  Using 2D thermal structure models and pseudo-2D chemical kinetics models, we
explore how atmospheric temperatures and composition change as a function of
altitude and longitude within the equatorial regions of close-in transiting
Neptune-class exoplanets at different distances from their host stars. Our
models predict that the day-night stratospheric temperature contrasts increase
with increasing planetary effective temperatures T_eff; atmospheric composition
also changes significantly with T_eff. Horizontal transport-induced quenching
is very effective in our simulated exo-Neptune atmospheres, acting to
homogenize the vertical profiles of species abundances with longitude at
stratospheric pressures where infrared observations are sensitive. Our models
have important implications for planetary emission observations as a function
of orbital phase with the Ariel mission. Cooler solar-composition exo-Neptunes
with T_eff = 500-700 K are predicted to have small variations in infrared
emission spectra with orbital phase, making them less robust phase-curve
targets for Ariel. Hot solar-composition exo-Neptunes with T_eff > 1300 K
exhibit strong variations in infrared emission with orbital phase but are
arguably less interesting from an atmospheric chemistry standpoint, with
spectral signatures being dominated by a small number of species whose
abundances are expected to be constant with longitude and consistent with
thermochemical equilibrium. Solar-composition exo-Neptunes with T_eff =
900-1100 K reside in an interesting intermediate regime, with infrared phase
curve variations being affected by both temperature and composition variations.
This interesting intermediate regime shifts to smaller temperatures as
atmospheric metallicity is increased, making cool higher-metallicity
Neptune-class planets appropriate targets for Ariel phase-curve observations.
