Ozone pollution in the North China Plain spreading into the late-winter haze season

Abstract:

China is suffering from serious and increasing surface ozone pollution. The situation is worst in the North China Plain in summer. The governmental Clean Air Action starting in 2013 focused initially on winter haze particulate pollution episodes as its top mitigation priority. More recent research and regulation have included a focus on summertime ozone. Ozone in other seasons has received little attention, particularly for winter haze conditions when ozone concentrations have been very low.

Here we show that the abrupt 60% decrease of nitrogen oxides (NO_x) emissions following the COVID-19 lockdown in January 2020 causes a switch to fast ozone production during winter haze episodes in the North China Plain, with ozone concentrations approaching the air quality standard. We reproduce this switch with the GEOS-Chem model, where high emissions of volatile organic compounds (VOCs) drive ozone-forming photochemistry through formaldehyde photolysis. This switch to an ozone-producing regime in January–February following the COVID-19 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 NO_x (30% decrease) while VOC emissions have remained constant.

Our results call for stringent year-round VOC emission controls, as NO_x emissions continue to decrease, to avoid further spreading of severe ozone pollution into the winter-spring season. This result has important implications for developing joint ozone-haze air pollution control strategy in China to protect both public health and crop production.