Concentration\nand Isotopic Composition of Atmospheric\nAmmonia Throughout the Urban Mixing Layer in Response to Extreme Emission\nReduction

Here, we investigate the vertical\ndistribution of ammonia (NH<sub>3</sub>) and its nitrogen isotopic\ncomposition (δ<sup>15</sup>N-NH<sub>3</sub>) at nine heights\nalong the Shanghai Tower (632 m\na.g.l.), the world’s highest in situ research platform in urban\nareas that we recently established. Both the NH<sub>3</sub> levels\nand δ<sup>15</sup>N-NH<sub>3</sub> values, at all heights, were\nhighly responsive to China’s COVID-19 shutdown, and N isotopic\nshifts were consistent with the shutdown-associated reduction of combustion-related\nNH<sub>3</sub> emissions in early 2020. Despite the fact that the\nNH<sub>3</sub> source partitioning did not greatly change along the\nvertical transect, we observed that the abundance of NH<sub>3</sub> continuously increased from the ground to the upper mixing layer\n(∼570 m). Supported by chemical transport-model simulations\nand auxiliary field measurements, our data indicate that vertical\ntransport of urban NH<sub>3</sub> emissions represents an important\nmodulating control with regards to the observed vertical pattern of\nNH<sub>3</sub> concentrations and δ<sup>15</sup>N-NH<sub>3</sub>.