Aerosol Ammonium in the Urban Boundary Layer in Beijing: Insights from Nitrogen Isotope Ratios and Simulations in Summer 2015

This study investigates the concentrations and δ15N values of NH4 + in PM2.5 at three heights (8, 120, and 260 m) on a 325 m tower in urban Beijing. NH4 + concentrations were lower during the Parade Blue Period (August 20 to September 3, 2015) due to strict air pollution control and favorable meteorological conditions, while δ15N-NH4 + values were higher because the regional transport of agricultural sources (lower ratios of δ15N-NH4 +) was less significant. Vertical profiles of NH4 + increased with height while δ15N-NH4 + decreased, indicating an enhanced contribution from regional transport at high altitudes. The results of the “MixSIAR” isotopic mixing model indicate that agricultural emission produces 47% of the ground-surface NH4 + and reaches 51–56% at high altitudes. Results from a source-oriented air quality model and the 2016 MEIC emission inventory suggest that non-agricultural NH3 emissions are likely underestimated and NH3 slip from selective catalytic reduction processes should be included to explain the observed source contributions.