  Understanding the planetary envelope composition of sub-Neptune-type
exoplanets is challenging due to the inherent degeneracy in their interior
composition scenarios. Particularly, the planetary envelope's H2O/H2 ratio,
which can also be expressed as the O/H ratio, provides crucial insights into
its original location relative to the ice line during planetary formation.
Using self-consistent radiative transfer modeling and a rate-based automatic
chemical network generator combined with 1D photochemical kinetic-transport
atmospheric modeling, we investigate various atmospheric scenarios of temperate
sub-Neptunes, ranging from H2-dominated to H2O-dominated atmospheres with
equilibrium temperatures (Teq) of 250-400 K. This study includes examples such
as K2-18 b (Teq = 255 K), LP 791-18 c (Teq = 324 K), and TOI-270 d (Teq = 354
K). Our models indicate that the atmospheric CO2/CH4 ratio can be used to infer
the deep-interior H2O/H2 ratio. Applying this method to recent JWST
observations, our findings suggest K2-18 b likely has an interior that is 50%
highly enriched in water, exceeding the water content in a 100xZ scenario and
suggesting a planetary formation mechanism involving substantial accretion of
ices. In contrast, our model suggests that approximately 25% of TOI-270 d's
interior is composed of H2O, which aligns with the conventional metallicity
framework with a metallicity higher than 100xZ. Furthermore, our models
identify carbonyl sulfide (OCS) and sulfur dioxide (SO2) as strong indicators
for temperate sub-Neptunes with at least 10% of their interior composed of
water. These results provide a method to delineate the internal composition and
formation mechanisms of temperate sub-Neptunes (Teq < ~500 K) via atmospheric
characterization through transmission spectroscopy.
