Samsung Advanced Institute of Technology

 Surface ozone is a severe air pollution problem in the North China Plain, home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HOx) that can overcome the radical titration caused by high emissions of nitrogen oxides (NOx) from fuel combustion. Ozone is very low during winter haze (particulate) pollution episodes due to direct titration by emitted NO.  However, the abrupt 60％ decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-hour average (MDA8) ozone of 60？70 parts per billion (ppb) and peroxyacetyl nitrate (PAN) levels of 4 ppb. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone and PAN is driven by HOx radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NOx emissions. Formaldehyde levels are very high in the North China Plain because of high emissions of volatile organic compounds (VOCs). This switch to an ozone-producing regime in January？February following the lockdown illustrates a more general tendency over 2013？2019 of higher winter-spring ozone in the North China Plain, and increasing association of high ozone with winter haze events in late winter, as emission controls have targeted NOx (30％ decrease) while VOC emissions have remained constant. This calls for stringent year-round VOC emission controls as NOx emissions continue to decrease to avoid further spreading of severe ozone pollution into the winter-spring season.