Photochemical modeling of Titan's atmosphere

We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984, Astrophys. J. Suppl. 55, 465-506), the most significant recent change in the reactions used is the updated methane photodissociation scheme (Mordaunt et al. 1993, J. Chem. Phys. 98(3), 2054-2065). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990, Icarus, 85, 43-57) using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992, Icarus 100, 512-526). We have run the model in both steady-state and diurnal modes, with 62 speices involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.