Experimental and theoretical simulations of Titan's VUV photochemistry

A new reactor, named APSIS (Atmospheric Photochemistry SImulated by Synchrotron), has been designed to simulate planetary atmospheric photochemistry [Peng et al. JGR-E. 2013, 118, 778]. We report here a study focusing on Titan's upper atmosphere. A nitrogen-methane gas flow was irradiated by a continuous 60-350 nm VUV beam provided by the DISCO line at SOLEIL synchrotron radiation facility. The production of C2-C4 hydrocarbons as well as several nitriles (HCN, CH3 CN and C2N2) was detected by in situ mass spectrometry, in agreement with Cassini's INMS observations at Titan, and ex situ GC-MS of a cryogenic experiment. We compared the mass spectra with those obtained by a plasma experiment [Carrasco et al. Icarus. 2012, 219, 230] and with another synchrotron-based experiment [Imanaka and Smith. PNAS. 2010, 107, 12423], and with the in situ measurements of the INMS instrument onboard Cassini probing the neutral content of Titan's upper atmosphere. In spite of lower photochemical production efficiency and different environmental conditions, the APSIS reactor seems to simulate Titan's neutral composition rather well. To interpret these experimental data, we developed a fully coupled ion-neutral photochemical model of the reactor, with uncertainty management, based on the neutral model of Hébrard et al. [J. Photochem. Photobiol. A. 2006, 7, 211], the model of ion chemistry of Plessis et al. [J. Chem. Phys. 2010, 133, 134110], and a new representation of photolysis cross-sections and branching ratios [Gans et al. Icarus. 2013, 223, 330]. Compared to the measurements, the production in Cn blocks is in good agreement. Ion chemistry and the full dissociative recombination scheme have been demonstrated to be important features of the model. The photolysis was confirmed to be globally influential by sensivity analysis. We observed the importance of the addition of small (C1 or C2) units in molecular growth, as well as 3 growth families, promoted by C2H2, C2H4 and C2H5/C2H6, respectively. Among the three, the C2H2 family, in which the growth pathways of unsaturated species via ion chemistry are the most efficient, is clearly prominent. Our model was also used to interpret the results of the INMS data and Imanaka and Smith's experiments. Through variants of the reference model of the APSIS experiments, we showed that low pressure and low temperature favor the growth of unsaturated species. These conditions are fulfilled in Titan's ionosphere. The INMS neutral spectrum, in which there is mainly the signal of unsaturated species, can be well reproduced by our simulated MS. Compared to the experimental MS of the APSIS experiments and Imanaka and Smith's experiments, the simulated MS systematically underestimate the intensities of the saturated part of each band. After the consideration of the recombinations catalyzed by the reactor's walls, we improved the simulated MS significantly. This suggests the existence of wall effects in the laboratory simulation setups of atmospheric chemistry, leading to an overestimation of the saturated products compared to Titan's chemical products