Modeling δ 13C and δ D of Stratospheric Methane: Implications for Kinetic Isotope Effects and the Isotopic Composition of Tropospheric Methane

New measurements of δ D and δ ¹³C in stratospheric CH₄ from 78 whole air samples collected aboard the NASA ER-2 aircraft during the STRAT, POLARIS, and SOLVE field campaigns are compared with model results from the Lawrence Livermore National Laboratory 2-D model. Although uncertainties in stratospheric transport are not small, the effect of these uncertainties on the isotopic compositions is likely small compared to the current range of experimental values for the kinetic isotope effects (KIEs) for the ¹³C and D isotopomers of CH₄ and their associated experimental uncertainties. Thus, by comparing the new δ ¹³C and δ D observations from the stratosphere with various model scenarios that vary the KIEs for OH, Cl, and O(¹D), the uncertainty in the laboratory KIEs may be reduced. Furthermore, latitudinal and seasonal trends in the observations are compared with modeled variability. In addition, model results predict the influence of the KIEs of stratospheric sinks on the δ ¹³C and δ D of CH₄ in the free troposphere, which is of importance in inverse models that use isotopic compositions to derive the magnitude and distribution of methane sources to the atmosphere.