An isotopic approach for understanding the CH3Br budget of the atmosphere

The atmospheric budget of methyl bromide (CH3Br), 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 CH3Br 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 CH3Br 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 CH3Br from the three largest manufacturers worldwide yield a weighted average of -54.4 parts per thousand relative to the Peedee Belemnite standard. This result suggests that industrial CH3Br is isotopically distinct and that the carbon isotopic composition of atmospheric CH3Br may indicate what fraction of atmospheric CH3Br is anthropogenic.