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The isotopic composition of molecular hydrogen (H2) produced byphotochemical oxidation of methane (CH4) and Volatile Organic Compounds(VOCs) is a key quantity in the global isotope budget of (H2). The manyindividual reaction steps involved complicate its investigation. Here wepresent a simplified structure-activity approach to assign isotope effects tothe individual elementary reaction steps in the oxidation sequence ofCH4 and some other VOCs. The approach builds on and extends the work byGerst and Quay (2001) and Feilberg et al. (2007b). The description is generalized andallows the application, in principle, also to other compounds. The idea isthat the C-H and C-D bonds – seen as reactive sites – have similar relativereaction probabilities in isotopically substituted, but otherwise identicalmolecules. The limitations of this approach are discussed for the reactionCH4+Cl. The same approach is applied to VOCs, which are importantprecursors of H2 that need to be included into models. Unfortunately,quantitative information on VOC isotope effects and source isotope signaturesis very limited and the isotope scheme at this time is limited to a stronglyparameterized statistical approach, which neglects kinetic isotope effects.Using these concepts we implement a full hydrogen isotope scheme in achemical box model and carry out a sensitivity study to identify thosereaction steps and conditions that are most critical for the isotopecomposition of the final H2 product. The reaction scheme is directlyapplicable in global chemistry models, which can thus include the isotopepathway of H2 produced from CH4 and VOCs in a consistent way

A consistent molecular hydrogen isotope chemistry scheme based on an independent bond approximation