Source Apportionment, Spatiotemporal Variations of Atmospheric Ammonia in China: Focus on Severe Haze Episodes in Urban Beijing

The reduction of ammonia emissions is urgently needed due to its role in haze pollution and nitrogen deposition. However, the spatial distribution and seasonal variations of ammonia across China are unclear. The observation gaps motivate us to design a study to investigate the atmospheric ammonia on a national scale. Based on the year round observation at 50 sites in China, we confirm that the abundant concentrations of ammonia spotted in typical agricultural regions of China, especially North China Plain. On urban scale, however, the relative contributions of individual ammonia sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric ammonia in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of ¹⁵N (expressed using δ¹⁵N values) of NH₄⁺ in fine particles varies with the development of haze episodes, ranging from −37.1‰ to −21.7‰ during clean/dusty days (relative humidity: ~40%), to −13.1‰ to +5.8‰ during hazy days (relative humidity: 70−90%). After accounting for the isotope exchange between NH₃ gas and aerosol NH₄⁺, the δ¹⁵N value of the initial NH₃ during hazy days is found to be −14.5‰ to −1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90% of the total NH₃ during hazy days in urban Beijing. On an annual basis, 40−52% of the “initial NH₃ concentrations” in urban Beijing arise from fossil fuel emissions, which are episodically enhanced by stagnant weather preceding the passage of cold fronts. This work demonstrates the analysis of δ¹⁵N values of aerosol NH₄⁺ to be a promising new tool for partitioning atmospheric NH₃ sources. These results provide strong evidence for the contribution of non-agricultural sources to NH₃ in urban regions and suggest the prioritization of controlling these emissions for haze regulation.