Chemical composition of simulated Titanʼ s midatmospheric aerosols

A large fraction of the major unsaturated species (C2H2 C2H4, HCN, and HC3N) with mixing ratios of about 3 x 10-6, 10-7-10-8, 3 x 10-7, and 10-9-10-10 reside in Titanʼ 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 large