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

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., O₃ and secondary particulate matter, PM) in many regions studied. Declines in NO_x emissions were largely responsible for the changes in O₃, driving decreases in O₃ concentrations in remote regions and increases in urban regions due to both decreases in O₃ titration by NO_x and also nonlinear changes in O₃ production. Lower NO_x levels also increased the levels of other oxidants (e.g., OH and O₃), 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 (SO₂ 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 O₃ and PM).