Diverse Response of Ozone to the COVID-19 Lockdown in Northern and Southern Regions of China

Abstract:

Significant increases in atmospheric ozone (O₃) concentrations have been reported in many cities during the COVID-19 outbreak when emissions of NO_x and other pollutants were drastically reduced. Here we present analysis of surface O₃ observations across China for periods before and during city lockdown and found O₃ decrease in the subtropical south during lockdown in contrast to O₃ increases in most of other regions of China. With numerical modeling, we show that the meteorological changes and emission reductions both contributed to the O₃ changes. The southward-shifted wind with increased temperature, enhanced planetary boundary layer height, decreased cloud fraction and precipitation favored the O₃ increase in north and central China, while the northward-shifted wind with decreased temperature and then biogenic VOC emissions increased cloud fraction and precipitation reduced O₃ in south China. As for the role of emission reduction, the decreased NO titration led to increasing O₃ across China at night; during daytime, the O₃ increased in the north due to larger NO_x reduction than VOC in the NO_x-saturated north, while the O₃ decreased in the south due to the comparably large reductions of NO_x and VOCs from transportation and industries. Our findings reiterate the importance of meteorology in assessing short-time variability of air quality and the nonlinear dependence of O₃ on NO_x and VOC. Our findings also suggest the need to control residential emissions in north and central China.