  Planet formation models suggest broad compositional diversity in the
sub-Neptune/super-Earth regime, with a high likelihood for large atmospheric
metal content (> 100 x Solar). With this comes the prevalence of numerous
plausible bulk atmospheric constituents including N2, CO2, H2O, CO, and CH4.
Given this compositional diversity there is a critical need to investigate the
influence of the background gas on the broadening of the molecular absorption
cross-sections and the subsequent influence on observed spectra. This
broadening can become significant and the common H2/He or "air" broadening
assumptions are no longer appropriate. In this work we investigate the role of
water self-broadening on the emission and transmission spectra as well as on
the vertical energy balance in representative sub-Neptune/super-Earth
atmospheres. We find that the choice of the broadener species can result in a
10 -- 100 parts-per-million difference in the observed transmission and
emission spectra and can significantly alter the 1-dimensional vertical
temperature structure of the atmosphere. Choosing the correct background
broadener is critical to the proper modeling and interpretation of transit
spectra observations in high metallicity regimes, especially in the era of
higher precision telescopes such as JWST.
