  We investigate the abiotic production of oxygen and its photochemical
byproduct ozone through water vapor photolysis in moist atmospheres of
temperate terrestrial exoplanets. The amount of water vapor available for
photolysis in the middle atmosphere of a planet can be limited by an
atmospheric cold-trap, the formation of which largely depends on the amount of
non-condensable gases. We study this effect using a photochemical model coupled
to a 1D radiative-convective equilibrium model in atmospheres with N2, CO2 and
H2O as the main constituents. We find that in atmospheres with a low N2
inventory, water vapor mixing ratios in the middle atmosphere can be over two
orders of magnitude higher compared to atmospheres with an Earth-like N2
inventory. Without a strong surface sink, the non-condensable oxygen can build
up rapidly, drying out the upper atmosphere. With a moderate surface sink, the
planet can approach a steady state with significant oxygen mixing ratios in
which oxygen production is balanced by surface uptake. We use a radiative
transfer model to study the spectroscopic fingerprint of these atmospheres in
transit observations. Spectral signatures of abiotic oxygen and ozone can be of
comparable magnitude as in spectra of Earth seen as an exoplanet. Middle
atmospheric water vapor is unlikely to be a usable indicator of the abiotic
origin of oxygen because of the influence of oxygen on the water vapor
distribution. This suggests that atmospheric oxygen and ozone cannot be used as
binary bioindicators and their interpretation will likely require atmospheric
and planetary models.
