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

Ozone (O) is a key oxidant and pollutant in the lower atmosphere. Significant increases in surface O have been reported in many cities during the COVID-19 lockdown. Here we conduct comprehensive observation and modeling analyses of surface O across China for periods before and during the lockdown. We find that daytime O decreased in the subtropical south, in contrast to increases in most other regions. Meteorological changes and emission reductions both contributed to the O changes, with a larger impact from the former especially in central China. The plunge in nitrogen oxide (NO) emission contributed to O increases in populated regions, whereas the reduction in volatile organic compounds (VOC) contributed to O decreases across the country. Due to a decreasing level of NO saturation from north to south, the emission reduction in NO (46%) and VOC (32%) contributed to net O increases in north China; the opposite effects of NO decrease (49%) and VOC decrease (24%) balanced out in central China, whereas the comparable decreases (45-55%) in these two precursors contributed to net O declines in south China. Our study highlights the complex dependence of O on its precursors and the importance of meteorology in the short-term O variability.