  Molecular oxygen (O2) paired with a reducing gas is regarded as a promising
biosignature pair for the atmospheric characterization of terrestrial
exoplanets. In circumstances when O2 may not be detectable in a planetary
atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O3),
the photochemical product of O2, could be used as a proxy to infer the presence
of O2. However, O3 production has a nonlinear dependence on O2 and is strongly
influenced by the UV spectrum of the host star. To evaluate the reliability of
O3 as a proxy for O2, we used Atmos, a 1D coupled climate/photochemistry code,
to study the O2-O3 relationship for "Earth-like" habitable zone planets around
a variety of stellar hosts (G0V-M5V) and O2 abundances. Overall, we found that
the O2-O3 relationship differed significantly with stellar hosts and resulted
in different trends for hotter stars (G0V-K2V) vs cooler stars (K5V-M5V).
Planets orbiting hotter host stars counter-intuitively experience an increase
in O3 when O2 levels are initially decreased from 100% Earth's present
atmospheric level (PAL), with a maximum O3 abundance occurring at 25-55% PAL
O2. As O2 abundance initially decreases, larger amounts of UV photons capable
of O2 photolysis reach the lower (denser) regions of the atmosphere where O3
production is more efficient, resulting in these increased O3 levels. This
effect does not occur for cooler host stars (K5V-M5V), as the weaker incident
UV flux does not allow O3 formation to occur at dense enough regions of the
atmosphere where the faster O3 production can outweigh a smaller source of O2
from which to create O3. Overall it will be extremely difficult (or impossible)
to infer precise O2 levels from an O3 measurement, however, with information
about the UV spectrum of the host star and context clues, O3 will provide
valuable information about potential surface habitability of an exoplanet.
