect, the opposite effects from the changes of SO42- and non-volatile cations played key roles in determining this minor pH trend, contributing to a change of +0.3 8 and -0.35 , respectively. Seasonal variations in aerosol pH were mainly driven by the temperature, while the diurnal variations were driven by both temperature and relative humidity. In the future, SO2 , NOx and NH3 emissions are expected to be further reduced by 86.9 %, 74.9 % and 41.7 % in 2050 according to the best health effect pollution control scenario (SSP1-26-BHE). The corresponding aerosol pH in eastern China is estimated to increase by ∼0.19 , resulting in 0.04 less NO3- and 0.12 less NH4+ partitioning ratios, which suggests that NH3 and NOx emission controls are effective in mitigating haze pollution in eastern China. [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) has been recognized as an important contributor to global warming due to its absorption of solar radiation with strong wavelength dependence. While early studies mostly consider biomass burning as a major emission source, a few recent studies have suggested coal combustion and vehicle exhaust as important contributors to BrC emissions. Considering dominant coal consumption, large residential biofuel usage, and the increasing number of vehicles, BrC pollution could result in remarkable regional climate effects in China. This study gives an overview of sources, chemical composition and absorption properties of BrC, focusing on BrC categorization and its absorption measurement techniques. The basic pollution features of BrC in China, including surface concentrations, column burden, sources and associated absorbing properties are then summarized and compared to other countries. BrC emissions as well as its optical emission at 365 nm from the primary sources in China are also summarized to provide a more straightforward understanding of the light‐absorbing properties of BrC in China. With a detailed comparison of existing publications, the study gives deeper insight into the uncertainties in BrC measurements and will improve the understanding of the climate effect of BrC. The review also recommends key research areas for further investigation of BrC in the future. Plain Language Summary: Brown carbon (BrC) is a collection of organic compounds with brownish color. It has been identified as an important component of atmospheric absorbing aerosols and plays a substantial role in climate change. However, there is still a great challenge to precisely evaluate the climate effect of BrC. The difficulties stem mainly from the lack of the estimates of BrC emissions as well as the high variability of its light absorption ability. This review summarizes BrC sources, absorption ability, and pollution levels in China. The large energy consumption in China with dominant coal combustion and substantial residential biomass burning makes China a potentially large BrC emission source region and explains the observed relatively high BrC pollution level. The contributions of fossil fuel combustion to BrC emission and light absorption are comparable to those from biomass burning in China, although with large uncertainties. During the last decade, BrC emission from biomass burning and coal combustion decreases in China due to the change in energy structure associated with the implementation of emission control measures. To better address the climate impacts of BrC, further efforts devoted to the measurements of BrC, in particular its absorption ability from different sources are in urgent need. Key Points: BrC concentrations and absorption in China are much higher than in the USA, but similar to IndiaEmission and absorbing ability of BrC from fossil fuel combustion are comparable to or even larger than those from biomass burningDuring the last decade, BrC emission from biomass burning and coal combustion decreases in China [ABSTRACT FROM AUTHOR]

In the past decade, ozone (O 3) pollution has become a severe environmental problem in China's major cities. Here, based on available observational records, we investigated the long-term trend of O 3 pollution in China during 2014–2020. The O 3 concentrations were slightly higher in urban areas than in non-urban areas. During these 7 years, the highest O 3 concentrations primarily occurred during summer in northern China, and during autumn or spring in southern China. Although O 3 precursors, including nitrogen oxides (NO x) and carbon monoxide (CO), continuously decreased, O 3 concentrations generally increased throughout the 7 years with a slower increasing rate after 2017. The long-term trend of O 3 concentrations differed across seasons, especially from 2019 to 2020, when O 3 concentrations decreased in summer and increased in winter. To analyse the causes of this observed trend, a photochemical box model was used to investigate the change in the O 3 sensitivity regime in two representative cities – Beijing and Shanghai. Our model simulations suggest that the summertime O 3 sensitivity regime in urban areas of China has changed from a VOC-limited regime to a transition regime during 2014–2020. By 2020, the urban photochemistry was in a transition regime in summer but in a VOC-limited regime in winter. This study helps to understand the distinct trends of O 3 in China and provides insights into efficient future O 3 control strategies in different regions and seasons. [ABSTRACT FROM AUTHOR]

With improving PM2.5 air quality, the tropospheric ozone (O3) has become the top issue of China's air pollution control. Here, we combine comprehensive observational data analysis with models to unveil the contributions of different processes and precursors to the change of O3 during COVID-19 lockdown in the Yangtze River Delta (YRD), one of the most urbanized megacity regions of eastern China. Despite a 44 to 47% reduction in volatile organic compounds (VOCs) and nitrogen oxides (NOx) emissions, maximum daily 8-h average (MDA8) ozone concentrations increase from 28 ppbv in pre-lockdown to 43 ppbv in lockdown period. We reproduce this transition with the WRF-Chem model, which shows that ~80% of the increase in MDA8 is due to meteorological factors (seasonal variation and radiation), and ~20% is due to emission reduction. We find that daytime photochemistry does not lead to an increase but rather a decrease of daytime O3 production during the lockdown. However, the reduced O3 production is overwhelmed by the weakened nitric oxide (NO) titration resulting in a net increase of O3 concentration. Although the emission reduction increases O3 concentration, it leads to a decrease in the Ox (O3 + NO2) concentration, suggesting reduced atmospheric oxidation capacity on a regional scale. The dominant effect of NO titration demonstrates the importance of prioritizing VOCs reduction, especially from solvent usage and the petrochemical industry with high emission ratios of VOCs/NOx. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) regulate atmospheric oxidation capacity, and the reactions of VOCs are key in understanding ozone formation and its mitigation strategies. When evaluating their impact, most previous studies did not fully consider the role of oxygenated VOCs due to limitations of measurement technology. By using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) combined with a gas chromatograph–mass spectrometer (GC–MS), a large number of oxygenated VOCs (OVOCs) have been quantified in Guangzhou city, China. Based on the new dataset, we demonstrate that constraints using OVOC observations are essential in modeling radical and ozone production, as modeled OVOCs can be substantially lower than measurements, potentially due to primary emissions and/or missing secondary sources. Non-formaldehyde (HCHO) OVOCs can contribute large fractions (22 %–44 %) of total RO x radical production, which are comparable to or larger than the contributions from nitrous acid and formaldehyde. Our results show that models without OVOC constraints using ambient measurements will underestimate the production rates of RO x and ozone, and they may also affect the determination of sensitivity regime in ozone formation. Therefore, a thorough quantification of photodegradable OVOC species is in urgent need to understand accurately the ozone chemistry and to develop effective control strategies. [ABSTRACT FROM AUTHOR]

The heterogeneous reaction of N2O5 on Cl-containing aerosols (heterogeneous N2O5 + Cl chemistry) plays a key role in chlorine activation, NOx recycling, and consequently O3 and PM2.5 formation. In this study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions combined with updated parameterizations for the heterogeneous N2O5 + Cl chemistry (i.e., the uptake coefficient of N2O5 (γN2O5) and the ClNO2 yield (φClNO2)) to investigate the impacts of chlorine chemistry on air quality in China, the role of the heterogeneous N2O5 + Cl chemistry, and the sensitivity of air pollution formation to chlorine emissions and parameterizations for γN2O5 and φClNO2. The model simulations are evaluated against multiple observational datasets across China and show significant improvement in reproducing observations of particulate chloride, N2O5 , and ClNO2 when including anthropogenic chlorine emissions and updates to the parameterization of the heterogeneous N2O5 + Cl chemistry relative to the default model. The simulations show that total tropospheric chlorine chemistry could increase annual mean maximum daily 8 h average (MDA8) O3 by up to 4.5 ppbv but decrease PM2.5 by up to 7.9 µgm-3 in China, 83 % and 90 % of which could be attributed to the effect of the heterogeneous N2O5 + Cl chemistry. The heterogeneous uptake of N2O5 on chloride-containing aerosol surfaces is an important loss pathway of N2O5 as well as an important source of O3 and hence is particularly useful in elucidating the commonly seen ozone underestimations relative to observations. The importance of chlorine chemistry largely depends on both chlorine emissions and the parameterizations for the heterogeneous N2O5 + Cl chemistry. With the additional chlorine emissions, the simulations show that annual MDA8 O3 in China could be increased by up to 3.5 ppbv. The corresponding effect on PM2.5 concentrations varies largely with regions, with an increase of up to 4.5 µgm-3 in the North China Plain but a decrease of up to 3.7 µgm-3 in the Sichuan Basin. On the other hand, even with the same chlorine emissions, the effects on MDA8 O3 and PM2.5 in China could differ by 48 % and 27 %, respectively, between different parameterizations. [ABSTRACT FROM AUTHOR]

China has made great efforts to monitor and control air pollution in the past decade. Comprehensive characterization and understanding of pollutants in three-dimensions are, however, still lacking. Here, we used data from an observation network consisting of 13 aerosol lidars and more than 1000 ground observation stations combined with a data assimilation technique to conduct a comprehensive analysis of extreme heavy aerosol pollution (HAP) over the North China Plain (NCP) from November–December 2017. During the studied period, the maximum hourly mass concentration of surface PM 2.5 reached ∼390 µ g m -3. After assimilation, the correlation between model results and the independent observation sub-dataset was ∼50 % higher than that without the assimilation, and the root mean square error was reduced by ∼40 %. From pollution development to dissipation, we divided the HAP in the NCP (especially in Beijing) into four phases: an early phase (EP), a transport phase (TP), an accumulation phase (AP), and a removal phase (RP). We then analyzed the evolutionary characteristics of PM 2.5 concentration during different phases on the surface and in 3-D space. We found that the particles were mainly transported from south to north at a height of 1–2 km (during EP and RP) and near the surface (during TP and AP). The amounts of PM 2.5 advected into Beijing with the maximum transport flux intensity (TFI) were through the pathways in the relative order of the southwest > southeast > east pathways. The dissipation of PM 2.5 in the RP stage (with negative TFI) was mainly from north to south with an average transport height of ∼1 km above the surface. Our results quantified the multi-dimensional distribution and evolution of PM 2.5 concentration over the NCP, which may help policymakers develop efficient air pollution control strategies. [ABSTRACT FROM AUTHOR]

Many studies have recently been done on understanding the sources and formation mechanisms of atmospheric aerosols at ground level. However, vertical profiles and sources of size-resolved particulate matter within the urban boundary layer are still lacking. In this study, vertical distribution characteristics of size-segregated particles were investigated at three observation platforms (ground level, 118 m, and 488 m) on the 610 m high Canton Tower in Guangzhou, China. Size-segregated aerosol samples were simultaneously collected at the three levels in autumn and winter. Major aerosol components, including water-soluble ions, organic carbon, and elemental carbon, were measured. The results showed that daily average fine-particle concentrations generally decreased with height. Concentrations of sulfate and ammonium in fine particles displayed shallow vertical gradients, and nitrate concentrations increased with height in autumn, while the chemical components showed greater variations in winter than in autumn. The size distributions of sulfate and ammonium in both seasons were characterized by a dominant unimodal mode with peaks in the size range of 0.44–1.0 µ m. In autumn, the nitrate size distribution was bimodal, peaking at 0.44–1.0 and 2.5–10 µ m, while in winter it was unimodal, implying that the formation mechanisms for nitrate particles were different in the two seasons. Our results suggest that the majority of the sulfate and nitrate is formed from aqueous-phase reactions, and we attribute coarse-mode nitrate formation at the measurement site to the heterogeneous reactions of gaseous nitric acid on existing sea-derived coarse particles in autumn. Case studies further showed that atmospheric aqueous-phase and heterogeneous reactions could be important mechanisms for sulfate and nitrate formation, which, in combination with adverse weather conditions such as temperature inversion and calm wind, led to haze formation during autumn and winter in the Pearl River Delta (PRD) region. [ABSTRACT FROM AUTHOR]

Submicron aerosol (PM1) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM1 and PM2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM1/PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM1/PM2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Plain Language Summary: Current air pollution studies rely largely upon aerosol mass spectrometers that provide real‐time measurements of submicron aerosol (PM1) species, and in many studies, PM1 aerosol species are used to validate those of PM2.5 in chemical transport models and estimate particle acidity and aerosol water content which are key parameters in studying heterogeneous reactions. However, the uncertainties in air pollution studies due to the chemical differences of PM1 and PM2.5 are poorly constrained, particularly in highly polluted environment, for example, China and India. We found that the changes in PM1/PM2.5 ratios as a function of relative humidity were largely different for primary and secondary aerosol species in highly polluted environment, which was likely driven by the changes in aerosol hygroscopicity and phase states. The chemical differences of PM1/PM2.5 ratios were also found to have negligible impacts on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Considering the rapid increases in the deployments of aerosol mass spectrometers worldwide, the results in this study are of great importance for a better understanding of the uncertainties in both modelling and observations. Key Points: We observed large chemical differences between PM1 and PM2.5 under high relative humidity periods in highly polluted environmentThe RH dependence of PM1/PM2.5 ratios of primary and secondary species was largely different due to different hygroscopicityThe chemical differences between PM1 and PM2.5 have negligible influences on particle acidity prediction, yet have a large impact on AWC [ABSTRACT FROM AUTHOR]

Topic 3 of the Model Inter-Comparison Study for Asia (MICS-Asia) Phase III examines how online coupled air quality models perform in simulating wintertime haze events in the North China Plain region and evaluates the importance of aerosol radiative feedbacks. This paper discusses the estimates of aerosol radiative forcing, aerosol feedbacks, and possible causes for the differences among the participating models. Over the Beijing–Tianjin–Hebei (BTH) region, the ensemble mean of estimated aerosol direct radiative forcing (ADRF) at the top of atmosphere, inside the atmosphere, and at the surface are -1.1 , 7.7, and -8.8 W m -2 during January 2010, respectively. Subdivisions of direct and indirect aerosol radiative forcing confirm the dominant role of direct forcing. During severe haze days (17–19 January 2010), the averaged reduction in near-surface temperature for the BTH region can reach 0.3–1.6 ∘ C. The responses of wind speeds at 10 m (WS10) inferred from different models show consistent declines in eastern China. For the BTH region, aerosol–radiation feedback-induced daytime changes in PM 2.5 concentrations during severe haze days range from 6.0 to 12.9 µ g m -3 (<6 %). Sensitivity simulations indicate the important effect of aerosol mixing states on the estimates of ADRF and aerosol feedbacks. Besides, black carbon (BC) exhibits a large contribution to atmospheric heating and feedbacks although it accounts for a small share of mass concentration of PM 2.5. [ABSTRACT FROM AUTHOR]

This study investigates the water supply risk of Panjiakou reservoir in the Luanhe River basin of China during 1956–2016 under environmental change. Since the monthly runoff series during 1956–2016 is a time series with change points, it is necessary to find a new stochastic streamflow series generation approach to preserve the statistical characteristics of the original series and to refine the reliability of water supply risk analysis. This paper improves a known stochastic streamflow simulation method of previous research to better reflect the characteristics of series with change points. And this paper simulates the monthly runoff series with change point of Panjiakou reservoir during 1956–2016 by three different methods, including Thomas–Fiering model, copula function stochastic simulation method, and copula function stochastic simulation method with the mixed distribution model. Among the three methods, the copula function stochastic simulation method with the mixed distribution model which is improved on the basis of copula function stochastic simulation method in this study performs best in simulating the observed monthly runoff series during 1956–2016, and the water supply risk indices including reliability (time-based and volume-based), resilience, vulnerability, drought risk index (DRI), and sustainability index (SUI) are evaluated for Panjiakou reservoir and analyzed by using the stochastic simulation results. By comparing with the previous studies, all indicators are between the corresponding results of 1956–1979 and 1980–2016 with stationary inflows; it can be seen that change point seriously affects the water supply risk of Panjiakou reservoir. These results make it easy to formulate water supply strategies and schemes in changing environment for water resources managers. [ABSTRACT FROM AUTHOR]

Non-methane volatile organic compounds (NMVOCs) are important ozone and secondary organic aerosol precursors and play important roles in tropospheric chemistry. In this work, we estimated the total and speciated NMVOC emissions from China's anthropogenic sources during 1990–2017 by using a bottom-up emission inventory framework and investigated the main drivers behind the trends. We found that anthropogenic NMVOC emissions in China have been increasing continuously since 1990 due to the dramatic growth in activity rates and absence of effective control measures. We estimated that anthropogenic NMVOC emissions in China increased from 9.76 Tg in 1990 to 28.5 Tg in 2017, mainly driven by the persistent growth from the industry sector and solvent use. Meanwhile, emissions from the residential and transportation sectors declined after 2005, partly offsetting the total emission increase. During 1990–2017, mass-based emissions of alkanes, alkenes, alkynes, aromatics, oxygenated volatile organic compounds (OVOCs) and other species increased by 274 %, 88 %, 4 %, 387 %, 91 % and 231 %, respectively. Following the growth in total NMVOC emissions, the corresponding ozone formation potential (OFP) increased from 38.2 Tg of O3 in 1990 to 99.7 Tg of O3 in 2017. We estimated that aromatics accounted for the largest share (43 %) of the total OFP, followed by alkenes (37 %) and OVOCs (10 %). Growth in China's NMVOC emissions was mainly driven by the transportation sector before 2000, while industry and solvent use dominated the emission growth during 2000–2010. Since 2010, although emissions from the industry sector and solvent use kept growing, strict control measures on transportation and fuel transition in residential stoves have successfully slowed down the increasing trend, especially after the implementation of China's clean air action since 2013. However, compared to large emission decreases in other major air pollutants in China (e.g., SO2 , NOx and primary PM) during 2013–2017, the relatively flat trend in NMVOC emissions and OFP revealed the absence of effective control measures, which might have contributed to the increase in ozone during the same period. Given their high contributions to emissions and OFP, tailored control measures for solvent use and industrial sources should be developed, and multi-pollutant control strategies should be designed to mitigate both PM 2.5 and ozone pollution simultaneously. [ABSTRACT FROM AUTHOR]

Topic 3 of the Model Inter-Comparison Study for Asia (MICS-Asia) Phase III examines how online coupled air quality models perform in simulating high aerosol pollution in the North China Plain region during wintertime haze events and evaluates the importance of aerosol radiative and microphysical feedbacks. A comprehensive overview of the MICS-Asia III Topic 3 study design, including descriptions of participating models and model inputs, the experimental designs and results of model evaluation, are presented. Six modeling groups from China, Korea and the United States submitted results from seven applications of online coupled chemistry-meteorology models. Results are compared to meteorology and air quality measurements, including data from the Campaign on Atmospheric Aerosol Research Network of China (CARE-China) and the Acid Deposition Monitoring Network in East Asia (EANET). The correlation coefficients between the multi-model ensemble mean and the CARE-China observed near-surface air pollutants range from 0.51 to 0.94 (0.51 for ozone and 0.94 for PM2:5) for January 2010. However, large discrepancies exist between simulated aerosol chemical compositions from different models. The coefficient of variation (SD divided by the mean) can reach above 1.3 for sulfate in Beijing and above 1.6 for nitrate and organic aerosols in coastal regions, indicating that these compositions are less consistent from different models. During clean periods, simulated aerosol optical depths (AODs) from different models are similar, but peak values differ during severe haze events, which can be explained by the differences in simulated inorganic aerosol concentrations and the hygroscopic growth efficiency (affected by varied relative humidity). These differences in composition and AOD suggest that future models can be improved by including new heterogeneous or aqueous pathways for sulfate and nitrate formation under hazy conditions, a secondary organic aerosol (SOA) formation chemical mechanism with new volatile organic compound (VOCs) precursors, yield data and approaches and a more detailed evaluation of the dependence of aerosol optical properties on size distribution and mixing state. It was also found that using the ensemble mean of the models produced the best prediction skill. While this has been shown for other conditions (for example, the prediction of high-ozone events in the US (Mc- Keen et al., 2005)), this is to our knowledge the first time it has been shown for heavy haze events. [ABSTRACT FROM AUTHOR]

In situ measurements of the physical, chemical, and optical properties of aerosols were carried out in Guangzhou city, China, from 1 to 31 July 2006 during the Pearl River Delta (PRD) Campaign. The light extinction coefficient of the ambient atmosphere, the aerosol scattering coefficient under dry conditions, the aerosol absorption coefficient under ambient conditions, NO2 concentration, and relative humidity (RH) were measured by transmissionmeter, an integrating nephelometer, a multi-angle absorption photometer (MAAP), a NO X analyzer, and an automatic meteorological station, respectively. Meanwhile, the molecular scattering coefficient was calculated by the Rayleigh scattering function using the US Standard Atmosphere. A method to calculate the aerosol hygroscopic growth factor f(RH), defined as the ratio of the aerosol scattering coefficient under a wet condition to that under a dry condition (40% RH), is proposed based on these optical parameters. The mean and standard deviation aerosol hygroscopic growth factors at 80% RH (f(RH)=80%) in Ganzhou were 2.04±0.28, 2.29±0.28, and 2.68±0.59 for urban aerosols, mixed aerosols, and marine aerosols, respectively, with the air mass classification being based on the air mass source region. The relationship between f(RH) and RH is fitted by empirical equations and the fitting parameters are calculated. The relationships between f(RH)=80% and total carbon mass fraction (TCF) in PM2.5, the water-soluble mass fraction (WSF) in PM10, and the sea-salt aerosol mass fraction (SSF) in PM10 reveal that the hygroscopic properties of the observed aerosol have a good positive correlation with the WSF and SSF, but have a negative correlation with the TCF. [Copyright &y& Elsevier]

The characteristics of wintertime volatile organic compounds (VOCs) in the North China Plain (NCP) region are complicated and remain obscure. VOC measurements were conducted by a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) at a rural site in the NCP from November to December 2018. Uncalibrated ions measured by PTR-ToF-MS were quantified and the overall VOC compositions were investigated by combining the measurements of PTR-ToF-MS and gas chromatography-mass spectrometer/flame ionization detector (GC-MS/FID). The measurement showed that although atmospheric VOCs concentrations are often dominated by primary emissions, the secondary formation of oxygenated VOCs (OVOCs) is non-negligible in the wintertime, i.e., OVOCs accounts for 42% ± 7% in the total VOCs (151.3 ± 75.6 ppbV). We demonstrated that PTR-MS measurements for isoprene are substantially overestimated due to the interferences of cycloalkanes. The chemical changes of organic carbon in a pollution accumulation period were investigated, which suggests an essential role of fragmentation reactions for large, chemically reduced compounds during the heavy-polluted stage in wintertime pollution. The changes of emission ratios of VOCs between winter 2011 and winter 2018 in the NCP support the positive effect of "coal to gas" strategies in curbing air pollutants. The high abundances of some key species (e.g. oxygenated aromatics) indicate the strong emissions of coal combustion in wintertime of NCP. The ratio of naphthalene to C8 aromatics was proposed as a potential indicator of the influence of coal combustion on VOCs. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study is to quantify the relation of aerosol chemical compositions and optical properties, and to assess the impact of relative humidity (RH) on atmospheric visibility and aerosol direct radiative forcing (ADRF). Mass concentration and size distribution of aerosol chemical compositions as well as aerosol optical properties were concurrently measured at Guangzhou urban site during the PRD (Pearl River Delta) campaign from 1 to 31 July, 2006. Gaseous pollutant NO2 and meteorological parameter were simultaneously monitored. Compared with its dry condition, atmospheric ambient extinction coefficient σ ext(RH) averagely increased about 51% and atmospheric visibility deceased about 35%, among which RH played an important role on the optical properties of water soluble inorganic salts. (NH4)2SO4 is the most important component responsible for visibility degradation at Guangzhou. In addition, the asymmetry factor g increased from 0.64 to 0.74 with the up-scatter fraction β decreasing from 0.24 to 0.19 when RH increasing from 40% to 90%. At 80% RH, the ADRF increased about 280% compared to that at dry condition and it averagely increased about 100% during the campaign under ambient conditions. It can be inferred that aerosol water content is a key factor and could not be ignored in assessing the role of aerosols in visibility impairment and radiative forcing, especially in the regions with high RH. [Copyright &y& Elsevier]

Abstract: The coagulation sink and its role in new particle formation are investigated based on data obtained during the PRIDE-PRD2004 campaign at Xinken of Pearl River Delta, China. Analysis of size distributions and mode contributions of the coagulation sink show that the observed higher load of accumulation mode particles impose a significant effect on the coagulation sink and result in higher coagulation sinks at Xinken despite of the lower total particle number compared with other areas. Hence it is concluded that the higher coagulation sink may depress the occurrence frequency of new particle formation events. The strategies targeting at controlling accumulation mode particles may have influences on the frequency of new particle formation events at this area. The factors affecting the coagulation sink are evaluated. The relatively lower ambient relative humidities may weaken the coagulation sink and facilitate the occurrence of new particle formation events during noontime, while the surmise of nucleation and growth involving organic matter may imply an actually higher coagulation sink than expected. These factors have a significant influence on the ultimate fate of the newly formed nuclei and new particle formation. A comparison of event and non-event days indicates that the coagulation sink is not the only decisive factor affecting new particle formation, other factors including the precursor vapors and photochemical activity are none the less important either. Competition of coagulation sink and high source rate leads to the occurrence of new particle formation events at Xinken. [Copyright &y& Elsevier]

Abstract: In situ measurements of size-resolved aerosol chemical compositions and its optical properties were concurrently carried out at an urban site in mega-city Beijing from October 24 to November 9, 2007. The main objective was to quantitatively study the relationship between aerosol chemical compositions and its hygroscopic properties, to estimate the influence of relative humidity (RH) on aerosol scattering coefficient and to quantitatively investigate visibility impairment due to particle hygroscopic growth. The hygroscopic factor of aerosol scattering coefficient (f(RH)), which is defined as the ratio of aerosol scattering coefficient at wet condition to that at dry condition (RH≤30%), was calculated with the measured aerosol optical properties at dry and ambient conditions. The relationship between f(RH) and RH was fitted by empirical equation and the fitting parameters were calculated. Meanwhile, f(RH) for externally mixed aerosols or internally mixed aerosols was modeled based on size segregated particulate chemical composition. The modeled f(RH) agreed well with the measured f(RH). Empirical formula for atmospheric visibility based on mass concentration of PM2.5 and f(RH) was proposed. The result of this study is not only proven that RH, in addition to the mass concentration of PM2.5, played an important role on visibility impairment, but also provide practical aid for air quality control to improve the visibility in the megacity region of Beijing. [Copyright &y& Elsevier]

Abstract: Urbanization in China accompanies economic development and population growth. Changes in land use leads to changes in both meteorological and chemical fields. In this study, the impact of land use change in Jing-Jin-Ji (JJJ, indicating Beijing–Tianjin–Hebei) and Yangtze River Delta (YRD) areas on meteorology and ozone concentrations are studied using the WRF–Chem model. Land use change is represented by two different land cover data sets: USGS and MODIS. Monthly-average simulations show that urbanization causes an increase in 2-m temperature by maximum 2.4 °C and 3.2 °C in these two areas, respectively. Wind speed simulations suggest a decrease (average 1.2 m s−1) in daytime for YRD, and in nighttime for JJJ. Dew point differences show a dry effect over both areas, with maximum −3 °C in JJJ and −2.4 °C in YRD. Planetary Boundary Layer (PBL) height increases by 400 m (maximum in JJJ) and 600 m (maximum in YRD) for daytime, and nighttime increases are less than 100 m. Daytime ozone concentrations in JJJ increase by 20 ppb due to urbanization, while in YRD the difference is around 5 ppb. Compared to observations, mean errors in urban areas was improved when using updated land use information by 14.2% and 35.6%, and in suburban areas by 5.8% and 10.7%. Updating land use data set in air quality modeling is important in application to regions with rapid urbanization such as China. The effects due to land use change can be as large as those due to 20% increase in emissions. [Copyright &y& Elsevier]

Continuous measurements of aerosol number size distribution in the range of 3nm–10μm were performed in Pearl River Delta (PRD), China. These measurements were made during the period of 3 October to 5 November in 2004 at rural/coastal site, Xinken (22°37′N, 113°35′E, 6m above sea level), in the south suburb of Guangzhou City (22°37′N, 113°35′E, 6m above sea level), using a Twin Differential Mobility Particle Sizer (TDMPS) combined with an Aerodynamic Particle Sizer (APS). The aerosol particles at Xinken were divided into four groups according to the observation results: nucleation mode particles (3–30nm), Aitken mode particles (30–130nm), accumulation mode particles (130–1000nm) and coarse mode particles (1–10μm). Concentrations of nucleation mode, Aitken mode and accumulation mode particles were observed in the same order of magnitude (about 10,000cm−3), among which the concentration of Aitken mode particle was the highest. The Aitken mode particles usually had two peaks: the morning peak may be caused by the land–sea circulation, which is proven to be important for transporting aged aerosols back to the sampling site, while the noon peak was ascribed to the condensational growth of new particles. New particle formation events were found on 7 days of 27 days, the new particle growth rates ranged from 2.2 to 19.8nmh−1 and the formation rates ranged from 0.5 to 5.2cm−3 s−1, both of them were in the range of typical observed formation rates (0.01–10cm−3 s−1) and typical particle growth rates (1–20nmh−1). The sustained growth of the new particles for several hours under steady northeast wind indicated that the new particle formation events may occur in a large homogeneous air mass. [Copyright &y& Elsevier]

As one of the most polluted regions in China, East China is suffering from regional andmixed air pollution. Black carbon (BC) internally mixed with other aerosol components canaggravate regional air pollution through aerosol-radiation interactions. Optical properties ofBC are largely affected by its mixing state and particle size distribution. However, dominantprocesses and components of BC coating, especially during different seasons orpollution events, have not been systematically described. In this study, one-year singleparticle BC measurement was conducted to firstly study the temporal variations of BCmixing states and size distribution. Moreover, machine learning was performed toanalyze dominant aerosol components contributing to BC coating. Typical episodeswere then selected to evaluate impacts of aging process on BC light absorptionproperties. Lagrangian modeling and chemical signature observed at the site werealso analyzed to discuss the characteristics of BC-aging during pollution episodes. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the most important light-absorbing species in the atmosphere and has a strong positive direct radiative forcing. Recent studies found that BC-containing particles may also modify the structure of planetary boundary layer and thus enhance haze pollution, especially in densely populated areas, for instance, in China. It is however difficult to quantify these effects due to their complex mixing state, hygroscopicity and the high spatial variability of concentration. During an intensive field campaign conducted in the North China Plain in a cold season, the mass size distribution and mixing state of BC was measured with a single particle soot photometer (SP2). During a severe haze event, the BC mass concentration exceeded 15 ug m-3. Stemming from the strong aging process under high relative humidity condition, freshly emitted BC particle was observed to turn into internally mixing within a short time scale. To understand the variation of the hygroscopicity of BC during its aging process, the hygroscopic growth factor of BC-containing particles was measured with a combination of a hygroscopic tandem differential mobility analyzer and a SP2. The particle hygroscopic growth factor was found to be correlated with the volume fraction of BC. During haze events, a fraction of the more-hygroscopic mode particles was observed to contain BC cores; while a minor fraction of the hydrophobic mode particles contained no BC. [ABSTRACT FROM AUTHOR]

In this study, we performed simultaneous measurements of aerosol hygroscopicity and particle phase chemical composition at a suburban site over the Pearl River Delta Region in the late summer of 2016. By using a self-assembled Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA), hygroscopic growth factor (HGF) of particles of four different sizes (30 nm, 60 nm, 100 nm, 145 nm) particles was obtained at 90% RH. A dominant mode of more hygroscopic (MH) particles together with a small mode of less hygroscopic (LH) particles were observed, suggesting our aerosol population was external mixing. The HGF of less hygroscopic (LH) mode particles increased, while their number fractions decreased during the daytime indicating these LH mode particles in the early morning or late afternoon could be possibly from local automobile exhaust during the rush hours. By using the chemical composition obtained from an Aerodyne Quadruple Aerosol Chemical Speciation Monitor (ACSM), the hygroscopicity of ambient aerosols can also be derived. From the closure analysis between the HTDMA-measured HGF and the ACSM-derived hygroscopicity, hygroscopic growth factor of organic compounds (HGForg) was estimated to be 1.26 for 145 nm particles and a simple linear relationship between HGForg and the oxidation level inferred from the O:C ratio of the organic material was suggested. Compared with the results from other environments, HGForg obtained from our measurements appeared to be less sensitive to the variation of its oxidation level, which is similar to the observations in the urban atmosphere of other megacities in China. [ABSTRACT FROM AUTHOR]

Severe ozone (O 3 ) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NO x ) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions ( F HONO ) enhanced by nitrogen fertilizer application at an agricultural site. The observed F HONO significantly improves model performance in predicting atmospheric HONO and leads to regional O 3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NO x , by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O 3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O 3 pollution and climate change.

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 (PM 2.5 ). Although severe indoor pollution contributes to indoor PM 2.5 concentrations (23%) to a similar extent as severe haze contributes to ambient PM 2.5 concentrations (21%), the former's contribution to premature deaths was significantly higher. Furthermore, residential emissions contributed more in the higher PM 2.5 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.

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implications at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heterogeneous conversion of NO 2 was more efficient on aerosol surfaces than on ground, highlighting the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 ± 0.26, 0.97 ± 0.47 and 1.45 ± 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (NO 2 heterogeneous reaction and particulate nitrate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 ± 0.71), (1.75 ± 1.26) and (1.82 ± 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O 3 photolysis (i.e., (0.004 ± 0.004), (0.006 ± 0.007) and (0.0035 ± 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxidation capacity and hence promote the formation of secondary aerosols and pollutants., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflict of interest., (Copyright © 2022. Published by Elsevier B.V.)

Atmospheric nitrous acid (HONO) is an important precursor of atmospheric hydroxyl radicals. Vehicle emissions and heterogeneous reactions have been identified as major sources of urban HONO. Here, we report on HONO emissions from residential natural gas (RNG) for water and space heating in urban areas based on in situ measurements. The observed HONO emission factors (EFs) of RNG heating vary between 6.03 and 608 mg·m -3 NG, which are highly dependent on the thermal load. The highest HONO EFs are observed at a high thermal load via the thermal NO homogeneous reaction. The average HONO EFs of RNG water heating in winter are 1.8 times higher than that in summer due to the increased thermal load caused by the lower inlet water temperatures in winter. The power-based HONO EFs of the traditional RNG heaters are 1085 times and 1.7 times higher than those of gasoline and diesel vehicles that meet the latest emission standards, respectively. It is estimated that the HONO emissions from RNG heaters in a typical Chinese city are gradually close to emissions from on-road vehicles when temperatures decline. These findings highlight that RNG heating is a non-negligible source of urban HONO emissions in China. With the continuous acceleration of coal-to-gas projects and the continuous tightening of NO x emission standards for vehicle exhaust, HONO emissions from RNG heaters will become more prominent in urban areas. Hence, it is urgently needed to upgrade traditional RNG heaters with efficient emission reduction technologies such as frequency-converted blowers, secondary condensers, and low-NO x combustors.

Accurate estimation on reaction nitrogen (N r ) deposition is highly demanded for assessing the impacts on the environment and human beings. This study investigated the wet deposition of inorganic nitrogen (IN) in mainland China by measurements from over 500 sites from five observational networks/databases and ensemble results of eleven chemical transport models (CTMs). Each data source has its focus and limitations and together formed a comprehensive view over China. But the inconsistency among different sources may hinder the appropriate usage of data. Model evaluation results demonstrated the models' deficiency in simulating the wet NO 3 deposition over Southeast China (40 % underestimation) and showed an overall underestimation of wet NH - deposition over Southeast China (40 % underestimation) and showed an overall underestimation of wet NH 4 + deposition over the hotspot regions (5-60 % underestimation). A synthesis of this study and twelve reference studies was conducted to quantify the national amount of wet IN deposition. The estimations by CTMs ranged 2.4-3.9 Tg(N) yr -1 for wet NO y deposition and 4-6.7 Tg(N) yr -1 for wet NH x deposition, after adjusting the results with 10-19 % underestimations in wet NO y deposition and 1-40 % underestimations in wet NH x deposition. The estimations by ground observations ranged 7.1-9 Tg(N) yr -1 for wet NO y deposition and 8-13.1 Tg(N) yr -1 for wet NH x deposition, which were 20-275 % higher than the estimation by CTMs, but the results were strongly influenced by the abundances and representative of measurements. Studies using statistical techniques to interpolate site observations predicted 3-5.5 Tg(N) yr -1 for wet NO y deposition and 3.9-7.2 Tg(N) yr -1 for wet NH x deposition. This approach benefited from high accuracy and good robustness of the statistical models, but the uncertainty in the interpolation methods could be a potential drawback., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

This study investigates the impact of aerosol liquid water content (ALWC) and related factors, i.e., relative humidity (RH), aerosol mass concentration (PM 2.5 ), and aerosol hygroscopicity, on aerosol optical properties, based on field measurements made in the Pearl River Delta (PRD) region of China at the surface (1 November 2019 to 21 January 2020) and in the upper boundary layer (the 532-m Guangzhou tower from 1 February to 21 March 2020). In general, temporal variations in the ambient aerosol backscattering coefficient (β p ) and ALWC followed each other. However, the surface β p and 532-m β p had generally opposite diurnal variation patterns, caused by dramatic differences in PM 2.5 and ambient RH between the surface and the upper boundary layer. The ambient 532-m RH was systematically higher than the surface RH, with the latter having a much pronounced diurnal cycle than the former. The surface PM 2.5 concentration was systematically higher than the PM 2.5 concentration at 532 m, and their diurnal cycle patterns were overall opposite. These dramatic differences reveal that the atmospheric variables, i.e., ambient RH and the PM 2.5 concentration in the upper boundary layer, cannot be directly represented by the same variables at the surface. Vertical variability should be considered. Clear differences in the sensitivities of aerosol light scattering to ambient RH, PM 2.5 , and aerosol hygroscopicity between the two levels were found and examined. Aerosol chemical composition played a minor role in causing the differences between the two levels. In particular, β p was more sensitive to PM 2.5 at the surface level but more to the ambient RH in the upper boundary layer. The larger contribution of aerosol loading to the variability in β p at the surface implies that local emission controls can decrease β p and further improve atmospheric visibility effectively at the surface during winter in the PRD region., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Secondary aerosol (SA) frequently drives severe haze formation on the North China Plain. However, previous studies mostly focused on submicron SA formation, thus our understanding of SA formation on supermicron particles remains poor. In this study, PM 2.5 chemical composition and PM 10 number size distribution measurements revealed that the SA formation occurred in very distinct size ranges. In particular, SA formation on dust-dominated supermicron particles was surprisingly high and increased with relative humidity (RH). SA formed on supermicron aerosols reached comparable levels with that on submicron particles during evolutionary stages of haze episodes. These results suggested that dust particles served as a medium for rapid secondary organic and inorganic aerosol formation under favorable photochemical and RH conditions in a highly polluted environment. Further analysis indicated that SA formation pathways differed among distinct size ranges. Overall, our study highlights the importance of dust in SA formation during non-dust storm periods and the urgent need to perform size-resolved aerosol chemical and physical property measurements in future SA formation investigations that are extended to the coarse mode because the large amount of SA formed thereon might have significant impacts on ice nucleation, radiative forcing, and human health.

The Chinese "coal-to-gas" and "coal-to-electricity" strategies aim at reducing dispersed coal consumption and related air pollution by promoting the use of clean and low-carbon fuels in northern China. Here, we show that on top of meteorological influences, the effective emission mitigation measures achieved an average decrease of fine particulate matter (PM 2.5 ) concentrations of ∼14% in Beijing and surrounding areas (the "2+26" pilot cities) in winter 2017 compared to the same period of 2016, where the dispersed coal control measures contributed ∼60% of the total PM 2.5 reductions. However, the localized air quality improvement was accompanied by a contemporaneous ∼15% upsurge of PM 2.5 concentrations over large areas in southern China. We find that the pollution transfer that resulted from a shift in emissions was of a high likelihood caused by a natural gas shortage in the south due to the coal-to-gas transition in the north. The overall shortage of natural gas greatly jeopardized the air quality benefits of the coal-to-gas strategy in winter 2017 and reflects structural challenges and potential threats in China's clean-energy transition., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Isocyanic acid (HNCO) is a potentially toxic atmospheric pollutant, whose atmospheric concentrations are hypothesized to be linked to adverse health effects. An earlier model study estimated that concentrations of isocyanic acid in China are highest around the world. However, measurements of isocyanic acid in ambient air have not been available in China. Two field campaigns were conducted to measure isocyanic acid in ambient air using a high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) in two different environments in China. The ranges of mixing ratios of isocyanic acid are from below the detection limit (18 pptv) to 2.8 ppbv (5 min average) with the average value of 0.46 ppbv at an urban site of Guangzhou in the Pearl River Delta (PRD) region in fall and from 0.02 to 2.2 ppbv with the average value of 0.37 ppbv at a rural site in the North China Plain (NCP) during wintertime, respectively. These concentrations are significantly higher than previous measurements in North America. The diurnal variations of isocyanic acid are very similar to secondary pollutants (e.g., ozone, formic acid, and nitric acid) in PRD, indicating that isocyanic acid is mainly produced by secondary formation. Both primary emissions and secondary formation account for isocyanic acid in the NCP. The lifetime of isocyanic acid in a lower atmosphere was estimated to be less than 1 day due to the high apparent loss rate caused by deposition at night in PRD. Based on the steady state analysis of isocyanic acid during the daytime, we show that amides are unlikely enough to explain the formation of isocyanic acid in Guangzhou, calling for additional precursors for isocyanic acid. Our measurements of isocyanic acid in two environments of China provide important constraints on the concentrations, sources, and sinks of this pollutant in the atmosphere.

Secondary organic aerosol (SOA) constitutes a large fraction of organic aerosol worldwide, however, the formation mechanisms in polluted environments remain poorly understood. Here we observed fast daytime growth of oxygenated organic aerosol (OOA) (with formation rates up to 10 μg m -3 h -1 ) during low relative humidity (RH, daytime average 38 ± 19%), high RH (53 ± 19%), and fog periods (77 ± 13%, fog occurring during nighttime with RH reaching 100%). Evidence showed that photochemical aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime OOA formation during the periods with nighttime fog, while both photochemical aqSOA and gas-phase SOA (gasSOA) formation were important during other periods with the former contributing more under high RH and the latter under low RH conditions, respectively. Compared to daytime photochemical aqSOA production, dark aqSOA formation was only observed during the fog period and contributed negligibly to the increase in OOA concentrations due to fog scavenging processes. The rapid daytime aging, as indicated by the rapid decrease in m , p -xylene/ethylbenzene ratios, promoted the daytime formation of precursors for aqSOA formation, e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA formation such as OH radical and H 2 O 2 also bear fast daytime growth features even under low solar radiative conditions. The simultaneous increases in ultraviolet radiation, photooxidant, and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. We also found that biomass burning emissions can promote photochemical aqSOA formation by adding to the levels of aqueous-phase photooxidants and aqSOA precursors. Therefore, future mitigation of air pollution in a polluted environment would benefit from stricter control on biomass burning especially under high RH conditions.

Highly oxygenated molecules (HOMs) play an important role in the formation and evolution of secondary organic aerosols (SOA). However, the abundance of HOMs in different environments and their relation to the oxidative potential of fine particulate matter (PM) are largely unknown. Here, we investigated the relative HOM abundance and radical yield of laboratory-generated SOA and fine PM in ambient air ranging from remote forest areas to highly polluted megacities. By electron paramagnetic resonance and mass spectrometric investigations, we found that the relative abundance of HOMs, especially the dimeric and low-volatility types, in ambient fine PM was positively correlated with the formation of radicals in aqueous PM extracts. SOA from photooxidation of isoprene, ozonolysis of α- and β-pinene, and fine PM from tropical (central Amazon) and boreal (Hyytiälä, Finland) forests exhibited a higher HOM abundance and radical yield than SOA from photooxidation of naphthalene and fine PM from urban sites (Beijing, Guangzhou, Mainz, Shanghai, and Xi'an), confirming that HOMs are important constituents of biogenic SOA to generate radicals. Our study provides new insights into the chemical relationship of HOM abundance, composition, and sources with the yield of radicals by laboratory and ambient aerosols, enabling better quantification of the component-specific contribution of source- or site-specific fine PM to its climate and health effects.

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

Size-resolved aerosols were continuously collected by a Nano Sampler for 13 days at an urban site in Beijing during winter 2012 to measure the chemical composition of ambient aerosol particles. Data collected by the Nano Sampler and an ACSM (Aerodyne Aerosol Chemical Speciation Monitor) were compared. Between the data sets, similar trends and strong correlations were observed, demonstrating the validity of the Nano Sampler. PM₁₀ and PM₂.₅ concentrations during the measurement were 150.5 ± 96.0 μg/m³ (mean ± standard variation) and 106.9 ± 71.6 μg/m³, respectively. The PM₂.₅/PM₁₀ ratio was 0.70 ± 0.10, indicating that PM₂.₅ dominated PM₁₀. The aerosol size distributions showed that three size bins of 0.5-1, 1-2.5 and 2.5-10 μm contributed 21.8%, 23.3% and 26.0% to the total mass concentration (TMC), respectively. OM (organic matter) and SIA (secondary ionic aerosol, mainly SO₄(2-), NO₃(-) and NH₄(+)) were major components of PM₂.₅. Secondary compounds (SIA and secondary organic carbon) accounted for half of TMC (about 49.8%) in PM₂.₅, and suggested that secondary aerosols significantly contributed to the serious particulate matter pollution observed in winter. Coal burning, biomass combustion, vehicle emissions and SIA were found to be the main sources of PM₂.₅. Mass concentrations of water-soluble ions and undetected materials, as well as their fractions in TMC, strikingly increased with deteriorating particle pollution conditions, while OM and EC (elemental carbon) exhibited different variations, with mass concentrations slightly increasing but fractions in TMC decreasing., (Copyright © 2014. Published by Elsevier B.V.)

A new instrument for measuring atmospheric nitrous acid (HONO) was developed, consisting of a double-wall glass stripping coil sampler coupled with ion chromatography (SC-IC). SC-IC is featured by small size (50 x 35 x 25 cm) and modular construction, including three independent parts: the sampling unit, the transfer and supporting unit, and the detection unit. High collection efficiency (> 99%) was achieved with 25 micromol/L Na2CO3 as absorption solution even in the presence of highly acidic compounds. This instrument has a detection limit of 8 pptv at 15 min time resolution, with a measurement uncertainty of 7%. Potential interferences from NO(x), NO2+SO2, NO2+VOCs, HONO+O3, HNO3, peroxyacetyl nitrite (PAN) and particle nitrite were quantified in laboratory studies and were found to be insignificant under typical atmospheric conditions. Within the framework of the 3C-STAR project, inter-comparison between the SC-IC and LOPAP (long path liquid absorption photometer) was conducted at a rural site in the Pearl River Delta. Good agreement was achieved between the two instruments over three weeks. Both instruments determined a clear diurnal profile of ambient HONO concentrations from 0.1 to 2.5 ppbv. However, deviations were found for low ambient HONO concentrations (i.e. < 0.3 ppbv), which cannot be explained by previous investigated interference species. To accurately determine the HONO budget under illuminated conditions, more intercomparison of HONO measurement techniques is still needed in future studies, especially at low HONO concentrations.