Air quality in Nanjing during COVID-19 lockdown period

In order to investigate the impact of COVID-19lockdown on air quality in Nanjing, the air pollutants observed from January 25 to February 10, in 2020 (COVID-19lockdown period) in Nanjing and its surrounding cities was analyzed. During the lockdown period with poor atmospheric diffusion conditions, the concentrations of PM2.5, PM10, NO2, SO2, and CO decreased obviously, with the value of 36, 44, 5, 22μg/m3 and 1.1mg/m3, whereas O3 increased by 4%. The net effectiveness of the emission reduction measures was calculated through comparisons of concentrations of air pollutants between and before COVID in the similar meteorological conditions. Concentrations of PM2.5, PM10, SO2, NO2 and CO decreased by 41.7%, 45.3%, 14.3%, 43.5% and 18.2%, respectively, whereas O3 increased by 4.8%. Compared to capital cities of the Yangtze River Delta in the same period, the largest decline of SO2 and the medium decline of the other pollutions were appeared in Nanjing. The diurnal variation concentration of PM2.5 and PM10 changed from double peak to single peak, due to the disappearance of nighttime sub-peak of particle. The concentration of O3 increased significantly at night, which was resulted from that sharp reduction of traffic sources weaken the titration reaction of NO to O3. The peak of O3 during the daytime depended on the variation of the ratio of VOCs to NOx due to the emission control. © 2021, Editorial Board of China Environmental Science. All right reserved.