  Atmospheric chemistry models have shown molecular oxygen can build up in
CO2-dominated atmospheres on potentially habitable exoplanets without an input
of life. Existing models typically assume a surface pressure of 1 bar. Here we
present model scenarios of CO2-dominated atmospheres with the surface pressure
ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We
use a one-dimensional photochemistry model to calculate the abundance of O2 and
other key species, for outgassing rates ranging from a Venus-like volcanic
activity up to 20x Earth-like activity. The model maintains the redox balance
of the atmosphere and the ocean, and includes the pressure dependency of
outgassing on the surface pressure. Our calculations show that the surface
pressure is a controlling parameter in the photochemical stability and oxygen
buildup of CO2-dominated atmospheres. The mixing ratio of O2 monotonically
decreases as the surface pressure increases at the very high outgassing rates,
whereas it increases as the surface pressure increases at the lower-than-Earth
outgassing rates. Abiotic O2 can only build up to the detectable level, defined
as 1e-3 in volume mixing ratio, in 10-bar atmospheres with the Venus-like
volcanic activity rate and the reduced outgassing rate of H2 due to the high
surface pressure. Our results support the search for biological activities and
habitability via atmospheric O2 on terrestrial planets in the habitable zone of
Sun-like stars.
