Chemical composition of simulated Titan's midatmospheric aerosols

Abstract

A large fraction of the major unsaturated species (C2H2 C2H4, HCN, and HC3N) with mixing ratios of about 3 × 10−6, 10−7–10−8, 3 × 10−7, and 10−9–10−10 reside in Titan's atmosphere between 150 and 500 km (Vinatier et al., 2009; Coustenis et al., 2007) before they condense near the tropopause. A large flux of medium‐wavelength UV penetrates down to these levels, resulting in the polymerization of these unsaturated compounds and the formation of aerosols. We performed our experiments on aerosol formation at these altitudes where both abundances and solar UV flux are high, bearing in mind that additional photolysis occurs at both higher and lower altitudes. In the gas phase, C2H2 photolysis results in unsaturated C4 species that, on further addition of C2, form the cyclic benzene. These gas‐phase intermediates are consumed when an acetylene‐poor gas mixture is irradiated for a long time, giving rise to larger solid‐state species, mainly by addition of C2, followed by further cyclization. The largest species formed was the condensed 5‐ring benzpyrene, not the 7‐ring coronen molecule, which could have been detected there. Another fraction of the polymers consists of polyvinyl and vinyl acetylene chains, which are cross‐linked due to their labile π electrons and form an insoluble solid matrix. This explains the reduction of the C:H ratio from the condensed aromatics of 1.2 to the measured C:H = 1.013 ± 0.001 of the polymers.