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

Highlights: • The O₃ decreased in the south but increased in most other regions during lockdowns. • Meteorological changes and emission reductions both contributed to O₃ changes. • The O₃ changes in different regions depended on the levels of NO_x and VOC reductions. Ozone (O₃) is a key oxidant and pollutant in the lower atmosphere. Significant increases in surface O₃ have been reported in many cities during the COVID-19 lockdown. Here we conduct comprehensive observation and modeling analyses of surface O₃ across China for periods before and during the lockdown. We find that daytime O₃ decreased in the subtropical south, in contrast to increases in most other regions. Meteorological changes and emission reductions both contributed to the O₃ changes, with a larger impact from the former especially in central China. The plunge in nitrogen oxide (NO_x) emission contributed to O₃ increases in populated regions, whereas the reduction in volatile organic compounds (VOC) contributed to O₃ 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 O₃ 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 these two precursors contributed to net O₃ declines in south China. Our study highlights the complex dependence of O₃ on its precursors and the importance of meteorology in the short-term O₃ variability.