1. Modeling the isotopic composition of H 2 with the TM5 model using a new photochemical scheme for production from CH 4 and VOCs Gerben Pieterse, Maarten Krol and Thomas Röckmann Atmospheric Physics and Chemistry Group, Institute for Marine and Atmospheric research (IMAU), Utrecht, The Netherlands. MODEL SETUP The TM5 model is a global offline chemistry–transport model with two- way nested zooming capability (Krol et al. 2005). The model is operated on a basic horizontal resolution of 6°- 4° globally with the possibility to zoom to 1°- 1° over regions of interest. We have recently included molecular hydrogen (H 2 ) into the model, including its isotopologue HD. Surface emissions are used from EDGAR/HYDA datasets where possible. Photochemical sources are calculated based on the modified carbon bond mechanism (CBM). The deposition scheme is based on Sanderson et al., 2003. Isotope signatures for the surface sources are taken similar to the model study by Price et al., 2007. For the photochemical sources, we use either the bulk number given by Price et al., 2007, or the full photochemical mechanism based on functional group theory as described in the companion poster. Table 1 presents the budget of H 2 and the isotope signatures used in the model. Note that in the explicit photochemistry run, the average of the two photochemical pathways for formaldehyde destruction from Feilberg et al., 2007 is used. The budget cannot be closed at present using the individual rate constants. RESULTS Figure 3 and 4 show the very first results of the new global H 2 and HD modeling with TM5. Concentrations and isotope values are in a reasonable range, the interhemispheric gradient is modeled roughly correctly. Nevertheless, some unexpected features indicate that there are still some model errors (concentration minima in South America, Africa and the maritime continent up to Japan. Isotope minima over the VOC production regions. The latter indicate that isotope scheme as implemented at present has deficiencies and the investigation of such features will actually lead to interesting scientific results. Note that at the moment it is too early to draw scientific conclusions, because many aspects of the code still have to be checked, for example the VOC source field (see below). ADDITIONAL FUTURE POSSIBILITIES 1)Model results will soon be compared to new data from the EUROHYDROS project. 2)Since isotope chemistry of intermediate species is explicitly included, it will also be possible to model the isotope content of H 2 CO. References Feilberg, K. L., Johnson, M. S., and Nielsen, C. J.: Relative reaction rates of hcho, hcdo, dcdo, (hcho)- 13 c, and (hcho)- 18 o with oh, cl, br, and no 3 radicals, J. Phys. Chem. A, 108, 7393-7398, 2004; Feilberg, K. L., Johnson, M. S., Bacak, A., Röckmann, T., and Nielsen, C. J.: Relative tropospheric photolysis rates of hcho and hcdo measured at the european photoreactor facility, Journal of Physical Chemistry A, 111, 9034-9046, 2007; Gerst, S., and Quay, P.: Deuterium component of the global molecular hydrogen cycle, J. Geophys. Res., 106, 5021-5031, 2001., Krol, M., Houweling, S., Bregman, B., van den Broek, M., Segers, A., van Velthoven, P., Peters, W., Dentener, F., and Bergamaschi, P.: The two-way nested global chemistry-transport zoom model tm5: Algorithm and applications, Atmospheric Chemistry and Physics, 5, 417-432, 2005., Novelli, P. C., Lang, P. M., Masarie, K. A., Hurst, D. F., Myers, R., and Elkins, J. W.: Molecular hydrogen in the troposphere: Global distribution and budget, J. Geophys. Res., 104, 30427-30444, 1999., Price, H., Jaegle, L., Rice, A., Quay, P., Novelli, P. C., and Gammon, R.: Global budget of molecular hydrogen and its deuterium content: Constraints from ground station, cruise, and aircraft observations, Journal of Geophysical Research-Atmospheres, 112, -, 2007., Rahn, T., Eiler, J. M., Kitchen, N., Fessenden, J. E., and Randerson, J. T.: Concentration and dd of molecular hydrogen in boreal forests: Ecosystem-scale systematics of atmospheric H2, Geophys. Res. Lett., 29, doi:10.1029/2002GL015118, 2002., Rahn, T., Eiler, J. M., Boering, K. A., Wennberg, P. O., McCarthy, M. C., Tyler, S., Schauffler, S., Donnelly, S., and Atlas, E.: Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of h 2, Nature, 424, 918-921, 2003., Rhee, T. S., Brenninkmeijer, C. A. M., and Röckmann, T.: The overwhelming role of soils in the global atmospheric hydrogen cycle, Atmos. Chem. Phys., 6, 1611-1625, 2005., Rhee, T. S., Brenninkmeijer, C. A. M., and Röckmann, T.: Hydrogen isotope fractionation in the photolysis of formaldehyde, Atm. Chem. Phys., 8, 1353-1366, 2008.; Röckmann, T., Rhee, T. S., and Engel, A.: Heavy hydrogen in the stratosphere, Atmos. Chem. Phys., 3, 2015-2023, 2003. STABILITY OF THE FIRST MODEL VERSION AND DEVELOPMENT OF THE PHOTOCHEMICAL CODE When the bulk number (Price et al., 2007) for the isotopic composition of the photochemical source is used, the global  D values increase slightly, but not dramatically at the two stations Cape Grim, Tasmania and Barrow, Alasca(blue curve). The red and black curve show two implementations of a photochemical code. In the preliminary version, peroxy acetyl chemistry is not modeled correctly, global  D values drop (red curve). in the full version (black curve), the  D values stay approximately stable. SOURCE FIELDS Figures 5 and 6 show the global average fields of the main H 2 precursors CH 4 and isoprene (as indicator for VOC). The former is well validated and has been used in a recent inverse modeling study, the latter is very speculative and still has to be investigated. All VOC sources are initialized with the isotopic composition of CH 4.