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.