  In recent years, several potentially habitable, probably terrestrial
exoplanets and exoplanet candidates have been discovered. The amount of CO2 in
their atmosphere is of great importance for surface conditions and
habitability. In the absence of detailed information on the geochemistry of the
planet, this amount could be considered as a free parameter. Up to now, CO2
partial pressures for terrestrial planets have been obtained assuming an
available volatile reservoir and outgassing scenarios. This study aims at
calculating the allowed maximum CO2 pressure at the surface of terrestrial
exoplanets orbiting near the outer boundary of the habitable zone by coupling
the radiative effects of the CO2 and its condensation at the surface. These
constraints might limit the permitted amount of atmospheric CO2, independent of
the planetary reservoir. A 1D radiative-convective cloud-free atmospheric model
was used. CO2 partial pressures are fixed according to surface temperature and
vapor pressure curve. Considered scenarios cover a wide range of parameters.
Results show that for planets in the habitable zone around K-, G-, and F-type
stars the allowed CO2 pressure is limited by the vapor pressure curve and not
by the planetary reservoir. The maximum CO2 pressure lies below the CO2 vapor
pressure at the critical point of pcrit =73.8 bar. For M-type stars, CO2
pressures above pcrit are possible for almost all scenarios considered across
the habitable zone. This implies that determining CO2 partial pressures for
terrestrial planets by using only geological models is probably too simplified
and might over-estimate atmospheric CO2 towards the outer edge of the habitable
zone. (abridged)
