Steady-state box modelling of ammonia/um across the air-sea interface during winter in the Atlantic sector of the Southern Ocean

Abstract

Ammonia gas emissions have increased substantially since the preindustrial era due to agricultural activities and fossil fuel burning. The subsequent increases in deposition, even in remote regions, have led to multiple environmental consequences known as the ‘nitrogen cascade’. Investigating the anthropogenic effect on the nitrogen cycle requires quantification of natural sources. However, marine ammonia emissions, the largest natural source of ammonia, are difficult to quantify due to the dominance of continental sources and inaccessibility of remote regions. Previous work in coastal regions suggests that the marine ammonia source may have a unique isotopic signature as compared to anthropogenic/continental sources, but this has never been tested in a truly remote marine region. Here, we use a steady-state isotope box model and surface ocean measurements to investigate ammonia/um (NHx. cycling in the remote atmosphere of the Southern Ocean in order to characterize the isotopic composition of marine ammonia emissions. The model calculates the concentrations and isotopic compositions of NHx species in the ocean and atmosphere (seawater NHx, ammonia gas, and ammonium aerosols., and was initialised using measurements from a cruise to the marginal ice zone south of Africa during austral winter of 2017 onboard the R/V SA Agulhas II. Atmospheric NHx concentrations and isotopic compositions varied between the Sub-Antarctic, Polar Frontal, and Antarctic Zones. These differences were driven by seawater NHx concentrations and isotopic compositions, sea surface temperature, and wind speed. The correlation between the isotopic composition of seawater and atmospheric NHx demonstrates the predominance of the marine ammonia source in remote marine regions and suggests that isotopes can be used as a tracer for the marine ammonia source. Future work, including additional measurements of NHx and dimethyl sulphide and model validation, will improve our understanding of the surface ocean-lower atmosphere nitrogen cycle and its impact on marine aerosols and climate. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.