Reply on RC2

Abstract

The nationwide lockdown was imposed over India from 25 th March to 31 st May2020 with varied relaxations from phase-I to phase-IVto contain the spread of COVID-19. Thus emissions from industrial and transport sectorswere halted during lockdown (LD)which resulted in a significant reduction of anthropogenic pollutants. The first twolockdown phases werestrictly followed(phase-I and phase-II) and hence are considered as total lockdown (TLD) in this study. Satellite-based tropospheric columnar nitrogen dioxide (TCN) from the years 2015 to 2020, tropospheric columnar carbon monoxide (TCC) during 2019-2020 and aerosol optical depth (AOD550) from the years 2014 to 2020 during phase-I and phase-II LDand pre-LD periods were investigated with observations from Aura/OMI, Sentinel-5P/TROPOMI, and Aqua-Terra/ MODIS satellite sensors.To quantify lockdown induced changesin TCN, TCC, and AOD550, detailed statistical analysis was performed on de-trended data using student's paired statistical t-test.Results indicate that mean TCN levels over India showed a dip of 18% compared to the previous year and also against the 5-year mean TCN levelsduring the phase-I lockdown, which was found statistically significant (p-value <0.05) against the respective period. Furthermore, drastic changes in TCN levels were observed over hotspots namely the eastern region and urban cities. For example,there was a sharp decrease of 62%and 54% in TCN levels as compared to 2019 and against 5-years mean TCN levels over New Delhi with a p-value of 0.0002 (which is statistically significant) during total LD.The TCC levels werehigh in the North East (NE) region during the phase-I LD period, which is mainly attributed to the active fire counts in this region. However, lower TCC levels are observed in the same region due to the diminished fire counts during phase-II. Further, AOD550is reduced over the country by ~16 %(Aqua