METHYL-BROMIDE - OCEAN SOURCES, OCEAN SINKS, AND CLIMATE SENSITIVITY

The oceans play an important role in the geochemical cycle of methyl b romide (CH3Br), the major carrier of O-3-destroying bromine to the str atosphere. The quantity of CH3Br produced annually in seawater is comp arable to the amount entering the atmosphere each year from natural an d anthropogenic sources, The production mechanism is unknown but may b e biological. Most of this CH3Br is consumed in situ by hydrolysis or reaction with chloride. The size of the fraction which escapes to the atmosphere is poorly constrained; measurements in seawater and the atm osphere have been used to justify both a large oceanic CH3Br flux to t he atmosphere and a small net ocean sink, Since the consumption reacti ons are extremely temperature-sensitive, small temperature variations have large effects on the CH3Br concentration in seawater, and therefo re on the exchange between the atmosphere and the ocean, The net CH3Br flux is also sensitive to variations in the rate of CH3Br production. We have quantified these effects using a simple steady state mass bal ance model, When CH3Br production rates are linearly scaled with seawa ter chlorophyll content, this model reproduces the latitudinal variati ons in marine CH3Br concentrations observed in the east Pacific Ocean by Singh et al. [1983] and by Lobert et al. [1995], The apparent corre lation of CH3Br production with primary production explains the discre pancies between the two observational studies, strengthening recent su ggestions that the open ocean is a small net sink for atmospheric CH3B r, rather than a large net source. The Southern Ocean is implicated as a possible large net source of CH3Br to the atmosphere. Since our mod el indicates that both the direction and magnitude of CH3Br exchange b etween the atmosphere and ocean are extremely sensitive to temperature and marine productivity, and since the rate of CH3Br production in th e oceans is comparable to the rate at which this compound is introduce d to the atmosphere, even small perturbations to temperature or produc tivity can modify atmospheric CH3Br. Therefore atmospheric CH3Br shoul d be sensitive to climate conditions. Our modeling indicates that clim ate-induced CH3Br variations can be larger than those resulting from s mall (+/- 25%) changes in the anthropogenic source, assuming that this source comprises less than half of all inputs. Future measurements of marine CH3Br, temperature, and primary production should be combined with such models to determine the relationship between marine biologic al activity and CH3Br production. Better understanding of the biologic al term is especially important to assess the importance of non anthro pogenic sources to stratospheric ozone loss and the sensitivity of the se sources to global climate change.