Stable Carbon Isotopes of Background Atmospheric Methyl Bromide

Methyl bromide (CH3Br) constitutes the largest source of bromine atoms to the stratosphere and plays an important role in stratospheric ozone depletion. However the CH3Br atmospheric budget remains unbalanced. We utilize an isotopic mass balance to constrain the CH3Br atmospheric budget by analyzing the source signatures and the isotopic fractionations associated with sinks. Carbon isotope composition of background atmospheric CH3Br is needed to test this approach. Thus far, the carbon isotope composition of background atmospheric CH3Br was not analyzed due to its low atmospheric mixing ratio (10 pptv) and the relatively high amount of carbon needed to measure carbon isotope ratios. Therefore, preconcentration is necessary for isotopic analysis of atmospheric CH3Br. Here we report the first measurements of stable carbon isotope ratios from background tropospheric CH3Br. We developed a preconcentration technique for measuring the carbon isotope compositions of halogenated organic gases. We are able to sample 1100 Liters of air with a flow of 6.5L/min. We have solved the problem of separating CH3Br from N2 and O2 in high pumping flow and the problem of saturation of the gas chromatograph column by CO2. We show that it is possible to measure the delta13C values using a gas chromatograph isotope ratio mass spectrometer (GC-IRMS) for sample sizes down to 6 nanograms of carbon. This is the first time measurements of carbon isotope composition of atmospheric organic components have been achieved at a concentration of about 10 pptv.