Kinetic isotope effects, their use in studying atmospheric trace species: Case study, CH4 + OH

Many recent studies of atmospheric trace gases have included analyses of stable isotope compositions of key elements. One such example is the use of 13CH4 in studies of atmospheric methane and its sources and sinks. δ13CH4 measurements of background methane and its sources are used along with companion data of fluxes to help determine relative source strengths. A mass-weighted average composition of all sources should equal the mean δ13C of atmospheric methane corrected for any isotope fractionation effects in methane sink reactions. The principal sink for methane in the atmosphere is from its reaction with OH radicals. At NCAR the authors measured the carbon kinetic isotope effect for this reaction and have reported a value of 1.0054 ± 0.0009 for the ratio of the rate coefficients k12/k13. This value improves on earlier determinations make by the authors and others. A discussion of experimental techniques and difficulties is presented here. A mass-weighted hydrogen isotope balance similar to that described above for carbon could also be very useful in constraining the methane budget. They are in the process of determining the KCH4/KCH3D KIE value for reaction with OH at temperatures which can be found in the troposphere