  Seasonal variation is significant in Titan's atmosphere due to the large
change of solar insolation resulting from Titan's 26.7{\deg} axial tilt
relative to the plane of Saturn's orbit. Here we present an investigation of
hydrocarbon and nitrile species in Titan's upper atmosphere at 400-1200 km,
which includes the mesosphere and the lower thermosphere, over more than one
fourth of Titan's year (2006-2014, LS=318{\deg}-60{\deg}), using eighteen
stellar occultation observations obtained by Cassini/UVIS. Vertical profiles of
eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze
particles are retrieved from these observations using an instrument forward
model, which considers the technical issue of pointing motion. The Markov-chain
Monte Carlo (MCMC) algorithm is used to obtain the posterior probability
distributions of parameters in the retrieval, which inherently tests the extent
to which species profiles can be constrained. The results show that no change
of the species profiles is noticeable before the equinox, while the decrease of
atmospheric temperature and significant upwelling in the summer hemisphere are
found five terrestrial years afterwards. Altitude of the detached haze layer
decreases towards the vernal equinox then it disappears, and no reappearance is
identified within the time range of our data, which is consistent with
observations from Cassini/ISS. This study provides observational constraints on
the seasonal change of Titan's upper atmosphere, and suggests further
investigations of the atmospheric chemistry and dynamics therein.
