Stable carbon isotope signatures of background tropospheric chloromethane and CFC113

Samples of background air were collected in the lower troposphere of the Northern (high Arctic, northern Ontario, Vancouver and Houston) and Southern (Baring Head, New Zealand) Hemispheres over the period July 1999 until March 2001. These samples were analysed for the stable carbon isotope ratios of 1,1,1-trichlorotrifluoroethane (CFC113) and CH₃Cl using a gas chromatography-continuous flow on-line combustion isotope ratio mass spectrometry combination. For CH₃Cl the global average of the stable carbon isotope ratio is -36.2 ± 0.3‰ (error of mean). The average is based on 78 data points, standard deviation for all measurements is 2.3‰, and the 90% confidence interval is -35.8 to -36.6‰. However, the number of data points from the Southern Hemisphere is rather limited and thus this observation is not necessarily representative for the entire Southern Hemisphere. A simple isotopic budget of CH₃Cl shows the most important parameters needing to be defined are the kinetic isotope effect of CH₃Cl destruction by OH radicals and the source composition of CH₃Cl emitted by the oceans and biomass burning of C-4 plants. Present budgets of atmospheric CH₃Cl show a significant deficit in the source strength. We estimate that the average stable carbon isotope ratio of the additional CH₃Cl emissions required to balance the budget is -41.9 ± 7.8‰. The average CFC113 isotopic composition based on 38 measurements is -23.3 ± 1.6‰ (error of mean), σ = 9.6‰ with no significant difference between the hemispheres.