Ozone Anomalies in the Free Troposphere During the COVID‐19 Pandemic

Using the CAM‐chem Model, we simulate the response of chemical species in the free troposphere to scenarios of primary pollutant emission reductions during the COVID‐19 pandemic. Zonally averaged ozone in the free troposphere during Northern Hemisphere spring and summer is found to be 5%–15% lower than 19‐yr climatological values, in good agreement with observations. About one third of this anomaly is attributed to the reduction scenario of air traffic during the pandemic, another third to the reduction scenario of surface emissions, the remainder to 2020 meteorological conditions, including the exceptional springtime Arctic stratospheric ozone depletion. For the combined emission reductions, the overall COVID‐19 reduction in northern hemisphere tropospheric ozone in June is less than 5 ppb below 400 hPa, but reaches 8 ppb at 250 hPa. In the Southern Hemisphere, COVID‐19 related ozone reductions by 4%–6% were masked by comparable ozone increases due to other changes in 2020. Plain Language Summary The reduction in the emissions of primary air pollutants during the 2020 COVID‐19 pandemic has generated perturbations in the chemical state of the atmosphere. A global Earth system model that accounts for chemical, physical, and dynamical processes in the atmosphere and for the coupling between the atmosphere, the ocean and the land surface, indicates that the abundance of tropospheric ozone was significantly reduced during the pandemic in response to realistic scenarios of reduced emissions of primary pollutants associated with restrictions of air traffic and economic activities. These simulated findings are consistent with observed ozone anomalies during the summer of 2020. Key Points The ozone concentration in the northern extratropical free troposphere was 5%–15% lower in May and June 2020 relative to climatology A third of this anomaly is attributed to meteorological conditions including stratospheric Arctic air with abnormally low ozone The assumed reduction in surface and aircraft emissions associated with the COVID‐19 pandemic can explain an ozone anomaly of 4%–8%