Oceanic emissions of methyl halides and effect of nutrients concentration on their production: A case of the western Pacific Ocean (2°N to 24°N).

Methyl halides are important greenhouse gases responsible for the majority of the ozone layer depletion. This study investigated atmospheric and seawater methyl halides (CHCl, CHBr, and CHI) in the western Pacific Ocean between 2°N and 24°N. Increases in methyl halides in the atmosphere were likely to have originated from Southeast Asian regions. Elevated CHI concentrations in seawater were mainly produced photochemically from dissolved organic carbon. Maximum methyl halide and chlorophyll a levels in the upper water column (0-200 m) were linked to biological activity and downwelling or upwelling caused by cold and warm eddies. Ship-based incubation experiments showed that nutrient supplementation promoted methyl halide emissions. The elevated methyl halide production was associated with increases in phytoplankton such as diatoms. The mean fluxes of CHCl, CHBr, and CHI in study area of during the cruise were 82.91, 4.70, and 3.50 nmol m d, respectively. The estimated emissions of CHCl, CHBr, and CHI in the western Pacific Ocean accounted for 0.67%, 0.79% and 0.09% of global oceanic emissions, respectively, indicating that the open sea contribute insignificantly to the global oceanic emissions of these gases.