The Impact Of Anthropogenic Emission Reduction On Urban Atmospheric Oxidizability: A Case Study During The 2020 Epidemi

Regional compound air pollution events occur frequently in eastern China.Fine particulate matter(PM2.5)and ozone(O3)are the main pollutants in regional compound air pollution events.Atmospheric oxidizing capacity(AOC)is a fundamental driver of atmospheric chemistry to form complex air pollution,and has an important role in the formation of PM2.5 and O3.Atmospheric oxidizing capacity is influenced by complex factors such as oxidant concentrations,pollutant emission levels,and meteorological conditions,but the relationship between the response of AOC to key precursor emissions is currently unclear.The significant decline in anthropogenic emissions of air pollutants due to the COVID-19 lockdown in early2020 provides a study case for exploring AOC and precursor reduction.In this thesis,simulations of community multiscale air quality model(CMAQ)are used to identify key influences on AOC enhancement during the COVID-19 lockdown(23 January to 29 February in 2020)and assess the impact of AOC on secondary pollutant generation by selecting the Beijing-Tianjin-Hebei,Yangtze River Delta and Pearl River Delta regions and representative cities in each region(Shijiazhuang,Nanjing and Guangzhou)in China for in-depth analysis.The results show that :(1)The concentrations of major oxidants hydroxyl radical(OH),O3,and nitrogen oxygen radical(NO3)all increase significantly(14.8%～73.3%,1.6%～26.4%,and 37.9%～180%)in Shijiazhuang,Nanjing,and Guangzhou during the COVID-19 lockdown,while AOC also increase significantly(12.6%～60%)in three cities,representing the enhancement of AOC in the urban atmosphere.(2)The contributions of emission effect to the oxidants and AOC changes in all three cities are positive(63%～174%),indicating that emission change is the main influencing factor for AOC enhancement.In contrast,the contribution of meteorological effect to AOC in different cities differs somewhat: meteorological contributions to AOC in Shijiazhuang and Nanjing are positive(33% and 37%),but the contribution to AOC in Guangzhou is negative(-74%).Reducing the emissions of volatile organic compounds(VOCs)can effectively control the concentrations of AOC,O3 and OH in Shijiazhuang,Nanjing and Guangzhou,while reducing the emissions of nitrogen oxides(NOx)instead causes an increase in oxidants and AOC levels,which is the main reason for the increase in AOC in each city during the COVID-19 lockdown;reducing NOx emissions can significantly reduce the concentrations of secondary inorganic pollutants(SIA),while reducing emissions of VOCs can effectively control the concentration of secondary organic pollutants(SOA).(3)Due to the rise in atmospheric oxidizing capacity during the COVID-19 lockdown,nitrogen oxidation rate(NOR),sulfur oxidation rate(SOR),and VOCs oxidation rate(VOR)all increase to different degrees(5%～66.7%,18.5%～57.9%,and 12%～75%),which in turn promoted the conversion efficiency of primary pollutants to secondary pollutants and offset part of the impact of primary emission reduction,resulting in non-linear decrease in secondary pollutants compared to emissions during the COVID-19 lockdown.The analysis of SOA generated by different oxidation pathways shows that the increase in SOA concentration in Nanjing is mainly caused by the enhancement of OH,NO3,and O3 oxidation,while the decrease in SOA concentration in Shijiazhuang and Guangzhou is mainly from the reduction of SOA in the liquid phase oxidation pathway(-25% to 49%).