Short-Lived Air Pollutants and Climate Forcers Through the Lens of the COVID-19 Pandemic

Dramatic reductions in anthropogenic emissions during the lockdowns of the COVID-19 pandemic provide an unparalleled opportunity to assess responses of the Earth system to human activities. Here, we synthesize the latest progress in understanding changes in short-lived atmospheric constituents, that is, aerosols, ozone (O₃), nitrogen oxides (NO_x), and methane (CH₄), in response to COVID-19 induced emission reductions and the associated climate impacts on regional and global scales. The large-scale emission reduction in the transportation sector reduced near-surface particulate and ozone concentrations, with certain regional enhancements modulated by atmospheric oxidizing capacity and abnormal meteorological conditions. The methane increase during the pandemic is a combined effect of fluctuations in methane emissions and chemical sinks. Global net radiative forcing of all short-lived species was found to be small, but regionally, aerosol radiative impacts during the lockdowns were discernible near China and India. Aerosol microphysical effects on clouds and precipitation were reported from modeling assessments only, except for observed reductions in aircraft contrails. There exist moderate climatic impacts of the pandemic on regional surface temperature, atmospheric circulations, and ecosystems, mainly over populous and polluted areas. Novel methodologies emerge in the pandemic-related research to achieve the synergy between observations from multiple platforms and model simulations and to overcome the enormous hurdles and sophistication in detection and attribution studies. The insight gained from COVID-19 research concerning the complex interplay between emission, chemistry, and meteorology, as well as the unexpected climate forcing-responses relationships, underscores future challenges for cleaning up the air and alleviating the adverse impacts of global warming.