[Variations in PM_2.5 Composition and Sources During 2020-2021 COVID-19 Epidemic Periods in Nanjing]

To understand the changes in chemical composition and sources of PM_2.5 under the extreme reduction background during the COVID-19 epidemic periods in Nanjing, hourly observation results of PM_2.5 components (water-soluble inorganic ions, carbonaceous components, and inorganic elements) of two epidemic events from January to March 2020 and June to August 2021 were analyzed. In comparison to that during pre-epidemic periods, the concentration of NO₃^- during the two epidemic control periods decreased by 52.9% and 43.0%, respectively, which was larger than the decreases in NH₄⁺(46.4% and 31.6%) and SO₄^2-(33.8% and 16.5%). Since the observation site was located close to a main road, the decrease in elemental carbon (EC, 35.4% and 20.6%) was higher than that in organic carbon (OC, 11.1% and 16.2%). In reference to the variations in the characteristic ratios of the bulk components mentioned above, the epidemic control showed a more substantial influence on traffic emissions than industrial activities. The concentration time series of PM_2.5 major components over the epidemic periods indicated that NO from local traffic emissions had substantial contributions to the formation of NO₃^-, which led to local short-term PM_2.5 pollution. In addition, the positive matrix factorization (PMF) model was used to analyze the hourly observation data of PM_2.5 components. The seven identified factors were linked with metallurgy, firework and firecracker combustions, road traffic emissions, coal combustion, dust resuspension, secondary sulfate, and secondary nitrate. Because the nitrate was unstable under high temperature, the contribution of secondary nitrate to PM_2.5 during the epidemic control period of 2021 (summer, 21.2%) was much lower than that during the epidemic control period of 2020 (winter, 60.6%); however, the formation of secondary components always dominated the contribution of PM_2.5 sources. Therefore, emissions of NO and SO₂ should be further controlled to continuously reduce ambient PM_2.5 concentrations in Chinese cities.