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 C₄ + 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.