Analyzing aerosol variation during the COVID-19 pandemic lockdown using satellite data

The COVID-19 pandemic caused lockdowns worldwide throughout many cities, implementing numerous restrictions on human activity. Previous studies have shown that various anthropogenic pollutants have decreased in many regions from these lockdowns, which should translate into aerosol levels, measured by aerosol optical depth (AOD), also having a large reduction. However, AOD levels over metropolitan areas during the lockdown periods have not been sufficiently documented. Using NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) AOD data, we analyzed AOD level variances during the pandemic over 13 selected metropolitan areas in Europe, India, and China. We hypothesized that reduced human activities from the pandemic lockdowns would also result in a large decrease in AOD levels in metropolitan areas. So, we compared the average AOD levels during the lockdown to the previous five-year average during the city’s corresponding lockdown dates in order to investigate the statistical significance of variations. For European cities, there were few changes in AOD levels observed during the pandemic lockdown periods. As for the Indian cities, three showed slight decreases in AOD levels (~0.5%–7.4%) and one showed an 11% increase. However, compared to Indian and European cities, all of the cities in China showed noticeable reductions in AOD levels during the lockdown (~11%–30%). Statistical analysis did not show statistical significance for most of the cities due to the large variation of AOD, suggesting that the AOD level decrease attributed to the reduction of human activities does not exceed the range of natural AOD level variation.