Diverse response of surface ozone to COVID-19 lockdown in China

Abstract

<p>Ozone (O<sub>3</sub>) is a key oxidant and pollutant in the lower atmosphere. Significant increases in surface O<sub>3</sub> have been reported in many cities during the COVID-19 lockdown. Here we conduct comprehensive observation and modeling analyses of surface O<sub>3</sub> across China for periods before and during the lockdown. We find that daytime O<sub>3</sub> decreased in the subtropical south, in contrast to increases in most other regions. Meteorological changes and emission reductions both contributed to the O<sub>3</sub> changes, with a larger impact from the former especially in central China. The southward-shifted wind with increased temperature, enhanced planetary boundary layer height, decreased cloud fraction and precipitation favored the O<sub>3</sub> increase in north and central China, while the northward-shifted wind with decreased temperature and then biogenic volatile organic compounds (VOCs) emissions, increased cloud fraction and precipitation reduced O<sub>3</sub> in south China. As for the emission reduction, the drop in nitrogen oxide (NO<sub>x</sub>) emission contributed to O<sub>3</sub> increases in populated regions, whereas the reduction in VOCs contributed to O<sub>3</sub> decreases across the country. Due to a decreasing level of NO<sub>x</sub> saturation from north to south, the emission reduction in NO<sub>x</sub> (46%) and VOC (32%) contributed to net O<sub>3</sub> increases in north China; the opposite effects of NO<sub>x</sub> decrease (49%) and VOC decrease (24%) balanced out in central China, whereas the comparable decreases (45-55%) in the two precursors contributed to net O<sub>3</sub> declines in south China. Our study highlights the complex dependence of O<sub>3</sub> on its precursors and the importance of meteorology in the short-term O<sub>3</sub> variability.</p>