Seasonal variations of Titan's atmospheric composition

In order to investigate seasonal variations of the composition of Titan's l ow stratosphere, we developed a two-dimensional (latitude-altitude) photoch emical and transport model. Large-scale advection, hidden in the vertical e ddy diffusion for one-dimensional models, is accounted for explicitly. Atmo spheric dynamics is prescribed using results of independent numerical simul ations of the atmospheric general circulation. Both the mean meridional tra nsport and latitudinal mixing by transient planetary waves are taken into a ccount. Chemistry is based on 284 reactions involving 40 hydrocarbons and n itriles. Photodissociation rates are based on a three-dimensional descripti on of the ultraviolet flux. For most species, the model fits well the latit udinal variations observed by Voyager I giving for the first time a full an d self-consistent interpretation of these observations. In particular, the enrichment of the high northern latitudes is attributed to subsidence durin g the winter preceeding the Voyager encounter. Discrepancies are observed f or C2H4, HC3N, and C2N2 and are attributed to problems in the chemical sche me. Sensitivity to dynamical parameters is investigated. The vertical eddy diffusion coefficient keeps an important role for the upper atmosphere. The wind strength and horizontal eddy diffusion strongly control the latitudin al behavior of the composition in the low stratosphere, while mean concentr ations appear to be essentially controlled by chemistry. (C) 2001 Academic Press.