Influence of changes in anthropogenic and natural sources on global aerosol optical depth during COVID-19 lockdown: Ground-based observations, satellites, models

We analyze, in this study, for the first time, multi-source data - high-quality ground-based observations (Aerosol Robotic Network (AERONET)) and satellite (Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI)) data in combination with two high resolution model (Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2) and Copernicus Atmosphere Monitoring Service (CAMS)) outputs, and examine the changes in aerosol optical depth (AOD) across the globe during Corona Virus Disease-2019 (COVID-19) lockdown (2020) period. Statistical significance of the change in aerosol characteristics (AOD and species AODs) from normal (2017–2019 mean) to COVID (2020) period is calculated by two-tailed Student's t-test. AERONET AODs decreased by 20–40% during peak lockdown period in spring and summer of 2020 in most locations across the globe, and these changes are statistically significant. The observed magnitude of changes (increase/decrease) are consistent between AERONET and MODIS. Both global models (MERRA-2 and CAMS) capture well, observed AOD changes. OMI absorbing aerosol index captures increases in carbonaceous aerosols due to wildfires and dust. Global analysis reveals that changes in total AOD and species AODs are highest, and statistically significant during spring over Middle East, South and East Asia. Among all regions, change in dust AOD is maximum over South Asia during spring (>75%; −0.04 decrease in dust AOD in −0.05 change in total AOD) which is statistically significant. Black carbon (BC) and sea salt contribute ≤10% to AOD across the globe. Decrease in BC AOD is maximum over South Asia (50%) during spring 2020 providing a direct evidence for the impact of applied restrictions on anthropogenic activities. Decrease in AOD over North America, Europe, Russia, Middle East, South Asia, East Asia, and South East Asia led to global decrease in AOD. This quantitative documentation of changes in AODs due to reduction in aerosol components/species, their regional and seasonal variations are crucial in planning and supporting future mitigation efforts.