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

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 CH₃CI. 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 CH₃C₁. Including the coastal ocean emissions of CH₃Br and CH₃C₁ will increase the estimate of global oceanic emissions by 1%-9% and 1%-8%, respectively. [ABSTRACT FROM AUTHOR]