An isotopic approach for understanding the CH(3)Br budget of the atmosphere.

The atmospheric budget of methyl bromide (CH(3)Br), an ozone-depleting gas, is highly uncertain, because it has complex sources and sinks. Although oceans, biomass burning, and industrial production are identified as the major sources, the fraction of CH(3)Br that is contributed by each source is not well known. A mass-balance approach that exploits differences in the carbon isotopic signature (delta(13)C) of CH(3)Br sources and sinks may provide a means of reducing uncertainties in the atmospheric budget. This approach depends on the distinctiveness of industrially produced methyl bromide. Our delta(13)C measurements of industrial CH(3)Br from the three largest manufacturers worldwide yield a weighted average of -54.4 per thousand relative to the Peedee Belemnite standard. This result suggests that industrial CH(3)Br is isotopically distinct and that the carbon isotopic composition of atmospheric CH(3)Br may indicate what fraction of atmospheric CH(3)Br is anthropogenic.