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

Abstract

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 H2 molecules as well as H, CH, 1CH2, 3CH2, and CH3 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 C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance, minor C3 molecules do show substantial sensitivity to choice of quantum yield scheme. We find that the C3H4 isomers (methylacetylene, allene) and C3H6 (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 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 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 C2H6 (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. Copyright 2000 by the American Geophysical Union.