Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titan's atmospheric photochemistry

A new wavelength-dependent model for CH₄ photolysis branching ratios is proposed, based on the values measured recently by Gans et al. (Gans, B. et al. [2011]. Phys. Chem. Chem. Phys. 13, 8140-8152). We quantify the impact of this representation on the predictions of a photochemical model of Titan's atmosphere, on their precision, and compare to earlier representations. Although the observed effects on the mole fraction of the species are small (never larger than 50%), it is possible to draw some recommendations for further studies: (i) the Ly-α branching ratios of Wang et al. (Wang, J.H. et al. [2000]. J. Chem. Phys. 113, 4146-4152) used in recent models overestimate the CH₂:CH₃ ratio, a factor to which a lot of species are sensitive; (ii) the description of out-of-Ly-α branching ratios by the "100% CH₃" scenario has to be avoided, as it can bias significantly the mole fractions of some important species (C₃H₈); and (iii) complementary experimental data in the 130-140nm range would be useful to constrain the models in the Ly-α deprived 500-700km altitude range.