Gravity definitely plays a role in convection, which is the main form of dispersion. The air one exhales has different composition — more CO2, more water — whic

Gravity definitely plays a role in convection, which is the main form of dispersion. The air one exhales has different composition — more CO2, more water — which means it has different buoyancy. Usually, this means the parcel of air rises here on Earth, but falling of your face is fine too. No gravity means no convection. Without wind to cause turbulence, mixing is really slow, leaving molecular diffusion as the only process. Remember, that’s random, and CO2 will still accumulate near the point source with a Gaussian distribution.