Sensitivity studies of methane photolysis and its impact on hydrocarbon chemistry in the atmosphere of Titan

The photodissociation of methane at Lyman alpha (1216 Angstrom) has been th e focus of much scrutiny over the past few years. Methane photolysis leads to the formation of H-2 molecules as well as H, CH, (CH2)-C-1, (CH2)-C-3, a nd CH3 radicals, which promote the propagation of hydrocarbon chemistry. Ho wever, recent studies [Mordaunt et al., 1993; Romani, 1996; Smith and Rauli n, 1999] have not fully resolved the issue of methane photolytic product yi elds at this wavelength. We use a one-dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield sc hemes on the hydrocarbon chemistry of Titan's atmosphere, where Lyman a rad iation accounts for 75% of methane photolysis longward of 1000 Angstrom. Se nsitivity studies show that while simple hydrocarbons like C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formati on of more complex hydrocarbons, show virtually no variation in abundance, minor C-3 molecules do show substantial sensitivity to choice of quantum yi eld scheme. We find that the C3H4 isomers (methylacetylene, allene) and C3H 6 (propylene) display major variation in atmospheric mixing ratios under th e implementation of these schemes, with a maximum variation of approximatel y a factor of 5 in C3H4 abundance and approximately a factor of 4 for C3H6. In these cases our nominal scheme, recommended by Romani [1996], offers an intermediate result in comparison with the other schemes. We also find tha t choice of pathway for non-Lyman alpha methane absorption does affect hydr ocarbon chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C2H6 (ethane) abundance, a value within observational unc ertainty, is the largest divergence found for a wide range of possible non- Lyman alpha photofragment quantum yields. These results will have significa nce in future modeling and interpretation of observations of the atmosphere of Titan.