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

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 (NOx) 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 NOx saturation from north to south, the emission reduction in NOx (46%) and VOC (32%) contributed to net O-3 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-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. (C) 2021 Elsevier B.V. All rights reserved.