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

Abstract

Abstract. Concentrations of ammonia / ammonium (NH3/NH4 +) and its methyl-derivatives the methylamines (MAs) were determined in seawater, atmospheric, and rainwater samples during the U.K. Joint Global Ocean Flux Study ARABESQUE program in the Arabian Sea (AugustDecember 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 oligotrophic regimes. Calculations demonstrated that both coastal and offshore 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 base (sub-micron). Although photochemical destruction was calculated to be more significant for MAs than for NH3, removal of all species from the atmosphere was dominated by wet deposition. Atmospheric deposition was estimated to fuel less than 1% of the nitrogen requirements of "new" production in oligotrophic regions of the Arabian Sea. With aerosol scavenging calculated to contribute under 0.5% of rainwater NH4 + and MAs, the gas phase was inferred as the dominant source of rainwater MAs and NH4 +. However, equilibria calculations demonstrated rainwater to be undersaturated with respect to gaseous MA and NH3 concentrations, and that paradoxically it was not possible for Henry's law to hold for all analytes simultaneously.

Coupling between dimethylsulfide emissions and the ocean - atmosphere exchange of ammonia

“…Oxidation of NH 3(g) by OH radical is rather slow, and considered insignificant relative to other removal processes (wet and dry deposition, mostly from the particulate phase). [17,22] There is generally a disequilibrium observed between the atmosphere and ocean with respect to ammonia, [22,24,27,29,30] supporting the suggestion that air-sea equilibration is a relatively slow process in the system. Therefore, the major control on the concentration of NH 3(g) in the remote MBL (neglecting advection from other sources) must be the equilibrium between gas and particle phases; i.e.…”

Ocean–Atmosphere Interactions of Particles

“…The presence of monomethylammonium (MMA + ), dimethylammonium (DMA + ) and trimethylammonium (TMA + ) salts in marine aerosol particles was reported for the first time by Gibb et al (1999). Their presence was attributed to secondary production, suggesting the condensation of volatile alkyl amines, degassed from the sea, through acid-base reactions, in analogy with NH 4 + .…”

Size-resolved sulfate and ammonium measurements in marine boundary layer over the North and South Pacific

“…Since the concentration of the atmospheric total ammonia (aerosol NH 4 + + gaseous NH 3 ) was much lower in the oceanic air over the Pacific than in the air over the land, the land is the major contributor to the total ammonia in the oceanic air (Tsunogai and Ikeuchi, 1968;Tsunogai, 1971). Later works (Quinn et al, 1987;Quinn et al, 1990;Zhuang and Huebert, 1996;Lee et al, 1998;Gibb et al, 1999;Sorensen et al, 2003) estimated the air-sea flux of ammonia by comparing their concentrations in the air and ocean. These studies concluded that ocean surfaces are potential sources of ammonia in marine air.…”