Correlations and emission ratios among bromoform, dibromochloromethane, and dibromomethane in the atmosphere

Bromoform (CHBr3), dibromochloromethane (CHBr2Cl), and dibromomethane (CH2Br2) in the atmosphere were measured at various sites, including tropical islands, the Arctic, and the open Pacific Ocean. Up to 40 ppt of bromoform was observed along the coasts of tropical islands under a sea breeze. Polybromomethane concentrations were highly correlated among the coastal samples, and the ratios CH2Br2/CHBr3 and CHBr2Cl/CHBr3 showed a clear tendency to decrease with increasing CHBr3 concentration. These findings are consistent with the observations that polybromomethanes are emitted mostly from macroalgae whose growth is highly localized to coastal areas and that CHBr3 has the shortest lifetime among these three compounds. The relationship between the concentration ratios CHBr3/CH2Br2 and CHBr2Cl/CH2Br2 suggested a large mixing/dilution effect on bromomethane ratios in coastal regions and yielded a rough estimate of 9 for the molar emission ratio of CHBr3/CH2Br2 and of 0.7 for that of CHBr2Cl/CH2Br2. Using these ratios and an global emission estimate for CH2Br2 (61 Gg/yr (Br)) calculated from its background concentration, the global emission rates of CHBr3 and CHBr2Cl were calculated to be approximately 820(+/- 310) Gg/yr (Br) and 43(+/- 16) Gg/yr (Br), respectively, assuming that the bromomethanes ratios measured in this study are global representative. The estimated CHBr3 emission is consistent with that estimated in a very recent study by integrating the sea-to-air flux database. Thus the contribution of CHBr3 and CHBr2Cl to inorganic Br in the atmosphere is likely to be more important than previously thought.