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

Abstract

Measurements of methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) 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 CH 3 Br and CH 3 Cl in the coastal ocean. The mean atmospheric mixing ratios were 10.1 ppt for CH 3 Br and 517 ppt for CH 3 Cl. Surface seawater concentrations ranged from 0.8 pM to 5.0 pM for CH 3 Br and from 61.5 pM to 179 pM for CH 3 Cl. Biological processes contribute to the elevated seawater concentrations of CH 3 Br, while a combination of biological processes and terrestrial sources may contribute to elevated seawater concentrations of CH 3 Cl. Vertical distributions show highest concentrations of CH 3 Br and CH 3 Cl in the subsurface seawater below the mixed layer due to high degradation rates near the surface. Good correlation of the production rates between CH 3 Br and CH 3 Cl 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 CH 3 Br and 0.3–1.6 Gg yr −1 for CH 3 Cl. By simple extrapolation, the global coastal emissions were estimated at 0.5–3.6 Gg yr −1 for CH 3 Br and 19–98 Gg yr −1 for CH 3 Cl. Including the coastal ocean emissions of CH 3 Br and CH 3 Cl will increase the estimate of global oceanic emissions by 1%–9% and 1%–8%, respectively.