  A time-variable 1D photochemical model is used to study the distribution of
stratospheric hydrocarbons as a function of altitude, latitude, and season on
Uranus and Neptune. The results for Neptune indicate that in the absence of
stratospheric circulation or other meridional transport processes, the
hydrocarbon abundances exhibit strong seasonal and meridional variations in the
upper stratosphere, but that these variations become increasingly damped with
depth due to increasing dynamical and chemical time scales. At high altitudes,
hydrocarbon mixing ratios are typically largest where the solar insolation is
the greatest, leading to strong hemispheric dichotomies between the
summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures
and deeper, slower chemistry and diffusion lead to latitude variations that
become more symmetric about the equator. On Uranus, the stagnant, poorly mixed
stratosphere confines methane and its photochemical products to higher
pressures, where chemistry and diffusion time scales remain large. Seasonal
variations in hydrocarbons are therefore predicted to be more muted on Uranus,
despite the planet's very large obliquity. Radiative-transfer simulations
demonstrate that latitude variations in hydrocarbons on both planets are
potentially observable with future JWST mid-infrared spectral imaging. Our
seasonal model predictions for Neptune compare well with retrieved C2H2 and
C2H6 abundances from spatially resolved ground-based observations (no such
observations currently exist for Uranus), suggesting that stratospheric
circulation -- which was not included in these models -- may have little
influence on the large-scale meridional hydrocarbon distributions on Neptune,
unlike the situation on Jupiter and Saturn.
