A two-dimensional photochemical transport model is developed for Titan's stratosphere. Transformed Eulerian mean equations are used to determine the meridional circulation. Mechanical forcing is modeled using eddy and Rayleigh friction, while thermal forcing is provided by Newtonian and eddy heating. The model contains 117 photolysis, chemical, and condensation reactions for 22 hydrocarbon species along with atomic and molecular hydrogen. The method of conservation of second-order moments is used for the meridional and vertical advection calculations (Prather, M. J. 1986. J. Geophys. Res. 91, 6671-6681.). Coustenis and BΓ©zard (1995. Icarus 115, 126-140) analyzed the Voyager IRIS data to calculate hydrocarbon abundances from 53 β’ S to 70 β’ N. They determined that the ethane mixing ratio was approximately constant for all sampled latitudes. In the absence of meridional transport, photochemistry would yield higher concentrations of C 2 and C 3 hydrocarbon species near the equator than near the poles. Meridional advection results in enough tracer transport from low to high latitudes that the latitudinal distributions for ethane are much more uniform.
The model shows methane is well mixed in Titan's stratosphere. Model mixing ratios for ethane and propane agree well with Voyager IRIS data; however, the model overestimates the abundances of acetylene and diacetylene. The model abundance for methylacetylene is within the Voyager observational uncertainty, but the abundance for mid-southern latitudes is at the high end of this uncertainty. The model does not accurately reproduce the observed ethylene abundances.
Mid-latitude column densities (above 5 mbar) for ethane, propane, acetylene, and methylacetylene vary seasonal by 10, 14, 9.5, and 12%, respectively. Large seasonal variations for short-lived chemical species results in significant seasonal variations in the chemical production rates for organic polymers at middle and high latitudes.