  We present results from one-dimensional atmospheric simulations investigating
the effect of varying the carbon-to-oxygen (C/O) ratio on the thermal
structure, chemical composition and transmission and emission spectra, for
irradiated hydrogen-dominated atmospheres. We find that each of these
properties of the atmosphere are strongly dependent on the individual
abundances (relative to hydrogen) of carbon and oxygen. We confirm previous
findings that different chemical equilibrium compositions result from different
sets of element abundances but with the same C/O ratio. We investigate the
effect of this difference in composition on the thermal structure and simulated
spectra. We also simulate observations using the PandExo tool and show that
these differences are observationally significant with current (i.e. Hubble
Space Telescope) and future (i.e. James Webb Space Telescope) instruments. We
conclude that it is important to consider the full set of individual element
abundances, with respect to hydrogen, rather than the ratios of only two
elements, such as the C/O ratio, particularly when comparing model predictions
with observed transmission and emission spectra.
