Enhanced Nonagricultural Emissions of Ammonia Influence Aerosol Ammonium in an Urban Atmosphere: Evidence from Kinetic Versus Equilibrium Isotope Fractionation Controls on Nitrogen

Aerosol ammonium (NH₄⁺) is a critical component of particulate matter that affects air pollution, climate, and human health. Isotope-based source apportionment of NH₄⁺ is essential for ammonia (NH₃) mitigation but the role of kinetic vs equilibrium controls on nitrogen isotope (δ¹⁵N) fractionation between NH₃ and NH₄⁺ remains unresolved. Based on concurrent measurements of NH₃ and NH₄⁺ in winter Beijing, we observed that the difference of δ¹⁵N between NH₃ and NH₄⁺ on clean days (3.3 ± 13.3‰) was significantly lower than that during polluted periods (30.2 ± 11.8‰). This difference signified incomplete equilibrium fractionation and that kinetic fractionation may have occurred between NH₃ and NH₄⁺, especially during clean days. Assuming that kinetic and equilibrium fractionation occurred successively, the contribution of nonagricultural emissions to NH₄⁺ was apportioned to 60.3 ± 12.8%, higher than that of 50.4 ± 17.7% considering only equilibrium fractionation. These results indicate that the source apportionment of NH₄⁺ considering only equilibrium fractionation in the conventional scenario would underestimate the contribution of nonagricultural emissions by 16.4% (up to 33.1% on clean days). Our analysis highlighted the importance in considering both kinetic and equilibrium fractionation scenarios to improve the precision of NH₄⁺ source apportionment by using nitrogen isotopes.