Significant contributions of combustion-related sources to ammonia emissions.

Atmospheric ammonia (NH 3 ) and ammonium (NH 4 + ) can substantially influence air quality, ecosystems, and climate. NH 3 volatilization from fertilizers and wastes (v-NH 3 ) has long been assumed to be the primary NH 3 source, but the contribution of combustion-related NH 3 (c-NH 3 , mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH 3 and NH 4 + and established a robust method to differentiate v-NH 3 and c-NH 3 . We found that the relative contribution of the c-NH 3 in the total NH 3 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-NH 3 emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH 3 loading.