AN INVESTIGATION OF THE ATMOSPHERIC SOURCES AND SINKS OF METHYL-BROMIDE

Methyl Bromide (CH3Br) is a ubiquitous component of the atmosphere and has been implicated as an important player in the depletion of stratospheric ozone. Atmospheric CH3Br abundances, interhemispheric gradients, oceanic concentrations, man-made emissions, and removal processes have been analyzed and interpreted with the help of a simple box model and a 2-D global photochemical model. Its calculated atmospheric lifetime (T) of 1.7-1.9 years, based on reaction with OH radicals, is consistent with a global source of 90-110 Gg (10(9)g)/year. Consequences of a much shorter lifetime of 1.2 years, due to possible deposition/hydrolysis losses, are also considered. Available data are used to estimate a CH3Br source that is 35% (20-50%) man-made and 65% (80-50%) natural. Oceans are substantially supersaturated and provide the most important natural source of almost-equal-to 60 (40-80) Gg/year. Within the oceans 200-300 Gg/year of CH3Br may be produced. Indirect emissions from automobile exhaust and biomass burning can not be well quantified (1-10 Gg/year). A global trend of 0.1-0.2 ppt/year is predicted. Model results show significant vertical and seasonal variations in the atmospheric abundances and interhemispheric gradients of CH3Br. Substantial uncertainties in calibrations, source estimates, and deposition processes are present.