Experimental and theoretical study of hydrocarbon photochemistry applied to Titan stratosphere

None of the Titan photochemical models currently available have been able to reproduce the full set of stratospheric molecular mixing ratios inferred from observations. In order to assess how well reaction sets describe hydrocarbon chemistry, theoretical modeling predictions were compared to the results of a laboratory experiment. A CH4-C2H2 mixture was irradiated at 185 nm in an atmospheric simulation chamber and the evolution of the gas mixture was followed in situ and in real time by infrared spectroscopy. In parallel, a 0D theoretical model of the laboratory experiment was developed. A new reaction set describing Titan's chemistry was built and incorporated in the model. Lebonnois et al. [Lebonnois, S., Toublanc, D., Hourdin, F., Rannou, P., 2001. Icarus 152, 384-406] reaction set was also used for comparison. The presence of small amounts of atmospheric O2 in the experiment was properly accounted for and led us to suggest that oxygenated chemistry might be a source Of C2H4 in Titan's atmosphere. With Lebonnois et al. [Lebonnois, S., Toublanc, D., Hourdin, F., Rannou, P., 2001. Icarus 152, 384-406] reaction set, the model could not fit at all the experimental evolution of the compounds. This is explained by some of the choices made for crucial kinetic parameters such as the quantum yield of photolysis of C2H2. Also, the absence of some reactions led to the enhancement of pathways that would otherwise be negligible. For example, the lack of reactions between C4H4 and radicals induced an erroneously high photolysis rate for this species. With the reaction set built in this study, the model much better fits the experiment, especially when the soot, which includes C4H4, is recycled into C2H2. This shows that photochemistry of the larger species has a role in determining the lighter species concentrations and that considering that they are simply lost from the system is not a valid assumption. Including even an abridged set of C-4 + hydrocarbon reactions will be required in future photochemical models. Especially, photolysis rates and yields for C2H2, C4H2, and C4H4, are important parameters in need of a better determination. (c) 2006 Elsevier Inc. All rights reserved.