15 May 2021
Ozone (O 3) is a key oxidant and pollutant in the lower atmosphere. Significant increases in surface O 3 have been reported in many cities during the COVID-19 lockdown. Here we conduct comprehensive observation and modeling analyses of surface O 3 across China for periods before and during the lockdown. We find that daytime O 3 decreased in the subtropical south, in contrast to increases in most other regions. Meteorological changes and emission reductions both contributed to the O 3 changes, with a larger impact from the former especially in central China. The plunge in nitrogen oxide (NO x) emission contributed to O 3 increases in populated regions, whereas the reduction in volatile organic compounds (VOC) contributed to O 3 decreases across the country. Due to a decreasing level of NO x saturation from north to south, the emission reduction in NO x (46%) and VOC (32%) contributed to net O 3 increases in north China; the opposite effects of NO x 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 3 declines in south China. Our study highlights the complex dependence of O 3 on its precursors and the importance of meteorology in the short-term O 3 variability.