Classification and transformation of aerosols over selected Indian cities during reduced emissions under COVID-19 lockdown

An improved air quality around the globe and over India has been witnessed during the Covid-19 pandemic lockdown. Using surface observations of particulate matter and chemical species data and products from the MERRA-2 reanalysis Ångstrom exponent (α) and aerosol optical depth (AOD), this study documents the changes in atmospheric chemistry over the Indian subcontinent as a result of nationwide lockdown. Two major cities are selected in five Indian regions to cover a large spatial domain. A general shift from fine to coarse particle size, predominantly of dust type, in all regions is observed, which implies a lowered residence time of aerosol in the atmosphere during decreased anthropogenic emissions. For the studied period, Thiruvananthapuram is the cleanest city with marine origin aerosols and an average PM_2.5 concentration of 7.69±2.40µg/m³ in the last phase of nationwide lockdown. Over Delhi and Ahmedabad, industrial and vehicular emission play important role in influencing the air quality. The diurnal variation of O₃ and NO₂ and their interdependency on each other vary over space and time, with the sharp nighttime O₃ peak observed in the southern region for each lockdown phase. Biomass burning type aerosols decrease over the eastern region. In lockdown, NO₂ also shows a significant correlation with population density (R² = 0.75; p < 0.05), suggesting human mobility (and accordingly vehicular emissions) as the major contributor to NO₂ concentration in the atmosphere. The results of present study did not find any relationship between the ambient concentrations of pollutants to the cumulative increase in COVID-19 cases. However, there is a significant relationship with O₃ concentrations, and in turn with NO₂, which can be associated with respiratory ailments.