Effects of COVID-19 pandemic lockdown: A satellite data-based appraisal of air quality in Guwahati, Assam.

Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI) based data are used to evaluate the effects of the COVID-19 lockdown on the concentrations of pollutants such as aerosol optical depth (AOD) and tropospheric columns of nitrogen dioxide (NO) along with sulfur dioxide (SO) respectively for the period of January 2017 to September 2021 over the capital city of Assam, Guwahati. In India lockdown due to COVID-19 was first imposed from 24th March to 14th April as phase I and then it extended from 15th April to 3rd May as phase II in the year 2020. The concentration of all pollutants was usually fall during the lockdown period as compared to their average during the 5-year period over the study area. The results showed that Pre-monsoon (March-May) seasonal AOD for the pandemic year 2020 was decreased by ∼ 23% after lockdown as compared to same season of normal years over the study location. The seasonally averaged AOD reached its peak value in pre-monsoon (0.78 ± 0.09), followed by winter (0.59 ± 0.10) and monsoon (0.52 ± 0.05), with the minimum taking place in post-monsoon (0.38 ± 0.08) season. The monthly average AOD varies from its highest value (0.82 ± 0.18) in May to its lowest value (0.36 ± 0.10) in October for the study period over Guwahati. Tropospheric column NO exhibits same seasonality as AOD with highest value (0.21 × 10 molecules cm) in pre-monsoon and lowest value (0.13 × 10 molecules cm) in post-monsoon season which may be due to same source of origination of both NO and AOD. Conversely, SO does not vary much from the five-year average value during the lockdown period. Significant reduction in PM mass concentration value during Covid-19 lockdown months has been observed which indicates short term improvement of air quality over Guwahati.