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.