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Persistent winter aerosol pollution frequently occurs in the Sichuan Basin (SCB) due to its unfavourable weather conditions, such as low wind, wetness, and cloudiness. Based on long-term observational data analyses from 2015–2021, it has been found that the four representative stations in the SCB often simultaneously experience PM2.5 pollution accompanied by variations in meteorological conditions above 850 hPa , which indicates a connection between regional winter air pollution in the SCB and large-scale synoptic patterns. The dominant 850 hPa synoptic patterns of winter in the SCB were classified into six patterns using T-model principal component analysis: (1) strong high pressure in the north, (2) east high–west low (EHWL) pressure, (3) weak high pressure in the north, (4) a weak ridge of high pressure after the trough, (5) a low trough (LT), and (6) strong high pressure. Pattern 2 characterized by the EHWL pressure system, and Pattern 5, featured with LT, was identified as having key synoptic patterns for the beginning and accumulation of pollution processes. Pattern 1, characterized by a strong high pressure in the north, was the cleanest pattern associated with reduced PM2.5 concentrations. The EHWL and LT patterns were associated with a remarkably high cloud liquid content attributed to upper southerly winds introducing humid air. Clouds reduce solar radiation through reflection and scattering, resulting in more stable stratification and aerosol accumulation. This cloud radiation interaction (CRI) was more pronounced in the LT pattern due to denser isobaric lines and stronger southerly winds than in the EHWL pattern. Numerical simulation experiments utilizing WRF-Chem indicated that there is an upper-level heating during afternoon and surface cooling in the morning forced by the aerosol radiation interaction (ARI) under the EHWL and LT patterns. Additionally, strong surface cooling in the evening influenced by valley winds could be found. With wet and cloudy synoptic forcing, ARI directly affects the stability of the boundary layer and is modulated through CRI inhibition. For example, Chongqing exhibited lower PM2.5 concentrations and stronger ARI compared to the western and southern SCB due to lower cloud liquid content and weaker CRI inhibition on the ARI. The CRI inhibition caused a 50 % reduction in solar radiation and boundary layer height during the daytime under the LT pattern, which was larger than that under the EHWL pattern. This study comprehensively analysed the spatial disparities in cloud inhibition on the ARIs, their impacts on the boundary layer structure, and the discrepancies of these interactions under different synoptic patterns during pollution processes. The findings have important implications for the effective management of pollution processes in cloudy and foggy weather. [ABSTRACT FROM AUTHOR]

Winter persistent aerosol pollution frequently occurs in the Sichuan Basin (SCB) due to its unfavorable weather conditions, such as low wind, wetness, and cloudiness. Based on long–term observational data analyses from 2015–2021, it was found that the four representative stations in the SCB often simultaneously experienced PM2.5 pollution accompanied by variations in meteorological conditions above 850 hPa, which indicates a connection between regional winter air pollution in the SCB and large–scale synoptic patterns. The dominant 850 hPa synoptic patterns of winter SCB were classified into six patterns using T–model principal component analysis. Pattern 2, characterized by an east high west low (EHWL) pressure system, and Pattern 5, featuring a low trough (LT), were identified as key synoptic patterns for the beginning and accumulation of pollution processes. Pattern 1, characterized by a strong high pressure in the north, was the cleanest pattern associated with reduced PM2.5 concentrations. The EHWL and LT patterns were associated with a remarkably high cloud liquid content, which was attributed to upper southerly winds that introduced humid air and converted aerosols into fog/cloud drops. Clouds reduce solar radiation through reflection and scattering, resulting in more stable stratification and aerosol accumulation. This cloud radiation interaction (CRI) is more pronounced in the LT pattern due to denser isobaric lines and stronger southerly winds than in the EHWL pattern. Numerical simulation experiments using WRF-Chem showed afternoon upper–level heating and morning surface cooling forced by the aerosol radiation interaction (ARI) and evening strong surface cooling influenced by valley winds in the SCB. With wet and cloudy synoptic forcing, CRI directly affects the stability of the boundary layer and is modulated through ARI inhibition. For example, Chongqing showed lower PM2.5 concentrations and stronger ARI than the western and southern SCB due to thinner cloud liquid content and weaker CRI inhibition on ARI. The CRI inhibition caused a 50 % reduction in solar radiation and boundary layer height during the daytime under the LT pattern, which was larger than that under the EHWL pattern. This study comprehensively analyzed the cloud inhibition on ARIs and their impacts on the boundary layer structure under typical synoptic forcing during pollution processes, emphasizing the significant role of CRI inhibition in wet and cloudy regions. [ABSTRACT FROM AUTHOR]

Given the increasing complexity of the chemical composition of PM 2.5 , identifying and quantitatively assessing the contributions of pollution sources has played an important role in formulating policies to control particle pollution. This study provides a comprehensive assessment between PM 2.5 chemical characteristics, sources, and health risks based on sampling data conducted over 1 year (March 2018 to February 2019) in Nanjing. Results show that PM 2.5 exhibits a distinct variation across different seasons, which is primarily driven by emissions, meteorological conditions, and the chemical conversion of gaseous pollutants. First, the chemical mass reconstruction shows that secondary inorganic aerosols (62.5 %) and carbonaceous aerosols (21.3 %) contributed most to the PM 2.5 mass. The increasing oxidation rates of SO 2 and NO 2 from summer to winter indicate that the secondary transformation of gaseous pollutants is strongly positively correlated with relative humidity. Second, the positive matrix factorization (PMF) method shows that identified PM 2.5 sources include secondary inorganic aerosol source (SIS, 42.5 %), coal combustion (CC, 22.4 %), industry source (IS, 17.3 %), vehicle emission (VE, 10.7 %), fugitive dust (FD, 5.8 %), and other sources (1.3 %). The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and the concentration-weighted trajectory (CWT) analysis are used to further explore different spatial distributions and regional transport of sources. The concentrations (10–11 µ g m -3) of SIS and CC distribute in Nanjing and central China in winter. The concentrations (8–10 µ g m -3) of IS and VE are potentially located north of Jiangsu, Anhui, and Jiangxi. Finally, the health risk assessment indicates that the carcinogenic and non-carcinogenic risks of toxic elements (Cr, As, Ni, Mn, V, and Pb) mainly come from IS, VE, and CC, which are within the tolerance or acceptable level. Although the main source of pollution in Nanjing is SIS at present, we should pay more attention to the health burden of vehicle emissions, coal combustion, and industrial processes. [ABSTRACT FROM AUTHOR]

Given the increasing complexity of the chemical composition of PM2.5, identifying and quantitatively assessing the contributions of pollution sources has played an important role in formulating policies to control particle pollution. This study provides a comprehensive assessment between PM2.5 chemical characteristics, sources, and health risks based on sampling data conducted over one year (March 2018 to February 2019) in Nanjing. Results show that PM2.5 exhibits a distinct variation across different seasons, which is primarily driven by emissions, meteorological conditions, and chemical conversion of gaseous pollutants. First, the chemical mass reconstruction shows that secondary inorganic aerosols (SIA, 62.5 %) and carbonaceous aerosols (21.3 %) contributed most to the PM2.5 mass. The increasing oxidation rates of SO2 and NO2 from summer to winter indicate that the secondary transformation of gaseous pollutants is strongly positively correlated with relative humidity. Second, the positive matrix factorization (PMF) method shows that identified PM2.5 sources include SIA (42.5 %), coal combustion (CC, 22.4 %), industry source (IS, 17.3 %), vehicle emission (VE, 10.7 %), fugitive dust (FD, 5.8 %) and other sources (1.3 %). The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and the concentration-weighted trajectory (CWT) analysis are used to further explore different spatial distributions and regional transport of sources. High emissions (10-11 μg·m−3) of SIA and CC distribute in Nanjing and central China in winter. Moderate emissions (8-9 μg·m−3) of IS and VE are potentially located in the north of Jiangsu, Anhui, and Jiangxi. The PM2.5 pollution from long-range transport is attenuated by meteorological conditions and ocean air masses. Finally, the health risk assessment indicates that the carcinogenic and non-carcinogenic risks of toxic elements (Cr, As, Ni, Mn, V, and Pb) mainly come from IS, VE, and CC, which are within the tolerance or acceptable level. Although the main source of pollution in Nanjing is SIA at present, we should pay more attention to the health burden of vehicle emissions, coal combustion, and industrial processes. [ABSTRACT FROM AUTHOR]