  Planets residing in circumstellar habitable zones (CHZs) offer our best
opportunities to test hypotheses of life's potential pervasiveness and
complexity. Constraining the precise boundaries of habitability and its
observational discriminants is critical to maximizing our chances at remote
life detection with future instruments. Conventionally, calculations of the
inner edge of the habitable zone (IHZ) have been performed using both 1D
radiative-convective and 3D general circulation models. However, these models
lack interactive three-dimensional chemistry and do not resolve the mesosphere
and lower thermosphere (MLT) region of the upper atmosphere. Here we employ a
3D high-top chemistry-climate model (CCM) to simulate the atmospheres of
synchronously-rotating planets orbiting at the inner edge of habitable zones of
K- and M-dwarf stars (between $T_{\rm eff} =$ 2600 K and 4000 K). While our IHZ
climate predictions are in good agreement with GCM studies, we find noteworthy
departures in simulated ozone and HO$_{\rm x}$ photochemistry. For instance,
climates around inactive stars do not typically enter the classical moist
greenhouse regime even with high ($< 10^{-3}$ mol mol$^{-1}$) stratospheric
water vapor mixing ratios, which suggests that planets around inactive M-stars
may only experience minor water-loss over geologically significant timescales.
In addition, we find much thinner ozone layers on potentially habitable moist
greenhouse atmospheres, as ozone experiences rapid destruction via reaction
with hydrogen oxide radicals. Using our CCM results as inputs, our simulated
transmission spectra show that both water vapor and ozone features on moist
greenhouse atmospheres could be detectable by instruments NIRSpec and MIRI LRS
onboard the James Webb Space Telescope.
