Atmospheric Chemistry Insights from the Global COVID-19 Pandemic: A Review

High Resolution Image Download MS PowerPoint Slide The COVID-19 pandemic and resulting reductions in worldwide emissions, associated primarily with the transport sector, provided an unprecedented opportunity to explore the response of atmospheric chemistry and composition to large anthropogenic emissions perturbations. While air quality generally improved in early 2020, this was tempered by increased formation of secondary pollutants (e.g., O 3 and secondary particulate matter, PM) in many regions studied. Declines in NO x emissions were largely responsible for the changes in O 3, driving decreases in O 3 concentrations in remote regions and increases in urban regions due to both decreases in O 3 titration by NO x and also nonlinear changes in O 3 production. Lower NO x levels also increased the levels of other oxidants (e.g., OH and O 3 ), leading to a general increase in atmospheric oxidation in polluted urban regions. This enhanced oxidation promoted additional PM formation in some regions but was generally outweighed by decreases in primary PM and other secondary precursors (SO 2 and VOCs). The COVID-19 pandemic gave rise to large local perturbations in air quality but only modest reductions in the global abundance of short-lived climate forcers (including O 3 and PM).