Characterizing Remote Marine Ammonia Emissions: An Isotopic Approach in the Southern Ocean

Understanding the biogeochemical and climate impacts of anthropogenic ammonia emissions requires the quantification of natural emissions. The ocean is the largest natural source of ammonia, however, natural emissions are difficult to detect due to the dominance of anthropogenic emissions, even over most ocean basins. Nitrogen stable isotopes are a potential tracer for natural marine ammonia sources, as suggested by previous work on the isotopic composition of ammonium aerosols and rain in the Atlantic Ocean. However, the Atlantic Ocean is a region influenced by both marine and anthropogenic sources, complicating the identification of a pure marine ammonia source. The remote nature of the Southern Ocean renders it one of the only regions where marine ammonia emissions are expected to dominate over anthropogenic sources. In this work, we investigate ammonium aerosols in the remote atmosphere of the Southern Ocean to identify the concentration and isotopic signature of marine ammonia. Bi-weekly aerosol samples are collected as part of an ongoing field campaign at the Cape Point Global Atmosphere Watch station located on the extreme southwestern point of the African continent. Aerosols collected under clean marine conditions are identified using a combination of ²²²Rn concentrations and HYSPLIT air mass back trajectory analysis. Aerosols that originate from the open Southern Ocean have ammonium concentrations that range from 1.8 to 3.1 nmol m^-3. In addition, a steady state isotope box model for the surface ocean-lower atmosphere ammonia/um system was modified and incorporated into the global ocean Biogeochemical Flux Model. Initial results from a one year model run conducted for the Southern Ocean show that in addition to the expected daily and seasonal variation in concentrations, ammonium aerosol ¹⁵N/¹⁴N varies with a dynamic range of 7‰. We find that the ¹⁵N/¹⁴N of ocean-derived ammonia can be highly variable depending on wind speed, surface ocean ammonium ¹⁵N/¹⁴N, temperature, and pH. Additional work is currently ongoing to explore the applicability of stable isotopes as a tracer of marine vs. anthropogenic ammonia emissions.