When air is cooled what happens to the mass?

As temperature decreases, the density of air will increase. There is an inverse relationship between temperature and density. Cooler temperatures allow atoms and molecules of the gases in air to "pack more closely" and take up less space. As more material is taking up a given volume, the density of the material, the air in this case, increases. Scroll down to related links and look at "Density of air - Wikipedia". See: Importance of temperature.