Carbon and hydrogen isotopic compositions of stratospheric methane: 2. Two‐dimensional model results and implications for kinetic isotope effects

Abstract

New high‐precision measurements of the carbon and hydrogen isotopic compositions of stratospheric CH 4 made on whole air samples collected aboard the NASA ER‐2 aircraft are compared with results from the Lawrence Livermore National Laboratory 2‐D model. Model runs incorporating sets of experimentally determined kinetic isotope effects (KIEs) for the reactions of CH 4 with each of the oxidants OH, O( 1 D), and Cl are examined with the goals of determining (1) how well the 2‐D model can reproduce the observations for both the carbon and hydrogen isotopic compositions, (2) what factors are responsible for the observed increase in the apparent isotopic fractionation factors with decreasing methane mixing ratios, and (3) how sensitive the modeled isotopic compositions are to various experimentally determined KIEs. Bound by estimates of the effects of uncertainties in model chemistry and transport on isotopic compositions, we then examine the constraints the ER‐2 observations place on values for the KIEs. For the carbon KIE for reaction of CH 4 with O( 1 D), for example, the analysis of model results and observations favors the larger of the experimental values, 1.013, over a value of 1.001. These analyses also suggest that intercomparisons of results from different models using a given set of KIEs may be useful as a new diagnostic of model‐model differences in integrated chemistry and transport.