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This study investigates the concentrations and δ¹⁵N values of NH₄⁺ in PM_2.5 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 …