This study investigates the concentrations and δ 15 N values of NH 4 + in PM 2.5 at three heights (8, 120, and 260 m) on a 325 m tower in urban Beijing. NH 4 + 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 δ 15 N-NH 4+ values were higher because the regional transport of agricultural sources (lower ratios of δ 15 N-NH 4 + ) was less significant. Vertical profiles of NH 4 + increased with height while δ 15 N-NH 4 + 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 NH 4 + 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 NH 3 emissions are likely underestimated and NH 3 slip from selective catalytic reduction processes should be included to explain the observed source contributions.

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