Surface
ozone is a severe air pollution problem in the North China Plain, which is 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 has been very low during winter haze (particulate) pollution episodes. However, the abrupt 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-h average (MDA8)
ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of
ozone is driven by HOx radicals from photolysis of
formaldehyde, overcoming radical titration from the decreased NOx emissions.
Formaldehyde is produced by oxidation of reactive
volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an
ozone-producing regime in January-February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter-spring
ozone in the North China Plain and increasing association of high
ozone with winter haze events, as pollution control efforts have targeted NOx emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe
ozone pollution events into the winter-spring season.