Ocean-atmosphere exchange of methyl chloride: Results from NW Atlantic and Pacific Ocean studies

Measurements of methyl chloride concentrations and saturations in the NW Atlantic and Pacific Ocean from 40 degrees N to 40 degrees S are presented. Labrador Sea waters, analyzed in July, were at or below saturation, while warmer waters south of the Gulf Stream and all Pacific samples were consistently supersaturated. We find no correlation between methyl chloride concentrations and chlorophyll a, but much of the variation in methyl chloride partial pressure appears to be accounted for by variations in sea surface temperature. We estimate ocean-atmosphere fluxes of methyl chloride, using exchange velocities for spot wind speeds and climatological wind speeds. These indicate that none of the waters we sampled support the magnitude of fluxes which have hitherto been considered to be emitted by the ocean. A simple extrapolation yields a global efflux of 0.8 to 1.1 x 10(10) mol yr(-1) (0.4 to 0.6 Tg yr(-1)) supplied by warm waters and a high-latitude influx of 0.2 to 0.5 x 10(10) mol yr(-1) (0.1 to 0.3 Tg yr(-1)). The net global ocean source to the atmosphere is 0.3 to 0.9 x 10(10) mol yr(-1) (0.2-0.4 Tg yr(-1)), which accounts for 5 to 12% of the total required to balance loss by reaction with hydroxyl radicals. Unless its atmospheric reactivity has been overestimated, we conclude that other sources of methyl chloride make major contributions to its atmospheric budget.