Department of Earth System Science

The degradation rate constant of CH₃Br in the North Pacific Ocean was measured in surface seawater between September and October 1999, using a stable isotope (¹³CH₃Br) incubation technique. Total degradation rate constants ranged from 0.02 to 0.43 d⁻¹, decreasing in colder waters as a result of the temperature-dependence of chemical losses. Biological rate constants ranged from 0.01 to 0.20 d⁻¹. In subtropical waters (13-20°N), biological loss rate constants were small compared to chemical loss rate constants. North of Hawaii, biological processes played an increasingly significant role in CH₃Br degradation. In subpolar waters (40-58°N), biological losses dominated the removal of methyl bromide. Comparison of the measured loss rate constants with surface water CH₃Br concentrations suggest that the CH₃Br production rate is higher in warm, low latitude waters than in cold subpolar waters at this time of year. Diel studies revealed a midday maximum in biological degradation of methyl bromide.