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

The photodissociation of methane at Lyman α (1216 Å) has been the focus of much scrutiny over the past few years. Methane photolysis leads to the formation of H 2 molecules as well as H, CH, 1 CH 2 , 3 CH 2 , and CH 3 radicals, which promote the propagation of hydrocarbon chemistry. However, recent studies [ Mordaunt et al. , 1993; Romani , 1996; Smith and Raulin , 1999] have not fully resolved the issue of methane photolytic product yields at this wavelength. We use a one‐dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titan's atmosphere, where Lyman α radiation accounts for 75% of methane photolysis longward of 1000 Å. Sensitivity studies show that while simple hydrocarbons like C 2 H 2 (acetylene) and C 2 H 4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance, minor C 3 molecules do show substantial sensitivity to choice of quantum yield scheme. We find that the C 3 H 4 isomers (methylacetylene, allene) and C 3 H 6 (propylene) display major variation in atmospheric mixing ratios under the implementation of these schemes, with a maximum variation of approximately a factor of 5 in C 3 H 4 abundance and approximately a factor of 4 for C 3 H 6 . In these cases our nominal scheme, recommended by Romani [1996], offers an intermediate result in comparison with the other schemes. We also find that choice of pathway for non‐Lyman α methane absorption does affect hydrocarbon chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C 2 H 6 (ethane) abundance, a value within observational uncertainty, is the largest divergence found for a wide range of possible non‐Lyman α photofragment quantum yields. These results will have significance in future modeling and interpretation of observations of the atmosphere of Titan.