Aerosol Ammonium in the Urban Boundary Layer in Beijing: Insights from Nitrogen Isotope Ratios and Simulations in Summer 2015
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2019 American Chemical Society. This study investigates the concentrations and 15N 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 15N-NH4+ values were higher because the regional transport of agricultural sources (lower ratios of 15N-NH4+) was less significant. Vertical profiles of NH4+ increased with height while 15N-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.
