Change in nitrogen (N) cycle is one of the causes of surface-earth eco-environmental and climatic changes in the Anthropocene, which is related to the quality of water-soil-air environments, food production, ecosystem structure and function, and climatic feedback. Synthetic ammonia (NH3) is the main material for chemical N fertilizers and NH3 fuel is also a key material to achieving "carbon neutrality" in the field of energy and power. However, population growth and economic development since the industrial revolution have led to continuous increases in the concentration, pool, and deposition flux of atmospheric NH3 and ammonium (NH4+). A better understanding of the main terrestrial sources of atmospheric NH3 is critical for making emission mitigation strategies and assessing the effects of anthropogenic NH3 pollution.
Due to its high solubility and chemical reactivity, NH3 has a short atmospheric lifetime, but the atmospheric transport of NH4+ is obvious and extensive, resulting in complex and highly mixed sources of atmospheric NHx (the sum of NH3 and NH4+) at specific monitoring sites and underlying surfaces. The NH3 volatilization from fertilizers and wastes (denoted as v-NH3) has long been assumed to be the primary NH3 source at regional and global scales. In recent years, evidence from laboratory simulations, in-situ observations, satellite observations, and more accurate emission inventories points to that fossil-fuel combustion sources dominated by coal combustion (industrial coal combustion in the urban and surrounding areas, and domestic coal combustion in non-urban areas) and oil combustion (urban traffic as a regional hotspot, and vehicles are also widely distributed in non-urban areas), as well as biomass burning dominated by crop straw and wildfires, emit a large amount of NH3. However, it is difficult to accurately constrain the emission factors and intensity of the widely-existing combustion-related NH3 (c-NH3), leading to uncertainty in the fractional contribution and amount of c-NH3 to regional NH3 emissions.
Since the 1950s, stable N isotope (δ15N) has gradually been recognized as an effective tool for tracing or differentiating NH3 emission sources. Until now, site-based δ15N observations of atmospheric NHx have been conducted widely in high N-pollution regions of East Asia, North America, and Europe (Fig. 1). However, it remains a challenge to constrain the post-emission δ15N changes of NH3 due to transformations in the atmosphere, which prevents a quantitative assessment of source contributions to NH3 emissions.
Based on the above background, we constrained the N isotope effect between atmospheric NH3, particulate NH4+ (p-NH4+), or precipitation NH4+ (w-NH4+) and the initial NH3 mixture (i-NH3). Then, we reconstructed δ15N of i-NH3 (δ15Ni-NH3) for the isotopic observation sites in East Asia, North America, and Europe. We found that the δ15Ni-NH3 was higher in North America than in Europe and East Asia and generally decreased with time in the three study regions. These results indicate the relative proportion of c-NH3 in North America is higher than that in the other two regions, but the relative proportions of v-NH3 have been increasing during the past decades. Finally, in combination with the δ15N of major v-NH3 and c-NH3 emission sources and amounts of regional v-NH3 emissions, we estimated the relative contributions and amounts of major c-NH3 emissions in the three regions using δ15N mass-balance methods, showing 40±21% and 6.6±3.4 Tg N yr-1 in East Asia, 49±16% and 2.8±0.9 Tg N yr-1 in North America, and 44±19% and 2.8±1.3 Tg N yr-1 in Europe (Fig. 1). All these results reveal that c-NH3 is an important source of NH3 emissions and provide new data for regional c-NH3 and total NH3 emissions (Fig. 1). This work provides new scientific evidence for making current and future mitigation strategies of NH3 emissions, evaluating NHx deposition fluxes and effects.