Source Apportionment of Atmospheric Ammonia at 16 Sites in China Using a Bayesian Isotope Mixing Model Based on δ 15 N-NH x Signatures.

Reducing atmospheric ammonia (NH 3 ) emissions is critical to mitigating poor air quality. However, the contributions of major agricultural and non-agricultural source emissions to NH 3 at receptor sites remain uncertain in many regions, hindering the assessment and implementation of effective NH 3 reduction strategies. This study conducted simultaneous measurements of the monthly concentrations and stable nitrogen isotopes of NH x (gaseous NH 3 plus particulate NH 4 + ) at 16 sites across China. Ambient NH x 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 δ 15 N values of the initial NH 3 , source apportionment results indicated that non-agricultural sources (traffic and waste) and agricultural sources (fertilizer and livestock) contributed 54 and 46% to NH 3 at urban sites, 51 and 49% at rural sites, and 61 and 39% at background sites, respectively. Non-agricultural sources contributed more to NH 3 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 NH 3 across China, even in rural regions.