An improved estimate of the oceanic lifetime of atmospheric CH3Br

Previous estimates of the partial atmospheric lifetime of CH3Br with respect to degradation in the ocean have not fully accounted for co-variation of sea-surface and boundary layer properties. Here we substantially reduce uncertainty in this calculation by using a coupled, ocean-atmosphere box model and a tightly gridded data set of oceanic and atmospheric properties. The best estimate of the partial atmospheric lifetime of CH3Br with respect to the ocean is 2.7 y with a possible range, due mainly to the choice of computational procedures for critical terms, of 2.4 to 6.5 y. This range is about one-third of that estimated previously. The total atmospheric lifetime, based upon oceanic, atmospheric, and proposed soil losses with all of their uncertainties, is 0.8 (0.6 to 1.4) y. Only 28% of this total uncertainty is attributable to the uncertainty in oceanic loss.