Source Apportionment of Atmospheric Ammonia at 16 Sites in China Using a Bayesian Isotope Mixing Model Based on δ15N–NHx Signatures

Reducing atmospheric ammonia (NH3) emissions is critical to mitigating poor air quality. However, the contributions of major agricultural and non-agricultural source emissions to NH3 at receptor sites remain uncertain in many regions, hindering the assessment and implementation of effective NH3 reduction strategies. This study conducted simultaneous measurements of the monthly concentrations and stable nitrogen isotopes of NHx (gaseous NH3 plus particulate NH4+) at 16 sites across China. Ambient NHx concentrations averaged 21.7 ± 19.6 μg m–3 at rural sites, slightly higher than those at urban (19.2 ± 6.0 μg m–3) and three times of those at background (7.0 ± 6.9 μg m–3) sites. Based on revised δ15N values of the initial NH3, source apportionment results indicated that non-agricultural sources (traffic and waste) and agricultural sources (fertilizer and livestock) contributed 54 and 46% to NH3 at urban sites, 51 and 49% at rural sites, and 61 and 39% at background sites, respectively. Non-agricultural sources contributed more to NH3 at rural and background sites in cold than warm seasons, arising from traffic and waste, but were similar across seasons at urban sites. We concluded that non-agricultural sources need to be addressed when reducing ambient NH3 across China, even in rural regions.