  Understanding the surface temperature and interior structure of
cold-to-temperate sub-Neptunes is critical for assessing their habitability,
yet direct observations are challenging. In this study, we investigate the
impact of water condensation on the atmospheric compositions of sub-Neptunes,
focusing on the implications for JWST spectroscopic observations. By modeling
the atmospheric photochemistry of two canonical sub-Neptunes, K2-18 b and LHS
1140 b, both with and without water condensation and with and without thick
atmospheres, we demonstrate that water condensation can significantly affect
the predicted atmospheric compositions. This effect is driven by oxygen
depletion from the condensation of water vapor and primarily manifests as an
increase in the C/O ratio within the photochemically active regions of the
atmosphere. This change in composition particularly affects planets with thin
H$_2$-dominated atmospheres, leading to a transition in dominant nitrogen and
carbon carriers from N$_2$ and oxygen-rich species like CO/CO$_2$ towards
heavier hydrocarbons and nitriles. While our models do not fully account for
the loss mechanisms of these higher-order species, such molecules can go on to
form more refractory molecules or hazes. Planets with thin H2-rich atmospheres
undergoing significant water condensation are thus likely to exhibit very hazy
atmospheres. The relatively flat JWST spectra observed for LHS 1140 b could be
consistent with such a scenario, suggesting a shallow surface with extensive
water condensation or a high atmospheric C/O ratio. Conversely, the JWST
observations of K2-18 b are better aligned with a volatile-rich mini-Neptune
with a thick atmosphere.
