Ocean-atmosphere exchange and atmospheric speciation of ammonia and methylamines in the region of the NW Arabian Sea

Concentrations of ammonia / ammonium (NH3/NH4+) and its methyl-derivatives the methylamines (MAs) were determined in seawater, atmospheric, and rainwa ter samples during the U.K. Joint Global Ocean Flux Study ARABESQUE program in the Arabian Sea (August December 1994). In seawater, concentrations of NH3/NH4+ were shown to be 10-100 times greater than those of MAs, of which monomethylamine was the most abundant. Concentrations of all analytes were highest in the productive coastal waters off Oman, and lowest in offshore o ligotrophic regimes. Calculations demonstrated that both coastal and offsho re waters vented NH3 to atmosphere, and that while coastal waters could act as sources or sinks of atmospheric MAs, offshore waters were a consistent sink. NH3/NH4+ was the dominant atmospheric base, with MAs contributing 16- 20% of total measured gaseous base and 5.9-12% of total measured aerosol ba se (sub-micron). Although photochemical destruction was calculated to be mo re significant for MAs than for NH3, removal of all species from the atmosp here was dominated by wet deposition. Atmospheric deposition was estimated to fuel less than 1% of the nitrogen requirements of "new" production in ol igotrophic regions of the Arabian Sea. With aerosol scavenging calculated t o contribute under 0.5% of rainwater NH4+ and MAs, the gas phase was inferr ed as the dominant source of rainwater MAs and NH4+. However, equilibria ca lculations demonstrated rainwater to be undersaturated with respect to gase ous MA and NH3 concentrations, and that paradoxically it was not possible f or Henry's law to hold for all analytes simultaneously.