CO2 is only a "green house" gas. Green house gas by definition is a gas that absorbs long wave (infrared) thermal radiation that is emmitted from the ground, while allow the shorter thermal radiation from the sun passing through. The effect has nothing to do with the ozone layer. CO2 is allegedly responsible for global warming.www.ddhw.com
There is only less than 0.03 % (by volume) of CO2 in the atmosphere. At 1 atm, it is a gas from above ~-80 DegC. Yes, it is heavier than O2 and N2 (air), and you will find more CO2 close to the ground. But, don't worry, something takes it as food. We are not going to drawn in a sea of carbondioxide.