Seasonal and spatial variations in methyl chloride in NW Atlantic waters

Abstract

[1] Methyl chloride concentrations were measured in the upper 200 m of the water column of the NW Atlantic during three cruises along the same track in spring, summer and fall of 2003. Distinct seasonality was apparent, with the surface waters being either undersaturated or close to equilibrium with the atmosphere in spring, but with supersaturation of the warmer waters in summer and fall. Cooler waters at the more northerly stations were always undersaturated, thus representing a continual sink for atmospheric methyl chloride. Even on an annual basis, the concentration anomaly ( the difference between measured concentration and equilibrium with the atmosphere) was strongly dependent on sea surface temperature (SST). This empirical relationship can be used to extrapolate fluxes globally or to estimate the influence of global warming on ocean-to-atmosphere fluxes of methyl chloride. The global flux of methyl chloride to the atmosphere estimate based on the full-year relationship between concentration anomaly and SST is 17 Gmol/y, and is reduced to 6 Gmol/y if separate seasonal relationships are used. It appears that the ocean component of the flux is highly sensitive to temperature, but the actual source of the methyl chloride in ocean waters remains largely unknown.