Impact of COVID-19 restrictions on the concentration and source apportionment of atmospheric ammonia (NH(3)) across India

The wide spread of the coronavirus disease (COVID-19) has significantly influenced human activities around the world, providing a unique opportunity to investigate the response of air pollution to anthropogenic emission reduction. Compared with numerous studies on conventional air pollutants, atmospheric ammonia (NH(3)) that has matched sources from both anthropogenic and natural emissions has been rarely investigated. Here we assess impacts of the COVID-19 lockdown on ambient NH(3) variation across India, one of the most severe NH(3) pollution region in the world. The role of meteorology in shaping emission contribution to NH(3) pollution and respective contribution of each emission source to ambient NH(3) before and after the COVID-19 outbreak were investigated using the XGBoost algorithm coupled with WRF-Chem model. Results showed that ambient NH(3) concentrations in the seven major cities (Hyderabad, Bengaluru, Chennai, Delhi, Lucknow, Kolkata and Mumbai) decreased by 2.1–53.8 % whereas in Ahmedabad increased by 20.3 % during the COVID-19 lockdown period. Obvious decrease in NH(3) in Indo-Gangetic Plain (−17.1 %) was mainly driven by favorable meteorology, whereas the slight decline in NH(3) in South India was mainly driven by epidemic-related emission control (−8.56 %). Source appointment results showed that the contribution of industrial emission (Ind) to ambient NH(3) in most megacities showed a decreasing trend (between 11 % and 26 %) during the lockdown period. However, the reduction effect was mostly compensated by increasing contributions (15–25 %) of residential emission (Res) or agricultural soil emission (Ags). Particularly, in Ahmedabad and Lucknow ambient NH(3) increased by 20.3 % and 12 % during the lockdown period, the reduction effect of Ind on ambient NH(3) (−23 % and −11 %, respectively) was absolutely compensated by enhanced contribution of Res (24 %) and Ags (12 %), respectively. Our results highlight the importance of eliminating residential and agricultural NH(3) emissions especially in North India.