| As our country’s urbanization process accelerates and economic development continues to grow,increased fuel consumption and a rising number of motor vehicles have resulted in air pollution becoming a serious challenge.Factors influencing atmospheric particulate matter（PM）include mass concentration,particle size,particle size distribution,optical properties,chemical composition,and atmospheric lifetime.Therefore,a deeper understanding of the physical characteristics and chemical composition of atmospheric PM can contribute to a better understanding of their environmental behavior and climate effects.The concentration of fine particulate matter(PM2.5)in regions such as Beijing-Tianjin-Hebei and its surrounding areas remains persistently high,with frequent episodes of severe pollution during the autumn and winter seasons.As the capital of China,Beijing’s pollution issues have drawn significant attention.The outbreak of the COVID-19 pandemic has also brought about major changes in various industries,including the daily lives of citizens,which in turn directly affect emissions of pollutants and their source contributions.Our research team has established atmospheric environmental monitoring sites in Beijing,where we have collected and analyzed chemical compositions of PM2.5 samples from autumn and winter seasons in 2019 to 2021.This has provided reliable data for systematically investigating the interannual variation trends and seasonal patterns of PM2.5 mass concentrations in Beijing before and after the COVID-19pandemic,and identifying the sources and health risks of PM based on their chemical compositions.We have also analyzed the impacts of meteorological factors on PM2.5 in this region,which provides robust support for urban pollution control.The main research findings are as follows:（1）The average concentration of PM2.5 in Beijing during the autumn of 2019 was59.23±40.48μg/m3,while the average concentration during winter was 63.26±33.93μg/m3.The number of days in 2019 when PM2.5 concentration exceeded the first-level limit value（35μg/m3）for daily average concentration accounted for 67.86%of the total sampling days.In autumn 2020,the average concentration of PM2.5 in Beijing was 38.07±32.10μg/m3,while the average concentration during winter was 47.56±13.80μg/m3.The number of days in 2020 when PM2.5 concentration exceeded the first-level limit value for daily average concentration accounted for 53.85%of the total sampling days.In autumn 2021,the average concentration of PM2.5 in Beijing was 13.77μg/m3,and the PM2.5 concentration did not exceed the first-level limit value for daily average concentration during the sampling period.The number of clean days increased in 2020,and the number of pollution days significantly decreased during the autumn and winter seasons,with further reduction in pollution during autumn 2021.The occurrence of the COVID-19 pandemic has resulted in significant changes in various industries,including the daily lives of citizens,which has affected pollutant emissions and also reflects the effectiveness of pollution control measures in Beijing.The proportion of secondary ions(SO42-,NO3-,NH4+)and carbonaceous components in PM2.5 was around 30%and 20%,respectively.Secondary ions were the largest component in PM2.5.Enrichment factor analysis showed that Mg,Al,Na,K,Ba,and Ca were mainly from natural sources,while As,Cd,Zn,Ni,Cr,Pb,Cu,and Ag were mainly from anthropogenic emissions.（2）The results of the positive matrix factorization（PMF）receptor model showed that the source contributions to PM2.5 during the sampling period in 2019 were coal combustion（32%）,secondary sources（28%）,traffic-related sources（21%）,industry I（8%）,industry II（8%）,and dust（3%）.The source contributions to PM2.5 during the sampling period in 2020 were traffic-related sources（28%）,secondary sources（22%）,coal combustion（21%）,industry I（19%）,industry II（7%）,and dust（3%）.Coal combustion had a significant impact on PM2.5 pollution in Beijing.The contribution of traffic-related sources increased in 2020,likely due to the increase in personalized travel in Beijing during the COVID-19 pandemic,which resulted in increased traffic-related emissions.Industrial sources were significantly influenced by regional transport.The differences in air mass trajectory and potential pollution source regions were the main reasons for the differences in industrial sources between the two sampling years.During autumn,the potential pollution sources of PM2.5 in Beijing were mainly distributed in the southern direction,including Shijiazhuang and Jinan areas,while during winter,they were mainly distributed in the northwest direction.（3）In terms of exposure pathways,the carcinogenic risk（CR）and non-carcinogenic risk（HQ）values of metal elements in PM2.5 through ingestion and dermal contact pathways were similar to or even higher than those through inhalation pathway.Among the metal elements,As contributed the most to the total CR values（～80%）and total HQ values（70%for children,41%for adults）through all three pathways.The total carcinogenic and non-carcinogenic risk values of metal elements in winter was greater than that in autumn,which has a greater potential impact on human health.Attention should be paid to the carcinogenic risk caused by metal elements in PM2.5 in Beijing.For non-carcinogenic risks,more attention should be paid to their impact on children.Reducing emissions from coal combustion not only reduces PM2.5 concentrations but also reduces health risks. |
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