Simulation of the isotopic composition of molecular hydrogen in the stratosphere

Molecular hydrogen has both source and sink in the stratosphere yielding an approximately constant mixing ratio of about 0.5 ppmv. The stratospheric source and sink processes of H2 can therefore be investigated using the isotopic composition. Stratospheric observations of isotopic composition of molecular hydrogen showed deuterium enrichment of H2 with a tight correlation with CH4 decrease that is rather independent of latitude (Röckmann et al., 2003). We present simulations of the hydrogen isotope ratios of H2 and CH4 in the stratosphere. A consistent hydrogen isotope chemistry scheme for the whole CH4 oxidation chain was incorporated into the chemistry module of the Chemical Lagrangian Model of the Stratosphere (CLaMS). The simulations are performed using a simplified representative box model approach. The results are compared with simultaneous observations of the isotopic composition of CH4 and H2. We also show the sensitivity with respect to uncertainties in the individual branching ratios and fractionation constants. As many isotope fractionation factors and branching ratios of the CH4 oxidation chain are not known with high accuracy, the simultaneous comparison of the deuterium enrichment of both CH4 and H2 and their correlation with CH4 is important for the evaluation of the different sensitivities.