  One of the desired outcomes of studying exoplanetary atmospheres is to set
decisive constraints on exoplanet formation theories. Astronomers often speak
of the "metallicity" in broad terms. When modeling the bulk metallicity,
workers refer to the elemental iron abundance (Fe/H). By contrast, when
exo-atmospheric astronomers speak of the "metallicity" derived from analysing
low-resolution Hubble and Spitzer spectrophotometry, they are referring to the
elemental abundances of oxygen (O/H), carbon (C/H) and nitrogen (N/H): in
decreasing order of importance, since spectra from the Hubble Wide Field Camera
3 are primarily sensitive to water with secondary contributions from hydrogen
cyanide and ammonia, while Spitzer photometry is sensitive to methane and
carbon monoxide. From retrieving for the water abundances, workers such as
Kreidberg et al. (2014), Wakeford et al. (2017, 2018), Arcangeli et al. (2018)
and Mansfield et al. (2018) have published figures that show the "metallicity"
versus the masses of the exoplanets. Even if we assume that bulk and
atmospheric elemental abundances are equal, the motivation behind this research
note is to demonstrate that the conversion between the water abundance and O/H
is not straightforward. "Water at solar metallicity" is a temperature- and
pressure-dependent statement, because there is no single value of the water
volume mixing ratio corresponding to solar values of O/H and C/H. Overall, the
conversion factor between the water volume mixing ratio and O/H is not unity
and depends on temperature, pressure, O/H, C/H and C/O, even without invoking
disequilibrium chemistry (photochemistry and/or atmospheric mixing). It remains
unproven that disequilibrium chemistry or condensation will always return a
conversion factor of unity. It is thus a fair statement to say that the
"metallicity" estimates inferred from atmospheric retrievals are
model-dependent.
