ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-7001-2011 Global modelling of H<sub>2</sub> mixing ratios and isotopic compositions with the TM5 model Pieterse G. 1 Krol M. C. 1 2 Batenburg A. M. 1 Steele L. P. 3 Krummel P. B. 3 Langenfelds R. L. 3 Röckmann T. 1 Institute for Marine and Atmospheric Research Utrecht (IMAU), Utrecht, The Netherlands Department of Meteorology and Air Quality at Wageningen University, Wageningen, The Netherlands Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Australia 20 07 2011 11 14 7001 7026 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/7001/2011/acp-11-7001-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/7001/2011/acp-11-7001-2011.pdf

The isotopic composition of molecular hydrogen (H<sub>2</sub>) contains independent information for constraining the global H<sub>2</sub> budget. To explore this, we have implemented hydrogen sources and sinks, including their stable isotopic composition and isotope fractionation constants, into the global chemistry transport model TM5. For the first time, a global model now includes a simplified but explicit isotope reaction scheme for the photochemical production of H<sub>2</sub>. We present a comparison of modelled results for the H<sub>2</sub> mixing ratio and isotope composition with available measurements on seasonal to inter annual time scales for the years 2001–2007. The base model results agree well with observations for H<sub>2</sub> mixing ratios. For δD[H<sub>2</sub>], modelled values are slightly lower than measurements. A detailed sensitivity study is performed to identify the most important parameters for modelling the isotopic composition of H<sub>2</sub>. The results show that on the global scale, the discrepancy between model and measurements can be closed by adjusting the default values of the isotope effects in deposition, photochemistry and the stratosphere-troposphere exchange within the known range of uncertainty. However, the available isotope data do not provide sufficient information to uniquely constrain the global isotope budget. Therefore, additional studies focussing on the isotopic composition near the tropopause and on the isotope effects in the photochemistry and deposition are recommended.

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