An Isotopic Approach for Determining the Industrial Fraction of the Total Methyl Bromide Source to the Atmosphere

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9729110 Goldstein Methyl bromide (CH3Br) is a trace gas that constitutes the largest source of bromine atoms to the stratosphere. While the role of bromine in stratospheric ozone loss has been well documented, the current understanding of methyl bromide sources suffers from major uncertainties. Unlike chlorofluorocarbons, methyl bromide is released by both industrial and non-industrial processes. The industrial source is dominated by agricultural fumigation while the largest non-industrial sources appear to be biological production in the oceans and inorganic production during biomass burning. The magnitude of the industrial source is known to within 50%, but the magnitudes of the non-industrial sources are highly uncertain. The industrial source fraction of methyl bromide is thus highly uncertain. A mass balance method based on carbon isotopes for determining the industrial source fraction will be developed in this project. Preliminary work has shown the isotopic signature of industrially produced methyl bromide to be very different from that expected for non-industrial sources of methyl bromide, which are likely to reflect the signature of their carbon source. Isotopic fractionations associated with the main sink mechanisms (OH oxidation, ocean degradation, and soil degradation) are likely to be small, because of the high molecular mass of methyl bromide. If these predictions are verified, it will be possible to exploit the large difference in source signatures to determine the industrial fraction of the total methyl bromide source to approximately 10%. New sampling protocols will be developed to preconcentrate methyl bromide, which occurs at very low (10 pptv) concentrations in the atmosphere. Gas Chromatography - Isotope Ratio Mass Spectrometry (GC-IRMS) will be used to measure the carbon isotope ratio of atmospheric methyl bromide. In the course of this project, the isotopic fractionation associated with the major sources (oceans, biomass burning, industrial) and sinks (OH oxi dation, ocean, and soil degradation) will be determined in collaboration with other research groups. If the assumptions on source and sink signatures can be verified, measurements of ambient air samples can be undertaken. If successful, these measurements will provide powerful new constraints on the methyl bromide budget, which are independent of the methods used to date.