The air-sea flux of ammonia
As the predominant naturally occurring base in the atmosphere, ammonia is important in particle formation associated with its neutralisation reactions. Its protonated form, ammonium, is biogeochemically important as an atmospheric redistribution vector for nitrogen. Ammonia is emitted to the atmosphere by the terrestrial biota, through human activity and potentially by the marine biota through air-sea flux. Thus it is important to be able to accurately quantify the flux of ammonia across the air-sea interface. During four research cruises in high latitude (>55° N) seas, seawater and atmospheric measurements were made to calculate the air-sea ammonia flux. All fluxes were found to be from air to sea, contradicting previous studies, which found that the ocean was generally a source of ammonia. The reason for the downward fluxes observed in this study is the strong temperature dependence of the Henry’s law coefficient for ammonia. Various approaches are taken to investigate the distributions and cycling of ammonia and ammonium in the atmosphere and ocean, and the process of air-sea flux. Evidence of significant accumulation of ammonium at the air-sea interface during a phytoplankton bloom in the NE Atlantic is presented, suggesting that biological activity in the ocean can potentially drive emission events in regions where the flux of ammonia is otherwise from air to sea. Using measured and estimated data a series of budgets of net global air-sea ammonia flux are constructed. These budgets highlight the shelf and coastal seas as being of potential importance in the net flux. They also suggest that, with respect to ammonia, the ocean and atmosphere globally are much closer to equilibrium in the present day than they were in pre-industrial times, when the ocean was probably a net source; or than they will be in the future, when the ocean is likely to be a sink for ammonia.
PhD Thesis, University of East Anglia, Norwich
http://researchpages.net/media/resources/2007/06/13/thesis.pdf