Enhanced\nNonagricultural Emissions of Ammonia Influence\nAerosol Ammonium in an Urban Atmosphere: Evidence from Kinetic Versus\nEquilibrium Isotope Fractionation Controls on Nitrogen

Aerosol\nammonium (NH<sub>4</sub><sup>+</sup>) is a critical component\nof particulate matter that affects air pollution, climate, and human\nhealth. Isotope-based source apportionment of NH<sub>4</sub><sup>+</sup> is essential for ammonia (NH<sub>3</sub>) mitigation but the role\nof kinetic vs equilibrium controls on nitrogen isotope (δ<sup>15</sup>N) fractionation between NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> remains unresolved. Based on concurrent measurements\nof NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> in winter Beijing,\nwe observed that the difference of δ<sup>15</sup>N between NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> on clean days (3.3 ±\n13.3‰) was significantly lower than that during polluted periods\n(30.2 ± 11.8‰). This difference signified incomplete\n equilibrium fractionation and that kinetic fractionation may have\noccurred between NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>, especially\nduring clean days. Assuming that kinetic and equilibrium fractionation\noccurred successively, the contribution of nonagricultural emissions\nto NH<sub>4</sub><sup>+</sup> was apportioned to 60.3 ± 12.8%,\nhigher than that of 50.4 ± 17.7% considering only equilibrium\nfractionation. These results indicate that the source apportionment\nof NH<sub>4</sub><sup>+</sup> considering only equilibrium fractionation\nin the conventional scenario would underestimate the contribution\nof nonagricultural emissions by 16.4% (up to 33.1% on clean days).\nOur analysis highlighted the importance in considering both kinetic\nand equilibrium fractionation scenarios to improve the precision of\nNH<sub>4</sub><sup>+</sup> source apportionment by using nitrogen\nisotopes.