UC Irvine

The oceans are important in the geochemical cycle of methyl bromide, as both a source of natural methyl bromide and a sink for anthropogenic methyl bromide. Air-sea exchange rate calculations are based on measured concentration differences across the air-sea surface, on various gas exchange wind speed relationships, and on the diffusivity of methyl bromide in seawater. In this study, the diffusivity of methyl bromide in pure water has been experimentally determined over the temperature range 5-20°C. The diffusivity varied from 9.85 x 10-6 cm2 s-1 at 5°C to 1.50 x 10-5 cm2 s-1 at 19.4°C. The values obtained in this study are ~ 8-35% higher than those derived from semi-empirical estimates. The diffusivity of methyl bromide in 3.5% NaCl solution was also measured at 13°C and found to be the same as the diffusivity measured in pure water. This is a surprising result given the viscosity differences between these two media. Schmidt numbers (Sc) for seawater have been calculated as a function of temperature from the pure water diffusivities. Schmidt numbers varied from 1585 at 5°C to 700 at 20°C.