Significant contributions of combustion-related sources to ammonia emissions

Atmospheric ammonia (NH₃) and ammonium (NH₄⁺) can substantially influence air quality, ecosystems, and climate. NH₃ volatilization from fertilizers and wastes (v-NH₃) has long been assumed to be the primary NH₃ source, but the contribution of combustion-related NH₃ (c-NH₃, mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH₃ and NH₄⁺ and established a robust method to differentiate v-NH₃ and c-NH₃. We found that the relative contribution of the c-NH₃ in the total NH₃ emissions reached up to 40 ± 21% (6.6 ± 3.4 Tg N yr^-1), 49 ± 16% (2.8 ± 0.9 Tg N yr^-1), and 44 ± 19% (2.8 ± 1.3 Tg N yr^-1) in East Asia, North America, and Europe, respectively, though its fractions and amounts in these regions generally decreased over the past decades. Given its importance, c-NH₃ emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH₃ loading.