This paper investigates the impact of the COVID-19 pandemic lockdown on urban air pollution in Nanjing during the summer of 2021.Using online monitoring data from the SORPES site at Nanjing University,we compared the concentrations of particulate matter and gaseous pollutants before,during,and after the lockdown.Additionally,we used the PMF model and the LPDM model to analyze the sources of volatile organic compounds (VOCs).The results show that,compared to the period before the lockdown,PM_2.5concentrations in Nanjing decreased by 40％-50％,while nitrate and organic matter concentrations decreased by 34.0％ and 16.5％,respectively.Urban areas experienced a significant increase in ozone concentration,approximately 50％.The two gaseous precursors of ozone exhibited opposite trends:NO_x concentrations dropped by 28％,while TVOCs increased by 49.6％.The OBM model and satellite data results indicate that ozone formation in Nanjing was in the VOC-limited regime during the lockdown.Air mass tracing results reveal that VOCs in Nanjing were jointly influenced by local and regional transport from surrounding cities,with increased contributions from the Shanghai-Suzhou-Changzhou-Zhenjiang and Tongling-Ma'anshan directions during the lockdown.
PMF source analysis results demonstrate that local VOCs in Nanjing primarily originate from motor vehicle emissions,plant sources,solvent sources,industrial production,and oil and gas volatile sources.Motor vehicle emissions exhibited the most significant variation among all sources,with a reduction of 15.1％ during the lockdown and an increase of 4.3％ after the lockdown.Oil and gas volatilization and solvent sources increased by 11.2％ and 1.7％,respectively,during the lockdown and decreased by 4.8％ and 4.3％,respectively,after the lockdown.