The reduction of ammonia (NH₃) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH₄⁺). However, the relative contributions of individual NH₃ sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH₃ 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. This work demonstrates the analysis of δ¹⁵N values of aerosol NH₄⁺ to be a promising new tool for partitioning atmospheric NH₃ sources, providing policy makers with insights into NH₃ emissions and secondary aerosols for regulation in urban environments.

Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ¹⁵N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ¹⁵N declined by 0.6–1.6‰. Examining patterns across different climate spaces, foliar δ¹⁵N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ¹⁵N of tree rings and leaves from herbarium samples over the past 75–150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.