  Due to the lockdown measures during the 2019 novel coronavirus (COVID-19)
pandemic, the economic activities and the associated emissions have
significantly declined. This reduction in emissions has created a natural
experiment to assess the impact of the emitted precursor control policy on
ozone (O$_3$) pollution, which has become a public concern in China during the
last decade. In this study, we utilized comprehensive satellite, ground-level
observations, and source-oriented chemical transport modeling to investigate
the O$_3$ variations during the COVID-19 in China. Here we found that the O$_3$
formation regime shifted from a VOC-limited regime to a NOx-limited regime due
to the lower NOx during the COVID-19 lockdown. However, instead of these
changes of the O$_3$ formation region, the significant elevated O$_3$ in the
North China Plain (40%) and Yangtze River Delta (35%) were mainly attributed to
the enhanced atmospheric oxidant capacity (AOC) in these regions, which was
different from previous studies. We suggest that future O$_3$ control policies
should comprehensively consider the synergistic effects of O$_3$ formation
regime and AOC on the O$_3$ elevation.
