Nitrogen Stable Isotopes Track Ammonia Emission Sources and Ammonium Aerosol Formation Processes in the Marine Boundary Layer

Abstract We measured the nitrogen isotopic composition of surface seawater and aerosol ammonium in the Weddell Sea region of the Southern Ocean to investigate the natural atmospheric cycle of marine ammonia emissions and aerosol ammonium formation in a region not overwhelmed by anthropogenic emissions. Seawater ammonium δ 15 N covered a narrow range (−6.9 to 2.4‰), in contrast to aerosol ammonium δ 15 N (−24.6 to 23.5‰). Aerosol values clustered in two distinct groups, one with low δ 15 N (−14.6 ± 7.1‰) and one with high δ 15 N (18.8 ± 6.7‰). The low δ 15 N aerosols are best explained by the surface ocean as the only source. By contrast, the higher δ 15 N aerosols are likely due to seabird emissions. This work demonstrates the potential of nitrogen isotopes to constrain the contribution of the ocean to natural ammonia emissions and to improve our understanding of aerosol ammonium formation away from anthropogenic sources.