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 NO, O and PM in two contrasting cities in India (Delhi and Hyderabad) using a boosted regression tree model to account for the influence of meteorology. The median NO and PM concentrations were observed to decrease after lockdown in both cities, up to 57% and 75% for PM and NO, respectively when compared to previous years. After normalization due to meteorology the calculated reduction after lockdown for PM was small (<8%) in both cities, and was likely less attributable to changes in local emissions, but rather due changes in background levels ( regional source(s)). The reduction of NO 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.