Global Changes in Secondary Atmospheric Pollutants During the 2020 COVID‐19 Pandemic

International audienceWe use the global Community Earth System Model to investigate the response of secondarypollutants (ozone O3, secondary organic aerosols SOA) in different parts of the world in response tomodified emissions of primary pollutants during the COVID-19 pandemic. We quantify the respectiveeffects of the reductions in NOx and in volatile organic carbon (VOC) emissions, which, in most cases,affect oxidants in opposite ways. Using model simulations, we show that the level of NOx has beenreduced by typically 40% in China during February 2020 and by similar amounts in many areas ofEurope and North America in mid-March to mid-April 2020, in good agreement with space and surfaceobservations. We show that, relative to a situation in which the emission reductions are ignored anddespite the calculated increase in hydroxyl and peroxy radicals, the ozone concentration increasedonly in a few NOx-saturated regions (northern China, northern Europe, and the US) during the wintermonths of the pandemic when the titration of this molecule by NOx was reduced. In other regions,where ozone is NOx-controlled, the concentration of ozone decreased. SOA concentrations decreasein response to the concurrent reduction in the NOx and VOC emissions. The model also shows thatatmospheric meteorological anomalies produced substantial variations in the concentrations of chemicalspecies during the pandemic. In Europe, for example, a large fraction of the ozone increase in February2020 was associated with meteorological anomalies, while in the North China Plain, enhanced ozoneconcentrations resulted primarily from reduced emissions of primary pollutants