Apportioning Atmospheric Ammonia Sources across Spatial and Seasonal Scales by Their Isotopic Fingerprint.

Mitigating ammonia (NH 3 ) emissions is a significant challenge, given its well-recognized role in the troposphere, contributing to secondary particle formation and impacting acid rain. The difficulty arises from the highly uncertain attribution of atmospheric NH 3 to specific emission sources, especially when accounting for diverse environments and varying spatial and temporal scales. In this study, we established a refined δ 15 N fingerprint for eight emission sources, including three previously overlooked sources of potential importance. We applied this approach in a year-long case study conducted in urban and rural sites located only 40 km apart in the Shandong Peninsula, North China Plain. Our findings highlight that although atmospheric NH 3 concentrations and seasonal trends exhibited similarities, their isotopic compositions revealed significant distinctions in the primary NH 3 sources. In rural areas, although agriculture emerged as the dominant emission source (64.2 ± 19.5%), a previously underestimated household stove source also played a considerably greater role, particularly during cold seasons (36.5 ± 12.5%). In urban areas, industry and traffic (33.5 ± 15.6%) and, surprisingly, sewage treatment (27.7 ± 11.3%) associated with high population density were identified as the major contributors. Given the relatively short lifetime of atmospheric NH 3 , our findings highlight the significance of the isotope approach in offering a more comprehensive understanding of localized and seasonal influences of NH 3 sources compared to emissions inventories. The refined isotopic fingerprint proves to be an effective tool in distinguishing source contributions across spatial and seasonal scales, thereby providing valuable insights for the development of emission mitigation policies aimed at addressing the increasing NH 3 burden on the local atmosphere.