Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titan’s atmospheric photochemistry
Résumé
A new wavelength-dependent model for CH4 photolysis branching ratios is proposed, based on the values measured recently by Gans et al. [Phys. Chem. Chem. Phys.13:8140 (2011)]. 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. [ J. Chem. Phys. 113:4146-4152 (2000)] used in recent models overestimates 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); (iii) complementary experimental data in the 130-140 nm range would be useful to constrain the models in the Ly-α deprived 500-700 km altitude range.