AN IMPROVED ESTIMATE OF THE OCEANIC LIFETIME OF ATMOSPHERIC CH3BR

Previous estimates of the partial atmospheric lifetime of CH3Br with r espect to degradation in the ocean have not fully accounted for co-var iation of sea-surface and boundary layer properties. Here we substanti ally reduce uncertainty in this calculation by using a coupled, ocean- atmosphere box model and a tightly gridded data set of oceanic and atm ospheric properties. The best estimate of the partial atmospheric life time 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 te rms, of 2.4 to 6.5 y. This range is about one-third of that estimated previously. The total atmospheric lifetime, based upon oceanic, atmosp heric, 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 t o the uncertainty in oceanic loss.