Assessing COVID-19 Lockdowns' Impacts on Global Urban PM₂.₅ Air Quality with Observations and Modeling

The regional lockdowns, implemented around the world over 2020-2022 to contain the rapid spread of the novel coronavirus disease 2019 (COVID-19), inadvertently created a natural laboratory for investigating the effect of reducing 15 anthropogenic emissions on urban air quality in unprecedentedly large temporal and spatial scales. In this study, we analyze multi-year surface PM₂.₅ observations in 21 cities around the globe to examine anomaly of daily PM₂.₅ concentrations during major COVID-19 lockdowns with respect to that in the pre-pandemic years. We then use a set of GEOS global aerosol transport modeling experiments to disentangle the effect of the lockdown emission reductions from other non-lockdown effects. Our analysis shows that no systematic reductions in PM₂.₅ are found in response to the lockdowns globally. In some locations, we 20 find the coincidences of an increasing stringency index and a decreasing of surface PM₂.₅, which often leads to the record low of PM₂.₅ over extensive period. These observations clearly suggest the positive impacts of COVID-19 lockdown-induced anthropogenic emission reductions on air quality. In other stations, however, the lockdown’s impacts could be masked by differing meteorology and the occurrence of dust and wildfire events. We also found that current satellite remote sensing of aerosol optical depth cannot be used to reliably discern the change of surface PM₂.₅ due to the COVID-19 lockdowns. Results of this study provide a preview of potential mixed effects on urban air quality when implementing air pollution control regulations such as transitioning gasoline and diesel-powered vehicles to electric vehicles.