  The geology of Earth and super-Earth sized planets will, in many cases, only
be observable via their atmospheres. Here, we investigate secondary volcanic
atmospheres as a key base case of how atmospheres may reflect planetary
geochemistry. We couple volcanic outgassing with atmospheric chemistry models
to simulate the growth of C-O-H-S-N atmospheres in thermochemical equilibrium,
focusing on what information about a planet's mantle fO$_2$ and bulk silicate
H/C ratio could be determined by atmospheric observation. 800K volcanic
atmospheres develop distinct compositional groups as the mantle fO$_2$ is
varied, which can be identified using sets of (often minor) indicator species:
Class O, representing an oxidised mantle and containing SO$_2$ and sulfur
allotropes; Class I, formed by intermediate mantle fO$_2$'s and containing
CO$_2$, CH$_4$, CO and COS; and Class R, produced by reduced mantles,
containing H$_2$, NH$_3$ and CH$_4$. These atmospheric classes are robust to a
wide range of bulk silicate H/C ratios. However, the H/C ratio does affect the
dominant atmospheric constituent, which can vary between H$_2$, H$_2$O and
CO$_2$ once the chemical composition has stabilised to a point where it no
longer changes substantially with time. This final atmospheric state is
dependent on the mantle fO$_2$, the H/C ratio, and time since the onset of
volcanism. The atmospheric classes we present are appropriate for the
closed-system growth of hot exoplanets, and may be used as a simple base for
future research exploring the effects of other open-system processes on
secondary volcanic atmospheres.
