Vertical Distribution of C3-hydrocarbons in the Stratosphere of Titan

Motivated by the recent detection of propene (C3H6) in the atmosphere of Titan, we use a one-dimensional Titan photochemical model with an updated eddy diffusion profile to systematically study the vertical profiles of the stable species in the C3-hydrocarbon family. We find that the stratospheric volume mixing ratio of propene (C3H6) peaks at 150 km with a value of 5 × 10, which is in good agreement with recent observations by the Composite Infrared Spectrometer on the Cassini spacecraft. Another important species that is currently missing from the hydrocarbon family in Titan's stratosphere is allene (CH2CCH2), an isomer of methylacetylene (CH3C2H). We predict that its mixing ratio in the stratosphere is about 10, which is on the margin of the detection limit. CH2CCH2 and CH3C2H equilibrate at a constant ratio in the stratosphere by hydrogen-exchanging reactions. Thus, by precisely measuring the ratio of CH2CCH2 to CH3C2H, the abundance of atomic hydrogen in the atmosphere can be inferred. No direct yield for the production of cyclopropane (c-C3H6) is available. From the discharge experiments of Navarro-González & Ramírez, the abundance of cyclopropane is estimated to be 100 times less than that of C3H6.