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Atmospheric oxidizing capacity (AOC) is an essential driving force of troposphere chemistry and self-cleaning, but the definition of AOC and its quantitative representation remain uncertain. Driven by national demand for air pollution control in recent years, Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research. This paper will give a brief review of these developments. First, AOC indexes were established that represent apparent atmospheric oxidizing ability (AOIe) and potential atmospheric oxidizing ability (AOIp) based on aspects of macrothermodynamics and microdynamics, respectively. A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing, and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country. In addition, the detection of ground or vertical profiles for atmospheric OH·, HO 2 ·, NO 3 · radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments. Moreover, laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O 3 and NO 2 , which are typical oxidants in the surface/interface atmosphere, and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies, multiphase and multi-interface conditions were obtained. Finally, based on the GRAPES-CUACE adjoint model improved by Chinese scholars, simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized. Normalized numerical simulations of AOIe and AOIp were performed, and regional coordination of AOC was adjusted. An optimized plan for controlling O 3 and PM 2.5 was analyzed by scenario simulation., 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 article. ", (Copyright © 2022. Published by Elsevier B.V.)

Luoyang is a typical heavy industrial city in China, with a coal-dominated energy structure and serious air pollution. Following the implementation of the clean air actions, the physicochemical characteristics and sources of PM 2.5 have changed. A comprehensive study of PM 2.5 was conducted from October 16, 2019 to January 23, 2020 to evaluate the effectiveness of previous control measures and further to provide theory basis for more effective policies in the future. Results showed that the aerosol pollution in Luoyang in autumn and winter is still serious with the average concentration of 91.1 μg/m 3 , although a large reduction (46.9%) since 2014. With the contribution of nitrate increased from 12.5% to 25.1% and sulfate decreased from 16.7% to 11.2%, aerosol pollution has changed from sulfate-dominate to nitrate-dominate. High NO 3 - /SO 4 2- ratio and the increasing of NO 3 - /SO 4 2- and EC concentration indicate coal combustion in Luoyang is still serious. The top three contributor sources were secondary inorganic aerosols (33.3%), coal combustion (13.6%), and industrial emissions (13.4%). Close-range transport from the western and northeastern directions were more important factors in air pollution in Luoyang during the sampling period. It is necessary to strengthen the control of coal combustion and reduce vehicle emissions in future policies.2.5 pollution in Luoyang, especially in the haze processes. Secondary inorganic ions contributed significantly to the enhancement of PM 2.5 during the pollution period. The high value of Cl - /Na + and EC concentration indicate coal combustion in Luoyang is still serious. The top three contributor sources were secondary inorganic aerosols (33.3%), coal combustion (13.6%), and industrial emissions (13.4%). Close-range transport from the western and northeastern directions were more important factors in air pollution in Luoyang during the sampling period. It is necessary to strengthen the control of coal combustion and reduce vehicle emissions in future policies., (Copyright © 2021. Published by Elsevier B.V.)

Submicron aerosol is of extensive concern not only due to its significant impact on air quality but also because it is detrimental to human health. In this study, we investigated the characteristics, sources and chemical processes of submicron aerosol based on real-time online measurements of submicron aerosols (NR-PM 1 ) during December 2015 at an urban site in Beijing. The average mass concentration of NR-PM 1 was 92.5±84.9 µg/m 3 , the hourly maximum was 459.1 µg/m 3 during the entire observation. The organic aerosol (OA) (55%) was the largest contributor to NR-PM 1 . The average mass concentration of PAHs was 0.217±0.247 µg/m 3 , exhibiting the highest concentration at night and the lowest levels in the daytime. The average mass concentration of organic nitrate was 2.52±2.36 µg/m 3 and that of inorganic nitrate was 7.62±8.22 µg/m 3 , accounting for 36% and 64%, respectively, of the total nitrate mass. Positive matrix factorization (PMF) differentiated the OA into five chemical components including LV-OOA, SV-OOA, COA, HOA and CCOA, accounting for 22%, 16%, 13%, 25% and 24% respectively, of the total OA. The average NR-PM 1 mass concentration on the heavy polluted days (HPD) was 182.8±70.2 µg/m 3 , which was approximately 9 times that on clean days (CD). The enhanced secondary formation of SNA was evident on HPD, especially the rapid increase of sulfate (23%) and nitrate (19%)., (Copyright © 2021. Published by Elsevier B.V.)

Several air pollution episodes occurred in Beijing before and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, during which air-pollution control measures were implemented. Within this autumn-winter transit season, domestic heating started. Such interesting period merits comprehensive chemical characterization, particularly the organic species, to look into the influence of additional heating sources and the control measures on air pollution. Therefore, this study performed daily and 6h time resolved PM 2.5 sampling from the 24th October to 7th December, 2014, followed by comprehensive chemical analyses including water-soluble ions, elements and organic source-markers. Apparent alterations of chemical profiles were observed with the initiation of domestic heating. Through positive matrix factorization (PMF) source apportionment modeling, six PM 2.5 sources including secondary inorganic aerosol (SIA), traffic emission, coal combustion, industry emission, biomass burning and dust were separated and identified. Coal combustion was successfully distinguished from traffic emission by hopane diagnostic ratio. The result of this study reveals a gradual shift of dominating sources for PM pollution episodes from SIA to primary sources after starting heating. BaP eq toxicity from coal combustion increased on average by several to dozens of times in the heating period, causing both long-term and short-term health risk. Air mass trajectory analysis highlights the regional influence of the industry emissions from the area south to Beijing. Control measures taken during APEC were found to be effective for reducing industry source, but less effective in reducing the overall PM 2.5 level. These results provide implications for policy making regarding appropriate air pollution control measures., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Stringent mitigation measures have reduced wintertime fine particulate matter (PM 2.5 ) concentrations by 42.2% from 2013 to 2018 in the Beijing-Tianjin-Hebei (BTH) region, but severe PM pollution still frequently engulfs the region. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM 2.5 , although the surface-measured NO 2 concentration has decreased by over 20%. The contributions of nitrogen oxides (NO X ) emissions mitigation to the nitrate and PM 2.5 concentrations and how to alleviate nitrate aerosols efficiently under the current situation still remains elusive. The WRF-Chem model simulations of a persistent and heavy PM pollution episode in January 2019 in the BTH reveal that NO X emissions mitigation does not help lower wintertime nitrate and PM 2.5 concentrations under current conditions in the BTH. A 50% reduction in NO X emissions only decreases nitrate mass by 10.3% but increases PM 2.5 concentrations by 3.2%, because the substantial O 3 increase induced by NO X mitigation offsets the HNO 3 loss and enhances sulfate and secondary organic aerosols formation. Our results are further consolidated by the occurrence of severe PM pollution in the BTH during the COVID-19 outbreak, with a significant reduction in NO 2 concentration. Mitigation of NH 3 emissions constitutes the priority measure to effectively lower the nitrate and PM 2.5 concentrations in the BTH under current conditions, with 35.5% and 12.7% decrease, respectively, when NH 3 emissions are reduced by 50%., Competing Interests: Declaration of competing interest The authors declare no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

A seven-year long-term comprehensive measurement of non-refractory submicron particles (NR-PM 1 ) in autumn and winter in Beijing from 2012 to 2018 was conducted to evaluate the effectiveness of the clean air actions implemented by the Chinese government in September 2013 on aerosols from different sources and chemical processes. Results showed that the NR-PM 1 concentrations decreased by 44.1% in autumn and 73.2% in winter from 2012 to 2018. Sulfate showed a much larger reduction than nitrate and ammonium in both autumn (55%) and winter (86%) and that nitrate even slightly increased by 15.8% in autumn. As a result, aerosol pollution in winter gradually changed from sulfate-rich to nitrate-rich with a sudden change after 2016 and the dominant role of nitrate in autumn was also strengthened after 2016. Among primary organic aerosol (OA) types, biomass burning OA and coal combustion OA exhibited the largest decline in autumn and winter, with reductions of 87.5% and 77.3%, respectively, while hydrocarbon-like OA (HOA) exhibited the smallest decline in both autumn (24.4%) and winter (37.1%). These significant changes in aerosol compositions were highly consistent with the much faster reduction of SO 2 (75-85%) than NOx (36-59%) and were mainly due to the clean air actions rather than the impact of meteorological conditions. What's more, the enhanced atmospheric oxidizing capacity, which was indicated by increased O 3 , altered the chemical processes of oxygenated OA (OOA), especially in autumn. Both of less-oxidized OOA (LO-OOA) and more-oxidized OOA showed elevated contributions in OA by 4% in autumn. The increased oxygen-to-carbon ratios of LO-OOA in autumn (from 0.42 to 0.58) and winter (from 0.44 to 0.52) indicated the enhanced atmospheric oxidizing capacity strengthened photochemical reactions and resulted in the increased oxidation degree of LO-OOA. This study demonstrates the effectiveness of the clean air actions for air quality improvement in Beijing., 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 © 2020. Published by Elsevier Ltd.)

China has large anthropogenic chlorine emissions from agricultural fires, residential biofuel, waste incineration, coal combustion, and industrial processes. Here we quantify the effects of chlorine on fine particulate matter (PM 2.5 ) and ozone air quality across China by using the GEOS-Chem chemical transport model with comprehensive anthropogenic emissions and detailed representation of gas-phase and heterogeneous chlorine chemistry. Comparison of the model to observed ClNO 2 , HCl, and particulate Cl - concentrations shows that reactive chlorine in China is mainly anthropogenic, unlike in other continental regions where it is mostly of marine origin. The model is successful in reproducing observed concentrations and their distributions, lending confidence in the anthropogenic chlorine emission estimates and the resulting chemistry. We find that anthropogenic chlorine emissions increase total inorganic PM 2.5 by as much as 3.2 μg m -3 on an annual mean basis through the formation of ammonium chloride, partly compensated by a decrease of nitrate because ClNO 2 formation competes with N 2 O 5 hydrolysis. Annual mean MDA8 surface ozone increases by up to 1.9 ppb, mainly from ClNO 2 chemistry, while reactivities of volatile organic compounds increase (by up to 48% for ethane). We find that a sufficient representation of chlorine chemistry in air quality models can be obtained from consideration of HCl/Cl - thermodynamics and ClNO 2 chemistry, because other more complicated aspects of chlorine chemistry have a relatively minor effect.

Ammonia (NH 3 ) emission inventories are an essential input in chemical transport models and are helpful for policy-makers to refine mitigation strategies. However, current estimates of Chinese NH 3 emissions still have large uncertainties. In this study, an improved inversion estimation of NH 3 emissions in China has been made using an ensemble Kalman filter and the Nested Air Quality Prediction Modeling System. By first assimilating the surface NH 3 observations from the Ammonia Monitoring Network in China at a high resolution of 15 km, our inversion results have provided new insights into the spatial and temporal patterns of Chinese NH 3 emissions. More enhanced NH 3 emission hotspots, likely associated with industrial or agricultural sources, were captured in northwest China, where the a posteriori NH 3 emissions were more than twice the a priori emissions. Monthly variations of NH 3 emissions were optimized in different regions of China and exhibited a more distinct seasonality, with the emissions in summer being twice those in winter. The inversion results were well-validated by several independent datasets that traced gaseous NH 3 and related atmospheric processes. These findings highlighted that the improved inversion estimation can be used to advance our understanding of NH 3 emissions in China and their environmental impacts.

Hourly concentrations of water-soluble inorganic ions (Na + , NH 4 + , K + , Mg 2+ , Ca 2+ , Cl - , NO 3 - and SO 4 2- ) in PM 2.5 and related reactive gases were measured with a Gas and Aerosol Collector combined with Ion Chromatography (GAC-IC) in urban Chengdu from April 17 to May 27, 2017, during which both haze and dust episodes occurred frequently. Nitrate was the most abundant ion in PM 2.5 and substantially increased during haze pollution with the NO 3 - /SO 4 2- mass ratio increasing from 0.78 during clean period to 1.1 during haze period. Aerosols in Chengdu were generally ammonium-rich, wherein ammonium nitrate was primarily formed through homogeneous gas-phase reactions and limited by the availability of HNO 3 , indicating that preferentially reducing the emissions of NOx could make for mitigating spring haze pollution in Chengdu. Backward trajectory clustering coupled with measured species and a potential source contribution function (PSCF) for PM 2.5 , PM 10 /PM 2.5 , sulfate, nitrate, ammonium, and Ca 2+ indicated that regionally transported pollutants from the southern and southeastern Sichuan Basin strongly contributed to springtime PM 2.5 pollution in Chengdu, but long-distance transport from northwestern China also contributed to dust pollution. Moreover, the treatment of urban fugitive dust in southern Sichuan is also important for reducing coarse particles in Chengdu. Therefore, the improvement of air quality in Chengdu, even in the Sichuan Basin, requires the regional joint emission reduction of particles and gaseous precursors across the entire Sichuan Basin, especially for cities located in southeastern Sichuan Basin., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Haze is a serious air pollution problem in China, especially in Beijing and surrounding areas, affecting visibility, public health and regional climate. In this study, the Weather Research and Forecasting-Chemistry (WRF-Chem) model was used to simulate PM2.5 (particulate matters with aerodynamic diameter≤2.5 μm) concentrations during the 2013 severe haze event in Beijing, and health impacts and health-related economic losses were calculated based on model results. Compared with surface monitoring data, the model results reflected pollution concentrations accurately (correlation coefficients between simulated and measured PM2.5 were 0.7, 0.4, 0.5 and 0.6 in Beijing, Tianjin, Xianghe and Xinglong stations, respectively). Health impacts assessments show that the PM2.5 concentrations in January might cause 690 (95% confidence interval (CI): (490, 890)) premature deaths, 45,350 (95% CI: (21,640, 57,860)) acute bronchitis and 23,720 (95% CI: (17,090, 29,710)) asthma cases in Beijing area. Results of the economic losses assessments suggest that the haze in January 2013 might lead to 253.8 (95% CI: (170.2, 331.2)) million US$ losses, accounting for 0.08% (95% CI: (0.05%, 0.1%)) of the total 2013 annual gross domestic product (GDP) of Beijing., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Air pollution has become a top environmental concern in China. In the present study, 9 years of PM10 and PM2.5 data from an urban monitoring station in Beijing was analyzed relative to other gaseous pollutants and several meteorological parameters. The levels and characteristics of particulate matters as well as the sources and factors affecting them were provided in the present study. During the 9-year observation period, PM10 and PM2.5 showed high levels, with annual mean values of 138.5 ± 92.9 and 72.3 ± 54.4 μg/m(3), respectively. Slight decreasing trends of annual mean PM10 and PM2.5 were observed; autumn was the main season that contributed to the decrease in annual mean PM10 and PM2.5. Higher values for both PM fractions were generally observed in the spring and summer months, respectively, whereas lower values were found in the summer and spring months, respectively. Pronounced diurnal variations were found for PM10 and PM2.5, which both displayed a bimodal pattern with peaks between 7:00 and 8:00 a.m. as well as 7:00 and 11:00 p.m.; a minimum generally appeared at approximately noon. The seasonal and diurnal variations in particulate concentration are mostly dominated by the seasonal and diurnal variability of boundary layer and source emissions. A principal components analysis revealed that both the traffic-related emissions and combustion sources were major contributions to the particles; their contributions ranged between 35.5-75.1%. Furthermore, a directional analysis shows a stronger association between particles and the southerly winds, the PM derived from sources south of Beijing, most likely secondary PM, significantly affects concentrations at the sampling site. To improve air quality in Beijing, mitigation measures including phasing out high energy-consuming industries and prioritize the use of clean energy sources should be designed to reduce emissions from both local and regional sources.

Using size-resolved filter sampling and chemical characterization, high concentrations of water-soluble ions, carbonaceous species and heavy metals were found in both fine (PM2.1) and coarse (PM2.1-9) particles in Beijing during haze events in early 2013. Even on clear days, average mass concentration of submicron particles (PM1.1) was several times higher than that previously measured in most of abroad urban areas. A high concentration of particulate matter on haze days weakens the incident solar radiation, which reduces the generation rate of secondary organic carbon in PM1.1. We show that the peak mass concentration of particles shifted from 0.43-0.65μm on clear days to 0.65-1.1μm on lightly polluted days and to 1.1-2.1μm on heavily polluted days. The peak shifts were also found for the following species: organic carbon, elemental carbon, NH4(+), SO4(2-), NO3(-), K, Cu, Zn, Cd and Pb. Our findings demonstrate that secondary inorganic aerosols (36%) and organic matter (26%) dominated the fine particle mass on heavily polluted days, while their contribution reduced to 29% and 18%, respectively, on clear days. Besides fine particles, anthropogenic chemical species also substantially accumulated in the coarse mode, which suggests that particles with aerodynamic diameter larger than 2.1μm cannot be neglected during severe haze events., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 μg m−3 of HMS, while samples from Singapore examined 9–18 months after collection reveal ~0.6 μg m−3 of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS‐Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co‐benefit of decreasing particulate sulfur. Plain Language Summary: Sulfur molecules are a major part of the tiny particles in air that are both a significant cause of death worldwide and exert a large influence on climate. Scientists have previously assumed that the sulfur in particle air pollution is in the form of sulfate. However, recent work has shown that a different sulfur molecule called hydroxymethanesulfonate may be confused for sulfate in the machines used for determining the chemical composition of particles. By reanalyzing observations of particle chemical composition and conducting new observations we find a significant global presence of hydroxymethanesulfonate. Using a computer simulation of global air pollution chemistry, we find that hydroxymethanesulfonate may comprise more than one quarter of sulfur in particles in many polluted regions. The computer simulation also indicates that in multiple regions the formation of hydroxymethanesulfonate is controlled by levels of formaldehyde. Controlling emissions of formaldehyde or emissions of the volatile organic compounds that can create formaldehyde in the atmosphere can therefore have the added co‐benefit of also reducing the levels of sulfur in particle air pollution. Key Points: New and reanalyzed observations suggest a ubiquitous global presence of hydroxymethanesulfonate (HMS) in particulate matterGEOS‐Chem simulations suggest HMS may comprise over 25% of particulate sulfur in many polluted regions, especially in continental winterReductions of formaldehyde and other volatile organic compounds may have the co‐benefit of reducing particulate sulfur [ABSTRACT FROM AUTHOR]

Although much attention has been paid to investigating and controlling air pollution in China, the trends of air-pollutant concentrations on a national scale have remained unclear. Here, we quantitatively investigated the variation of air pollutants in China using long-term comprehensive data sets from 2013 to 2017, during which Chinese government made major efforts to reduce anthropogenic emission in polluted regions. Our results show a significant decreasing trend in the PM2.5 concentration in heavily polluted regions of eastern China, with an annual decrease of ∼7% compared with measurements in 2013. The measured decreased concentrations of SO2, NO2 and CO (a proxy for anthropogenic volatile organic compounds) could explain a large fraction of the decreased PM2.5 concentrations in different regions. As a consequence, the heavily polluted days decreased significantly in corresponding regions. Concentrations of organic aerosol, nitrate, sulfate, ammonium and chloride measured in urban Beijing revealed a remarkable reduction from 2013 to 2017, connecting the decreases in aerosol precursors with corresponding chemical components closely. However, surface-ozone concentrations showed increasing trends in most urban stations from 2013 to 2017, which indicates stronger photochemical pollution. The boundary-layer height in capital cities of eastern China showed no significant trends over the Beijing–Tianjin–Hebei, Yangtze River Delta and Pearl River Delta regions from 2013 to 2017, which confirmed the reduction in anthropogenic emissions. Our results demonstrated that the Chinese government was successful in the reduction of particulate matter in urban areas from 2013 to 2017, although the ozone concentration has increased significantly, suggesting a more complex mechanism of improving Chinese air quality in the future. [ABSTRACT FROM AUTHOR]

Although much effort has been put into studying air pollution, our knowledge of the mechanisms of frequently occurring intense haze episodes in China is still limited. In this study, using 3 years of measurements of air pollutants at three different height levels on a 325 m Beijing meteorology tower, we found that a positive aerosol–boundary layer feedback mechanism existed at three vertical observation heights during intense haze polluted periods within the mixing layer. This feedback was characterized by a higher loading of PM 2.5 with a shallower mixing layer. Modelling results indicated that the presence of PM 2.5 within the boundary layer led to reduced surface temperature, relative humidity and mixing layer height during an intensive haze episode. Measurements showed that the aerosol–boundary layer feedback was related to the decrease in solar radiation, turbulent kinetic energy and thereby suppression of the mixing layer. The feedback mechanism can explain the rapid formation of intense haze episodes to some extent, and we suggest that the detailed feedback mechanism warrants further investigation from both model simulations and field observations. [ABSTRACT FROM AUTHOR]

PM 2.5 samples were collected simultaneously at three urban sites including Beijing (BJ), Tianjin (TJ), Shijiazhuang (SJZ) and one background site Xinglong (XL) from June 2014 till April 2015. Typical polar organic aerosol tracers including 2-methylglyceric acid, 2-methyltetrols, 3-hydroxyglutaric acid, anhydrous saccharides and phthalic acid were analyzed by gas chromatography-mass spectrometry. Both Isoprene secondary organic aerosol (SOA) tracers and pinene SOA tracers had higher mass concentrations at background site than those at urban sites. But they showed different seasonal variations. The former had the highest mass concentration in summer while the latter had the highest mass concentration in spring. Anhydrous saccharides and phthalic acids at urban sites had higher mass concentrations than those at background site and showed higher mass concentration in autumn and winter, respectively. Corresponding estimated secondary organic carbons (SOCs) showed similar variation trends with tracers. In general, biogenic sources had more influence on background site while urban sites were more affected by anthropogenic sources. [ABSTRACT FROM AUTHOR]

The "Campaign on Atmospheric Aerosol Research" network of China (CARE-China) is a long-term project for the study of the spatio-temporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term data sets from this project, including 3 years of observations of online PM2.5 mass concentrations (2012-2014) and 1 year of observations of PM2.5 compositions (2012-2013) from the CARE-China network. The average PM2.5 concentration at 20 urban sites is 73.2 µg m-3 (16.8-126.9 µg m-3), which was 3 times higher than the average value from the 12 background sites (11.2-46.5 µg m-3). The PM2:5 concentrations are generally higher in east-central China than in the other parts of the country due to their relatively large particulate matter (PM) emissions and the unfavourable meteorological conditions for pollution dispersion. A distinct seasonal variability in PM2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM2.5 were analysed at six paired urban and background sites located within the most polluted urban agglomerations -- North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) -- and the cleanest region of China -- the Tibetan Autonomous Region (TAR). The major PM2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO2-4 (17.7%, mineral dust (11.8 %), NO-3 (9.8 %), NH + 4 C4 (6.6 %), elemental carbon (EC) (6.0 %), Cl- (1.2 %) at 45% RH and unaccounted matter (20.7 %). Similar chemical compositions of PM2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO-3 (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 µg m-3 (Lhasa) to 23.3 µg m-3 (Shenyang), the SO2-4 ranged from 0.8 µg m-3 (Lhasa) to 19.7 µg m-3 (Chongqing), the NO-3 ranged from 0.5 µg m-3(Lhasa) to 11.9 µg m-3 (Shanghai) and the EC ranged from 1.4 µg m-3 (Lhasa) to 7.1 µg m-3 (Guangzhou). The PM2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting different contributions from regional anthropogenic or natural emissions and from long-range transport to background areas. Notable seasonal variations of PM2.5-polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM2.5 chemical compositions on polluted days was consistent for the urban and nearby background sites, where the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (31-57 %) of PM2.5 mass, suggesting fine-particle pollution in the most polluted areas of China assumes a regional tendency, and the importance of addressing the emission reduction of secondary aerosol precursors including SO2 and NOx. Furthermore, distinct differences in the evolution of [NO-3]/[SO2-4] ratio and OC=EC ratio on polluted days imply that mobile sources and stationary (coal combustion) sources are likely more important in Guangzhou and Shenyang, respectively, whereas in Beijing it is mobile emission and residential sources. As for Chongqing, the higher oxidation capacity than the other three cities suggested it should pay more attention to the emission reduction of secondary aerosol precursors. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes and lifetimes. [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]

To better understand the characteristics of the chemical compositions of aerosols comprised of water soluble ions (WSIs) and carbonaceous aerosol (CA) and their impacts on the visibility throughout the North China Plain (NCP), four experimental sampling campaigns were carried out between June 2013 and May 2014. The sampling sites were located in Beijing, Xiangshan, Tianjin, Shijiazhuang, and Qinhuangdao. The air pollution episodes mainly occurred during the autumn and winter in the NCP. With regard to different particle size distributions, the ratio of anions to cations in the fine size (0.64) was greater than that in the coarse size (0.54) in the NCP. Coefficients of divergence indicate that aerosol pollution had similar characteristics in the five cities of the NCP and that the pollutants were characterized by mutual influences and regional transfer processes. There were different non-linear correlations between the visibility and mass concentrations of PM 2.5 at different relative humidity (RH) conditions. When the RH was > 70%, the visibility was < 5 km; this shows that, due to hygroscopic growth in a higher RH, the increase in PM 2.1 resulted in a decrease in the visibility. The Interagency Monitoring of Protected Visual Environments (IMPROVE) method was used to estimate the light extinction coefficients (LEC) from the measured concentrations of chemical species. Organic matter (OM), ammonium nitrate (AN), and ammonium sulfate (AS) were the three dominant species that contributed to the LEC in Beijing and had the highest proportions of total contributions to the LEC in the summer, accounting for 32.2%, 25.9%, and 24.4%, respectively. The LEC of sea salts did not change much throughout the year and accounted for 4.1–5.3% in Beijing and 4.8–7.4% in Tianjin. According to the Ambient Air Quality Standard, the days were divided into pollutional days (PD) and attained days (AD). The increasing concentrations of AN, AS and OM resulted in an increase in the LEC in the NCP, which led to the occurrences of low visibility events during the autumn and winter. NH 4 + , NO 3 − , and SO 4 2 − mainly existed as fine size particles (diameter < 2.1 μm) in Beijing (73.5%, 80.7%, and 78.0% on PD and 63.3%, 79.4%, and 72.5% on AD, respectively) and Tianjin (81.0%, 80.6%, and 82.1% on PD and 71.5%, 44.3%, and 69.7% on AD, respectively). However, the CA originating mainly from fuel combustion also played an important part in the visibility impairment in the coarse size (2.1 < diameter < 9.0 μm). According to a positive matrix factorization (PMF) model, 37.1% and 26.5% of the LEC came from secondary aerosols on PD in Beijing and in Tianjin, respectively. In addition, biomass burning, fuel combustion and fugitive dust were also important contributing sources of the LEC in the NCP. [ABSTRACT FROM AUTHOR]

Exceedingly high levels of PM2.5 with complex chemical composition occur frequently in China. It has been speculated whether anthropogenic PM2.5 may significantly contribute to ice-nucleating particles (INP). However, few studies have focused on the ice-nucleating properties of urban particles. In this work, two ice-nucleating droplet arrays have been used to determine the atmospheric number concentration of INP (NINP) in the range from -6 to -25° in Beijing. No correlations between NINP and either PM2.5 or black carbon mass concentrations were found, although both varied by more than a factor of 30 during the sampling period. Similarly, there were no correlations between NINP and either total particle number concentration or number concentrations for particles with diameters > 500 nm. Furthermore, there was no clear difference between day and night samples. All these results indicate that Beijing air pollution did not increase or decrease INP concentrations in the examined temperature range above values observed in nonurban areas; hence, the background INP concentrations might not be anthropogenically influenced as far as urban air pollution is concerned, at least in the examined temperature range. [ABSTRACT FROM AUTHOR]

Size-segregated water-soluble ionic species (WSIs) were measured using an Anderson cascade impactor from Jul. to Aug. 2008 and from Dec. 2009 to Feb. 2010 in urban Beijing. The results showed that fine particles (PM 2.1 , Dp < 2.1 μm) accounted for ∼49% (summer) and ∼34% (winter) of the total particulate mass, and WSIs accounted for 23–82% of the mass concentration of PM 2.1 . Secondary inorganic aerosols (SIAs, the sum of SO 4 2− , NO 3 − and NH 4 + ) accounted for more than 30% of the fine particles, which were greatly elevated during particle pollution events (PM events), thereby leading to an alteration of the size distributions of SO 4 2− and NO 3 − to nearly single fine-mode distributions peaking at 0.65–2.1 μm. This finding suggests that heterogeneous aqueous reactions were enhanced at high RH values. SIAs also increased during dust events, particularly for coarse mode SO 4 2− , which indicated enhanced heterogeneous reactions on the dust surface. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, representing the three major sources of the particles. The formation of SO 4 2− was attributed primarily to in-cloud or aerosol droplet processes during summer (45%), and the heterogeneous reaction of SO 2 on mineral dust surfaces was an important formation pathway during winter (45%). The formation pathways of NO 3 − in fine particles were similar to those of SO 4 2− , where over 30% were formed by in-cloud processes. This work provides important field measurement-based evidence for understanding the formation pathway of secondary inorganic aerosols in the megacity of Beijing. [ABSTRACT FROM AUTHOR]

Particulate matter (PM) pollution is a serious issue that has aroused great public attention in Beijing. To examine the seasonal characteristics of aerosols in typical pollution episodes, water-soluble inorganic ions (SO 4 2 − , NO 3 − , NH 4 + , Cl − , K + , Na + , Ca 2 + and Mg 2 + ) in size-segregated PM collected by an Anderson sampler (equipped with 50% effective cut-off diameters of 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.65, 0.43 μm and an after filter) were investigated in four intensive campaigns from June 2013 to May 2014 in the Beijing urban area. Pronounced seasonal variation of TWSIs in fine particles (aerodynamic diameter less than 2.1 μm) was observed, with the highest concentration in summer (71.5 ± 36.3 μg/m 3 ) and the lowest in spring (28.1 ± 15.2 μg/m 3 ). Different ion species presented different seasonal characteristics of mass concentration and size distribution, reflecting their different dominant sources. As the dominant component, SO 4 2 − , NO 3 − and NH 4 + (SNA) in fine particles appeared to play an important role in the formation of high PM pollution since its contribution to the TWSIs and PM 2.1 mass increased significantly during pollution episodes. Due to the hygroscopic growth and enhanced secondary formation in the droplet mode (0.65–2.1 μm) from clean days to polluted days, the size distribution peak of SNA in the fine mode tended to shift from 0.43–0.65 μm to 0.65–2.1 μm. Relative humidity (RH) and temperature contributed to influence the secondary formation and regulate the size distributions of sulfates and nitrates. Partial correlation analysis found that high RH would promote the sulfur and nitrogen oxidation rates in the fine mode, while high temperature favored the sulfur oxidation rate in the condensation mode (0.43–0.65 μm) and reduced the nitrogen oxidation rate in the droplet mode (0.65–2.1 μm). The NO 3 − /SO 4 2 − mass ratio in PM 2.1 (73% of the samples) exceeded 1.0, suggesting that vehicle exhaust currently makes a greater contribution to aerosol pollution than stationary sources in Beijing. [ABSTRACT FROM AUTHOR]

To characterize the features of particle pollution, continuous measurements of particle number size distributions and chemical compositions were performed at an urban site in Beijing in January 2013. The particle number and volume concentration from 14 nm to 1000 nm were (37.4 ± 15.3) × 10 3 cm − 3 and (85.2 ± 65.6) μm 3 cm − 3 , respectively. N-Ait (Aitken mode) particles dominated the number concentration, whereas N-Acc (accumulation mode) particles dominated the volume concentration. Submicron particles were generally characterized by a high content of organics and SO 4 2 − , and a low level of NO 3 − and Cl − . Two types of pollution episodes were observed, characterized by the “explosive growth” (EXP) and “sustained growth” (SUS) of PM 2.5 . Fine particles greater than 100 nm dominated the volume concentration during the ends of these pollution episodes, shifting the maximum of the number size distribution from 60 nm to greater than 100 nm in a few hours (EXP) or a few days (SUS). Secondary transformation is the main reason for the pollution episodes; SO 4 2 − , NO 3 − and NH 4 + (SNA) accounted for approximately 42% (EXP) and greater than 60% (SUS) of the N-Acc particle mass increase. The size distributions of particulate organics and SNA varied on timescales of hours to days, the characteristics of which changed from bimodal to unimodal during the evolution of haze episodes. The accumulation mode (peaking at approximately 500–700 nm) was dominated by organics that appeared to be internally mixed with nitrate or sulfate. The sulfate was most likely formed via heterogeneous reactions, because the SOR was constant under dry conditions (RH < 50%) and began to increase when RH > 50%, suggesting an important contribution from heterogeneous reactions with abundant aerosol water under wet conditions. Finally, the correlations between [NO 3 − ]/[SO 4 2 − ] and [NH 4 + ]/[SO 4 2 − ] suggest that the homogenous reaction between HNO 3 and NH 3 dominated the formation of nitrate under conditions of lower aerosol acidity. Therefore, controlling the precursors of SNA will effectively help to reduce the fine particulate pollution during winter in Beijing. [ABSTRACT FROM AUTHOR]

Recent time series studies have indicated that daily mortality and morbidity are associated with particulate matters. However, about the relative effects and its seasonal patterns of fine particulate matter constituents is particularly limited in developing Asian countries. In this study, we examined the role of particulate matters and its key chemical components of fine particles on both mortality and morbidity in Beijing. We applied several overdispersed Poisson generalized nonlinear models, adjusting for time, day of week, holiday, temperature, and relative humidity, to investigate the association between risk of mortality or morbidity and particulate matters and its constituents in Beijing, China, for January 2005 through December 2009. Particles and several constituents were associated with multiple mortality or morbidity categories, especially on respiratory health. For a 3-day lag, the nonaccident mortality increased by 1.52, 0.19, 1.03, 0.56, 0.42, and 0.32 % for particulate matter (PM), PM, K, SO, Ca, and NO based on interquartile ranges of 36.00, 64.00, 0.41, 8.75, 1.43, and 2.24 μg/m, respectively. The estimates of short-term effects for PM and its components in the cold season were 1 ~ 6 times higher than that in the full year on these health outcomes. Most of components had stronger adverse effects on human health in the heavy PM mass concentrations, especially for K, NO, and SO. This analysis added to the growing body of evidence linking PM with mortality or morbidity and indicated that excess risks may vary among specific PM components. Combustion-related products, traffic sources, vegetative burning, and crustal component and resuspended road dust may play a key role in the associations between air pollution and public health in Beijing. [ABSTRACT FROM AUTHOR]

In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the 'Forming Mechanism and Control Strategies of Haze in China' group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollution episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei (Jing-Jin-Ji) area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PM mass concentrations in Beijing were 680 and 530 μg m, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depression of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. However, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the 'explosive growth' and 'sustained growth' of PM. Particularly, the abnormally high amount of nitric oxide (NO) in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide (SO) to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle's size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution. [ABSTRACT FROM AUTHOR]

Long-term CO 2 and PM 2.5 measurements in urban areas have important impacts on understanding the roles of urbanization in climate change and air pollution. From 2009 to 2017, CO 2 fluxes were measured by the eddy covariance (EC) system at a height of 140 m on the Beijing Meteorological Tower. The CO 2 fluxes followed a typical two-peak diurnal pattern all year round. The PM 2. 5 concentrations followed a similar diurnal pattern as the CO 2 fluxes in summer but a different diurnal pattern in winter (low in the day and high at night). On a seasonal time scale, both the CO 2 fluxes and the PM 2. 5 concentrations showed a pronounced seasonal variation (high in winter and low in summer). The spatial variations in CO 2 fluxes were dominated by the prevailing land use types within the flux footprint, particularly dense residential areas and heavy traffic roads. On both diurnal and annual time scales, the urban underlying surface was a net source of CO 2. The 9-year average annual total CO 2 flux was 36.4 kg CO 2 ·m−2 yr−1. Depending on the yearly prevailing wind direction, the effect of the heterogeneity correction on the annual total CO 2 fluxes based on the gap-filled dataset could reach up to 3.5%. Over the 9-year period, both the CO 2 fluxes and the PM 2.5 concentrations exhibited a declining interannual trend, and CO 2 fluxes could account for 64% of the interannual variability in PM 2.5 concentrations. In summer, emissions were more likely to control the interannual variability in PM 2.5 concentrations, whereas in winter, meteorological conditions had a greater impact on the interannual variability in PM 2.5 concentrations. Image 1 • The urban underlying surface in Beijing is a net source of CO 2 on multiple timescales. • The effect of the heterogeneity correction on the annual total CO 2 fluxes is within 5%. • Both the CO 2 fluxes and the PM 2.5 concentrations show a declining interannual trend. • There are huge potential co-benefits in carbon mitigation and air pollution abatement. [ABSTRACT FROM AUTHOR]

Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O 3) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O 3 and PM 2.5 episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O 3 and PM 2.5 episodes. The strong intensity of AOC is found at O 3 and PM 2.5 episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O 3 is main source of OH at O 3 episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM 2.5 episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O 3 budgets and sensitivity analysis indicates that O 3 production is transition regime (both VOC and NO x -limited) at O 3 episode. The heterogeneous reaction of hydroperoxy radicals (HO 2) on aerosol surfaces has significant influence on OH and O 3 production rates. The HO 2 uptake coefficient (γHO 2) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM 2.5 and O 3 pollution. [Display omitted] [ABSTRACT FROM AUTHOR]

Nitrate salts represent a major component of fine mode aerosols, which play an important role in air pollution worldwide. Based on on-line and off-line aerosol measurements in urban Beijing for both clean and haze conditions, we demonstrate that the absolute and relative concentrations of nitrate increased with visibility degradation (relative humidity), whereas the variations of organics tracked the patterns of mixing-layer height and temperature. We propose that the increase in the relative contribution of nitrate to PM 1 observed during the early stages of haze pollution was due to new particle formation, whereas the nitrate formed in PM 1-2.5 during the latter stages was due to heterogeneous formation and hygroscopic growth. The increasing trend of nitrate (and also sulfate and ammonium) but decreasing trends of organics during haze development, together with the increase of the NO 2 /SO 2 molar ratio with increasing proximity to downtown Beijing and with visibility degradation, provide further evidence that controlling NO x emissions should be a priority for improving air quality in mega cities. Additional large-scale investigation is required to adequately characterize the regional features of NO x -induced haze pollution in China. Such studies may provide insight into the formation of critical nuclei or the subsequent growth of freshly nucleated particles and advance our understanding of the role of nitrate in new particle formation. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive measurement was conducted to analyze the heaviest air pollution episodes, which occurred in January 2013 with the focus on particulate characterization and haze. Based on observation, five heavy pollution episodes were recorded, whose frequencies are far greater than in other years. Among the heavy pollution episodes, two distinct severe regional PM2.5 (particulate matter (PM) with an aerodynamic diameter ≤2.5 μm) pollution episodes were selected for investigation. One episode is characterized by an explosive growth in PM2.5 concentrations within several hours, which is mainly caused by the high local emission under stagnant conditions; the other episode is characterized by four-day consecutive increases in PM2.5 concentrations, which is largely attributed to a combination of slow regional transport and local accumulation. The PM compositions and concentrations including PM2.5, PM1 (PM with an aerodynamic diameter ≤1 μm), nitrate, sulfate, ammonium, organic carbon (OC) and elemental carbon (EC) as well as main gaseous precursor concentrations are analyzed in Beijing during the two episodes. Rapid gas-to-particle conversion, coagulation and collision of particles are discussed under high emission and stagnant weather conditions. High sulfur and nitrogen oxidation ratios were found, suggesting that additional production of sulfate and nitrate occurred during the pollution episodes. In addition, high levels of secondary particles were transported from surrounding regions via vertical mixing and horizontal transport during the polluted days. Notably, the stationary source is still a major contributor to the pollution episodes. Clear evidence is presented indicating that the secondary formation of particulate was one important mechanism in the formation of the heavy pollution episodes. To control the air pollution effectively, more stringent restriction measures on the SO2 and NO x emissions should be strictly executed at the regional scale. [Copyright &y& Elsevier]

Measurements of fine particles (PM2.5), coarse particles (PM10) and aerosol optical properties were carried out at a background station-Dinghushan in the Pearl River Delta (PRD) region from 2009 to 2012. It showed that the long-term mean concentrations of PM2.5 and PM10 were as high as (51±31) μg/m3 and (76±43) μg/m3, respectively. Particulate pollution in the PRD region was mitigated over the last four years, and the annual mean concentrations of PM2.5 was decreased to (39±25) μg/m3 in 2012 from (65±33) μg/m3 in 2009. The concentrations of PM2.5 and PM10 were low in summer and high in the other three seasons. The correlation between the daily concentrations of PM2.5 and PM10 was generally high (R >0.90). The long-term mean PM2.5/PM10 ratio reached up to 0.67, the ratio was high in autumn (0.71) and low in summer (0.57). Atmospheric visibility was poor, the long-term mean of aerosol optical depth (AOD) at 500nm was 0.91±0.40, and Ångström exponent was 0.97±0.36. The values of AOD were low in winter and summer, and high in spring and autumn. The correlation between particulate matter (PM) concentrations and AOD was high. The correlation coefficient in dry season (October–next March) was better than that was in wet season (April–September). In addition, the correlation also demonstrated great differences for different air masses. [ABSTRACT FROM AUTHOR]

With a decrease in fine particulate matter (PM 2.5) due to the implementation of air pollution prevention-control measures, ozone (O 3) has risen in China especially in urban areas recently. It becomes more and more important to control air combined pollution with high concentration of PM 2.5 and O 3. We analyzed the characteristics of air combined pollution in Beijing from 2014 to 2019 in this paper. Results indicated that the trough O 3 showed an upward trend of 4.5 μg m−3 yr−1, while the peak decreased by −1.1 μg m−3 yr−1. There were three typical atmospheric chemical stages in the interaction between O 3 and PM 2.5 in summer through observations. The first was an intense photochemical stage with the synchronous and rapid growth of O 3 and PM 2.5. The O 3 concentration varied from 68 μg m−3 to 206 μg m−3 when PM 2.5 ≤ 35 μg m−3. While secondary inorganic ions (SNA) ranged from 2 to 21 μg m−3 especially SO 4 2− (0.8–12 μg m−3). The second was an aerosol chemical generation stage with the rapid increase of PM 2.5 and SNA under high O 3 concentration. O 3 was maintained at a high concentration between 198 μg m−3 and 241 μg m−3 when PM 2.5 was within the range of 35–125 μg m−3. SNA increased from 12 μg m−3 to 96 μg m−3. The third was a high PM 2.5 pollution stage suppressing the generation of O 3 when PM 2.5 ≥ 125 μg m−3. O 3 began to decline with the growth of PM 2.5 and decreased from 224 μg m−3 to 130 μg m−3. Ultraviolet radiation (UV) decreased from 13 MJ m−3 in the first to 8 MJ m−3 in this stage. [Display omitted] • The O 3 trough increased by 4.5 μg/m3 and the peak decreased by −1.1 μg/m3 per year in Beijing. • There were three typical processes in the interaction between O 3 and PM 2.5 in summer day. • The photochemical process was intense in dry-sunny days with the synchronous growth of O 3 and PM 2.5. • Aerosol chemical process intensified with a rapid growth of PM 2.5 and SNA in high O 3. • O 3 began to decline with the growth of PM 2.5 when PM 2.5 attenuated UV in a high level. [ABSTRACT FROM AUTHOR]

Abstract: Evidence concerning the health risk of fine and coarse particles is limited in developing Asian countries. The modifying effect between particles and temperature and season also remains unclear. Our study is one of the first to investigate the acute effect of particles size fractions, modifying effects and interannual variations of relative risk in a developing megacity where particulate levels are extraordinarily high compared to other Asian cities. After controlling for potential confounding, the results of a time-series analysis during the period 2005&#x2013;2009 show that a 10 &#x03bc;g m&#x2212;3 increase in PM2.5 levels is associated with a 0.65% (95% CI: 0.29&#x2013;0.80%), 0.63% (95% CI: 0.25&#x2013;0.83%), and 1.38% (95% CI: 0.51&#x2013;1.71%) increase in non-accidental mortality, respiratory mortality, and circulatory mortality, respectively, while a 10 &#x03bc;g m&#x2212;3 increase in PM10 is similarly associated with increases of 0.15% (95% CI: 0.04&#x2013;0.22%), 0.08% (95% CI: 0.01&#x2013;0.18%), and 0.44% (95% CI: 0.12&#x2013;0.63%). We did not find a significant effect of PM2.5 &#x2013; 10 on daily mortality outcomes. Our analyses conclude that temperature and particulates, exposures to both of which are expected to increase with climate change, might act together to worsen human health in Beijing, especially in the cool seasons. The level of the estimated percentage increase assume an escalating tendency during the study period, in addition to having a low value in 2008, and after the Olympic Games, the values increased significantly as the temporary atmospheric pollution control measures were terminated mostly. [ABSTRACT FROM AUTHOR]

Abstract: The analysis of air pollution episodes greatly aids in understanding air pollution and formulating policies to improve air quality. In this study, variations in the distribution of PM10 concentrations were analyzed using data collected from the air quality monitoring network in northern China set up by the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP, CAS) and the air quality monitoring network of the Beijing Municipal Environmental Protection Bureau (BJEPB). The results showed that there were two severe regional PM10 pollution episodes recorded from October 27, 2009, to November 10, 2009. During these pollution episodes, the highest daily PM10 concentration reached 600 μg m−3 in Shijiazhuang, which is far above the daily PM10 limit of 150 μg m−3. These aerosol pollution episodes were accompanied by regional haze and low visibility ranges. Synoptic patterns and long-range transport were the main factors controlling the regional air pollution processes. Transport patterns affected by synoptic cycle resulted in the spatial and temporal variations of PM10 and PM2.5 during the period studied. Conditions with light winds, temperature inversion and low mixed layer heights contributed to the buildup of PM10 and PM2.5 as well as gas-to-particle processing. In Beijing, the concentrations of sulfate and nitrate accounted for ∼1/3 of the PM10 mass and ∼2/3 of the PM2.5 mass, part of which is attributed to the additional secondary conversion of SO4 2− from SO2 and NO3 − from NOx during high-pollution days. Stationary anthropogenic sources were still a major contributor to pollution in these two episodes based on the NOx/SO2 ratios. To address regional air pollution, more attention should be paid to the increases in emissions and the higher frequency of haze days, which are associated with degraded visibility, that have accompanied the rapid economic development in northern China. [Copyright &y& Elsevier]

Local weather conditions play important roles in surface fine particulate matter (PM 2.5) pollution episodes in metropolitan areas. Relative to surface meteorological factors, the roles of near-surface vertical structure of urban boundary layer in air pollution has yet to be fully understood, due to limitation of vertical observation. By using surface PM 2.5 observations and multi-layer (15 levels) vertical meteorological elements from a 325-m meteorological tower during 2004–2016, we investigated the different stages of 456 PM 2.5 Pollution Episodes (PPEs) and their associations with vertical urban boundary layer structure in Beijing. The average PM 2.5 concentration during PPEs is 141.2 μg m−3, twice the 13-year average (70.3 μg m−3). PPEs are more frequent and severe in autumn and winter, showing significant seasonal variations. For vertical wind, southerly (southwesterly) winds prevailed from 80 m to 320 m in the early stage of pollution episodes, transporting pollutants to Beijing. While strong northerly (northwesterly) winds prevailed in near-surface layer in the dissipation stages (DSs) and after PPEs, effectively removing pollutants. Owing to different thermal dynamic conditions, the starting hours of PPEs are various, showing that PPEs with stable atmospheric stratification occur most frequently at 1800 LST. Particularly, for long-lasting PPEs, shortwave (longwave) radiation at near-surface layer is weakened (enhanced) due to the existence of clouds and pollutant aerosols, attenuating the momentum flux and sensible heat flux, which further aggravates the accumulation of pollutants. Our findings provide a better understanding of the physical formation mechanism of high concentrations of the PM 2.5 pollution episodes and may aid in designing effective mitigation strategies for the urban environment and health. • Long-term records of multi-layer vertical WS, T, and RH are compared during different stages of PM 2.5 Pollution Episodes (PPEs). • Southerly winds prevailed from 80 to 320 m in the early stage of PPEs and northerly winds prevailed in the dissipation stages and after PPEs. • The starting hours of PPEs occurred most frequently at or 1 h before sunset, being explained by weak thermal and dynamic forcing. [ABSTRACT FROM AUTHOR]

Both PM 2.5 and visibility are important physical parameters to measure the air pollution condition. This paper first provided a new normalization approach for the relationship between PM 2.5 and visibility in heavy pollutant region, which realized a mathematical calculation for two different physical parameters based on non-dimensional data process (also called Min-Max normalization). The normalization relationship showed a power function (y = axb) with high correlation coefficients. The inflection points of the maximum power function curvature composed the theoretical threshold range of PM 2.5 for improving or deteriorating the visibility. In the case city of north China, the threshold range of PM 2.5 was 112–121 μg·m−3 corresponding to the visibility range 4.5–5.9 km. Within the threshold range, the mean concentration of main pollutant components was 106 μg·m−3. The mean extinction coefficient, b ext of 743.5 Mm−1, was mainly contributed by organic matter (34%), ammonium nitrate (19%), ammonium sulfate (18%) and elemental carbon (13%). When PM 2.5 was below the range, the visibility increased significantly with the slow decrease of PM 2.5. When PM 2.5 exceeded the range, the visibility reduced seriously due to the saturated extinction contribution of the main pollutants. The new normalization approach theoretically revealed the significance of threshold and how to improve visibility by controlling PM 2.5 at different pollution stages. Unlabelled Image • A new normalization approach was first applied to establish the mathematical relationship of PM 2.5 and visibility. • The normalization mathematical relationship was a power function with high correlation coefficients. • The maximum curvature of the function composed the theoretical threshold of PM 2.5 to improve or deteriorate visibility. • The extinction coefficients of aerosol components were quite different at different pollution stages. • The threshold range has guiding significance how to control PM 2.5 for the blue sky in heavy polluted cities. [ABSTRACT FROM AUTHOR]

Different chemical substances make different contributions to particles with different diameters. It is important to determine which species play important roles to control air pollution. In this study, the concentrations of water-soluble inorganic ions, elemental carbon, organic carbon and metal elements in PM 1 (particulate matter with a diameter less than 1 μm) and PM 2.5 (particulate matter with a diameter less than 2.5 μm) in the Shijiazhuang suburban area were measured by synchronous sampling for one month in the summer of 2016. The results showed that the chemical components of particulate matter are mainly SO 4 2−, NO 3 −, NH 4 + and organic matter (OM), accounting for 14%, 8%, 11%, and 11%, respectively, in PM 1 and 13%, 8%, 10%, and 9%, respectively, in PM 2.5. PM 1 and PM 2.5 have similar sources, including secondary nitrate, secondary sulfate, industrial sources, motor vehicle sources, dust, biomass combustion and coal burning, which contribute 29%, 30%, 10%, 9%, 8%, 8% and 6%, respectively, to PM 1 and 29%, 33%, 12%, 13%, 8%, 2%, and 4%, respectively, to PM 2.5. Based on the analysis of a typical pollution episode, the concentrations of PM 1 and PM 2.5 increase synchronously in the explosive growth stage, with nitrate acting as the key chemical component. In the most polluted period, the concentration of PM 2.5 continues to increase because of the increase in sulfate, while that of PM 1 is almost constant. The results show that the prevention and control of PM 1 and PM 2.5 should be different in different pollution stages. Image 1 • Nitrate in PM 1 dominated the explosive growth of air pollution events. • Sulfate in PM 2.5 sustained the most polluted periods of air pollution events. • Control measures should be adjusted in different pollution event periods. [ABSTRACT FROM AUTHOR]