Source apportionment of aerosol ammonium in the urban boundary layer of Beijing from tower-based observations

Ammonium (NH₄⁺) is an important component of PM_2.5, and atmospheric NH₄⁺ mainly comes from secondary reactions of NH₃. It significantly impacts air pollution, radiative forcing, and human health. Source apportionment of NH₄⁺ can help improve air quality through emission reductions. Previous studies have primarily focused on ground-level aerosols, and understanding the vertical characteristics of atmospheric NH₄⁺ in the atmospheric boundary layer can deepen our understanding of the sources and transport processes of NH₃/NH₄⁺, enhancing the accuracy of atmospheric model simulations. In this study, we collected PM_2.5 samples at 8, 120, and 260 m from the 325-meter meteorological tower at the Institute of Atmospheric Physics, Chinese Academy of Sciences (Beijing, China), conducted stable isotope analyses and source apportionment of atmospheric aerosol NH₄⁺ in summer and winter. The summer results show that the concentration of NH₄⁺ rises and its δ¹⁵N decreases as the sampling height increases, indicating that regional transport, especially from agricultural sources of NH₃/NH₄⁺ in the North China Plain, has a greater impact on high-altitude NH₄⁺ in Beijing. The source apportionment results from the stable isotope mixing model “MixSIAR” show that agricultural sources contribute 47% to NH₄⁺ in ground-level PM_2.5, and this increases to 51~56% at higher altitudes. Comparing the observational results with atmospheric chemistry modeling suggests that non-agricultural NH₃ emissions in Beijing may be significantly underestimated. Compared to summer, the vertical characteristics in winter are more complex. Still, overall, the concentration of NH₄⁺ increases with height, indicating that both local emissions and regional transport contribute significantly to NH₄⁺, with local emissions having a greater impact near the ground. Combustion-related NH₃ emissions, including fossil fuel sources, NH₃ slip, and biomass burning, contribute 60% to atmospheric NH₄⁺ during heavily polluted days in winter, exceeding the contributions from volatilization-related NH₃ emissions, including livestock breeding, N-fertilizer application, and human waste. In contrast, volatilization-related NH₃ emissions dominate on clean days. Biomass burning, especially bioapplication (combustion and use of straw and firewood), may be an important NH₃ source that has been overlooked. The study also used atmospheric chemical models to compare the effects of different emission reduction strategies on air pollution control. Compared to reducing a single pollutant (NH₃), the simultaneous reduction of NH₃ and other pollutants has a more significant effect on lowering PM_2.5 concentrations. To improve air quality, future policies could consider implementing simultaneous emission reductions of NH₃ and other pollutants for air pollution control.