Coastal Emissions of Methyl Bromide and Methyl Chloride Along the Eastern Gulf of Mexico and the East Coast of the U.S.

Methyl bromide and methyl chloride are the most abundant brominated and chlorinated gases in troposphere. They are the major contributors of natural bromine and chlorine to the stratosphere, destroying ozone catalytically. However, large uncertainties in their budgets remain. The ocean, a natural source and sink for these gases, is an important regulator on their atmospheric burden. Most of the previous investigations focused on the open ocean. There has been limited investigation of coastal regions, which are potentially highly productive regions for CH3Br and CH3Cl. In this study, we measured CH3Br and CH3Cl in seawater and air samples during the Gulf of Mexico and East Carbon (GOMECC) Cruise from 10 July to 4 August 2007. The mean observed atmospheric mixing ratios are 10.9 ppt for CH3Br and 526.4 ppt for CH3Cl. Enhanced seawater concentrations were observed in regions with major freshwater input. Combined saturation anomaly measurements and depth profile measurements suggest the two gases share some common sources but also have independent producers. After excluding the open ocean points, total coastal emissions of 0.05 Gg/y for CH3Br and 1.19 Gg/y for CH3Cl were calculated for the study region. By simple extrapolation, the global coastal emission of CH3Br is estimated as 3.0 Gg/y, accounting for ~4.8% of the global ocean emission, while the global coastal emission of CH3Cl is estimated as 68.6 Gg/y, accounting for ~13.5% of the global ocean emission.