Coastal emissions of methyl bromide and methyl chloride along the eastern Gulf of Mexico and the east coast of the United States

[1] Measurements of methyl bromide (CH3Br) and methyl chloride (CH3Cl) were made during the Gulf of Mexico and the East Coast Carbon (GOMECC) cruise in the summer of 2007. This was the first large-scale study of CH3Br and CH3Cl in the coastal ocean. The mean atmospheric mixing ratios were 10.1 ppt for CH3Br and 517 ppt for CH3Cl. Surface seawater concentrations ranged from 0.8 pM to 5.0 pM for CH3Br and from 61.5 pM to 179 pM for CH3Cl. Biological processes contribute to the elevated seawater concentrations of CH3Br, while a combination of biological processes and terrestrial sources may contribute to elevated seawater concentrations of CH3Cl. Vertical distributions show highest concentrations of CH3Br and CH3Cl in the subsurface seawater below the mixed layer due to high degradation rates near the surface. Good correlation of the production rates between CH3Br and CH3Cl suggests they have some common sources in the coastal ocean. After excluding the open ocean points, annual coastal emissions in the studied region were 0.01–0.06 Gg yr−1 for CH3Br and 0.3–1.6 Gg yr−1 for CH3Cl. By simple extrapolation, the global coastal emissions were estimated at 0.5–3.6 Gg yr−1 for CH3Br and 19–98 Gg yr−1 for CH3Cl. Including the coastal ocean emissions of CH3Br and CH3Cl will increase the estimate of global oceanic emissions by 1%–9% and 1%–8%, respectively.