Importance of meteorology and chemistry in determining air pollutant levels during COVID-19 lockdown in Indian cities.

Indian cities can experience severe air pollution, and the reduction in activity during the first national COVID-19 lockdown (2020) offered a natural experiment to study the contribution of local sources. The current work aimed to quantify the changes due to the lockdown in NOx, O3 and PM2.5 in two contrasting cities in India (Delhi and Hyderabad) using a boosted regression tree model to account for the influence of meteorology. The median NOx and PM2.5 concentrations were observed to decrease after lockdown in both cities, up to 57% and 75% for PM2.5 and NOx, respectively when compared to previous years. After normalization due to meteorology the calculated reduction after lockdown for PM2.5 was small (<8%) in both cities, and was likely less attributable to changes in local emissions, but rather due changes in background levels (i.e. regional source(s)). The reduction of NOx due to lockdown varied by site (on average 5-30%), likely reflecting differences in relative proximity of local sources to the monitoring site, demonstrating the key influence of meteorology on ambient levels post-lockdown. Ozone was observed to increase after lockdown at both sites in Delhi, likely due to changes in relative amounts of precursor concentrations promoting ozone production, suggesting a volatile organic compound (VOC)-limited regime in Delhi. Thus, the calculated reduction in air pollutants due to lockdown in the current work cannot be extrapolated to be solely from a reduction in emissions and instead reflects the overall change in ambient levels, as meteorology and atmospheric chemical processes also contributed.