The response of secondary chemical species to COVID-19 related emission distrubances

We use the global Community Earth System Model to investigate the response of secondary pollutants (ozone O₃, secondary organic aerosols SOA) in response to modified emissions of primary pollutants during the COVID-19 pandemic. We use an estimate of the reduction in surface and aircraft emissions to derive the changes in the chemical composition of the atmosphere. We quantify the respective effects of the reductions in NOx and in VOC emissions, which, in most cases, affect oxidants in opposite ways. Using model simulations, we show that, relative to a situation in which the emission reductions are ignored, the ozone concentration increased only in a few NOx-saturated regions during the winter months of the pandemic when the titration of this molecule by NO_x was reduced. In other regions, where ozone is NOx-controlled, the concentration of ozone decreased.  SOA concentrations decrease in response to the concurrent reduction in the NO_x and VOC emissions. Zonally averaged ozone concentrations in the free troposphere during Northern Hemisphere spring and summer were 5 to 15% lower than 19-year climatological values, in good quantitative agreement with ozone observations. We examine the response in free tropospheric ozone at different latitudes and specifically in the southern hemisphere, the tropics, the northern hemisphere mid-latitudes and polar region. About one third of this anomaly is attributed to the drastic reduction in air traffic during the pandemic, another third to reductions in surface emissions, the remainder to 2020 meteorological conditions, including the exceptional springtime Arctic stratospheric ozone depletion. We compare calculated changes in the vertical ozone profiles with values derived from ozone sonde observations.