CO2 changing states isn't proving what you think.
Here's real world data for earth.
The following data was measured at Station 42001 in the central Gulf.
In January, when air temperatures average 20.7 C (69F), the average pressure is 1019.1 mb. In June, when the average air temperature is 27.6C (81.7F), the average air pressure is 1015.0 mb. As you can see, air pressure does vary according to temperature. Cold air is more dense than warm air, i.e., it weighs more. As a result, it tends to sink. Warm air, on the other hand, is less dense. Therefore, it weighs less and tends to rise. Meteorologists say that warm air is buoyant.
National Data Buoy Center - Science Education - Does air pressure vary with respect to air temperature? - Answer
www.ndbc.noaa.gov