  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 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.
