Thermosphere/Exosphere
Hi SDF
Basically we're talking molecules and atoms on ballistic trajectories, separated out via their mass thanks to gravity. Ideal gas laws no longer in such a situation. What carbon dioxide does is reduce long-wave heating of the layer from surface thermal radiation, because the CO2 is very good at radiating it away - much better than N2/O2. Both Venus and Mars have cooler exospheres than Earth (300 & 200 K vs 1000 K) because their atmospheres are mostly CO2.
Adam