  Ozak et al. [2016] claimed that explicitly including the effect of CO$_2$
collisional line mixing (LM) in their radiative transfer calculations yield
CO$_2$ atmospheres that are more transparent to infrared radiation than when
spectra calculations are made using sub-Lorentzian line shapes. This would in
particular imply significantly colder surface temperatures (up to 15K) for
early Mars than estimated in previous studies. Here we show that the relative
cooling that Ozak et al. [2016] associated to the effect of collisional line
mixing is in fact due to a wrong choice of broadening species (air instead of
CO$_2$). We then calculated Line-by-Line spectra of pure CO$_2$ atmospheres
using a line-mixing model developed for self-broadened CO$_2$. Using the LMD
Generic model (in 1D radiative-convective mode), we find that calculations made
with the proper collisional line mixing model and with sub-Lorentzian line
shapes lead to differences between early Mars surface temperatures smaller than
2 Kelvins only.
