Concentration and Isotopic Composition of Atmospheric Ammonia Throughout the Urban Mixing Layer in Response to Extreme Emission Reduction

Here, we investigate the vertical distribution of ammonia (NH3) and its nitrogen isotopic composition (δ15N-NH3) at nine heights along the Shanghai Tower (632 m a.g.l.), the world’s highest in situ research platform in urban areas that we recently established. Both the NH3 levels and δ15N-NH3 values, at all heights, were highly responsive to China’s COVID-19 shutdown, and N isotopic shifts were consistent with the shutdown-associated reduction of combustion-related NH3 emissions in early 2020. Despite the fact that the NH3 source partitioning did not greatly change along the vertical transect, we observed that the abundance of NH3 continuously increased from the ground to the upper mixing layer (∼570 m). Supported by chemical transport-model simulations and auxiliary field measurements, our data indicate that vertical transport of urban NH3 emissions represents an important modulating control with regards to the observed vertical pattern of NH3 concentrations and δ15N-NH3.