The Role of the Ocean in the Atmospheric Budgets of Methyl Bromide, Methyl Chloride and Methane

Abstract

The ocean is both a source and a sink for atmospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl). It plays a significant role in their global biogeochemical cycling. In response to the Montreal Protocol, the atmospheric CH3Br is declining and the saturation state of CH3Br in the surface ocean is becoming more positive. Results from two large-scale transect studies in the eastern Pacific and the eastern Atlantic suggest that the ocean became near equilibrium with atmospheric CH3Br in 2010. Results from a "top-down" two-box model indicate that, if the remaining anthropogenic emissions are eliminated, atmospheric CH3Br is likely to drop to the pre-industrial level and the ocean would become a net source to atmospheric CH3Br.

This study also represents an effort to improve current understanding of the oceanic and atmospheric budgets of CH3Cl. The global net sea-to-air flux of CH3Cl was estimated at 335 (210 ? 480) Gg yr-1 with improved parameterizations on the solubility, seasonal saturation anomaly ? (sea surface temperature, wind speed) relationships and the use of an updated parameterization on gas transfer velocity. For the first time, we estimated the gross oceanic emission and gross oceanic uptake rates of CH3Cl in the surface ocean, which was 700 (490 to 920) Gg yr^-1 and -370 (-440 to -280) Gg yr^-1, respectively. The ocean accounts for 10 - 19 % in the global CH3Cl emission and 6 - 9 % in its global sinks.

Methane (CH4) is a potent greenhouse gas, which has a warming potential 72 times that of carbon dioxide over a 20 year time horizon. Gas hydrates are the largest CH4 reservoir in the planet. How much CH4 is transported from marine gas hydrates to the atmosphere is under debate. In this study, we investigated CH4 fluxes over three deepwater hydrocarbon seeps in the northern Gulf of Mexico using continuous air-sea flux measurements. Extrapolating the highest daily flux from this study to other deepwater seeps in the northern Gulf of Mexico suggests that CH4 fluxes to the atmosphere from the deepwater hydrocarbon seeps in this region are an insignificant source to atmospheric CH4 budget.