Atmospheric Ammonia in Beijing during the COVID-19 Outbreak: Concentrations, Sources, and Implications.

This study investigates the concentrations and δ 15 N values of NH 3 in Beijing during and after the 2020 COVID-19 lockdown. Higher NH 3 concentrations and lower δ 15 N-NH 3(measured) were observed at most sites in 2020 compared to 2017. Except for a site inside a tunnel, NH 3 concentrations did not increase significantly after the lockdown had ended compared to those during the lockdown, while δ 15 N-NH 3(measured) increased by 2.1-9.9‰. Nonagricultural sources (fossil fuel and urban waste) overall contributed 81% and 62% of NH 3 at on-road (tunnel interior) and nonroad (CAU) sites in 2020, respectively, comparable to those in 2017 (without significant difference). The contribution of nonagricultural sources slightly increased after the lockdown compared to the contribution during the lockdown at the nonroad site and hardly changed at the tunnel interior site. Our results suggest that (1) unfavorable meteorological conditions, especially lower boundary layer heights and changes in regional transport patterns, might play a more important role than reduced anthropogenic emissions in the temporal variations of Beijing NH 3 and (2) the effect of reduced anthropogenic emissions, during the COVID-19 outbreak or with the future implementation of emission control strategies, on atmospheric NH 3 can be better demonstrated by isotope-based source apportionment of NH 3 , rather than only by changes in NH 3 concentrations.