A GLOBAL 3-DIMENSIONAL ATMOSPHERE-OCEAN MODEL OF METHYL-BROMIDE DISTRIBUTIONS

A three-dimensional ocean-atmosphere model of the global methyl bromid e budget is presented including an analysis of the temporally and spat ially varying source and sink distributions. No correlation is found b etween oceanic net biological CH,Br production, implied by surface wat er saturation anomaly observations, and oceanic variables such as surf ace chlorophyll and primary production; therefore model ocean fluxes a re constrained directly by the saturation anomaly observations. The re sulting diagnosed biological production rates imply net production in the tropics and subtropics and net consumption at high latitudes. Resu lts from this semicoupled ocean-atmosphere model show substantial long itudinal variability in the atmospheric boundary layer CH,Br concentra tions with land-ocean contrasts of 1-6 ppt due to regional industrial and agricultural emissions on land and net fluxes into the ocean. Owin g to an imbalance in current understanding of the global budget, our s imulated mixing ratios of 3.5 to 6.5 ppt for the southern and northern hemispheres, respectively, are significantly lower than available mea surements. Sensitivity studies reducing the ocean and soil surface sin ks slightly improve the global mean CH,Br concentration but increase t he interhemispheric ratio further beyond that supported by observation s. Accordingly, an apparent terrestrial missing source of 89 - 104 kT yr(-1) is derived and applied to the model. This is of, the same order as the sum of all other sources in the model (85 kT yr(-1)). The hemi spheric distribution of the missing source is explored, indicating tha t 50 - 70% of this source occurs in the southern hemisphere and is lik ely to be biased toward tropical regions. Modeled seasonal variability in the interhemispheric ratio at specific monitoring sites agrees wel l with observations. The model-predicted vertical gradient of CH,Br th rough the troposphere and lower stratosphere is also presented.