Chemical Characteristics of Air Pollution with Significant Emission Changes During 2019-2021 in a Typical Industrial City of Eastern China

Abstract

Restrictions on emission controls were implemented in China to cope with the outbreak of the Coronavirus Disease 2019 (COVID-19), providing a chance to study the impacts of anthropogenic emissions on air quality. Subsequently, three-year measurements of PM2.5 chemical composition and light absorption were conducted in a typical megacity (Suzhou) of the Yangtze River Delta in China from 2019 to 2021. Overall, the annual observation period is divided into four stages: Normal Days, Chinese New Year, Lockdown, and Work Resumption. Our results show that the concentrations of chemical species (such as NO3-, SO42-, OC, EC) in PM2.5 and other gas species, including SO2, NO2, and CO from 2019 to 2021 decreased gradually, which were associated with emission reduction. During the four stages, the COVID-19 lockdown period had the lowest concentration of BC and PM2.5, indicating that strict emission control could significantly improve the air quality. The significant increase of O3 during the COVID-19 lockdown period is likely due to the reduced emission of NO from the traffic, which could weaken the effect of NO on O3 formation. The Normal Days in 2020 has the highest mass loadings of SO42-, NO3-, NH4+, OC, and EC, while the mass concentrations of SO42-, NO3-, NH4+, Cl-, and Na+ in the Lockdown were the lowest. Meanwhile, the aerosol light absorption has decreased yearly, and the significant decreases in light absorption were observed during the COVID-19 lockdown when compared to other stages. The highest frequency of Absorption Ångström exponent (AAE) ranges from 1.3 to 1.4, indicating that brown carbon played critical contributions to the light absorption. The mass concentration and fraction of fuel fossil were significantly higher than that of biomass burning during the whole campaign. The remarkable decline of fuel fossil during the COVID-19 lockdown can be attributed to the closure of factories and the reduction of vehicle emissions. The potential contribution source function analysis suggested that there was a significant regional difference in the potential sources of fossil fuel and biomass combustion. Our results highlight the strict emission control could provide a positive direction for air pollution mitigation campaigns.

Keywords: COVID-19, Yangtze River Delta, chemical composition, optical properties, source apportionment