Suggest 1 Way Of Adding Or Removing One Of The Gases In The Atmosphere?

You use fractional distillation, but you need a lab to perform this. I don't really know what the apparatus looks like, but I can explain what happens. Air in "put" into a sealed chamber. When the apparatus in activated the the temperature is slowly lowered and the pressure is increased. Firstly water condenses into water drops, which have to be removed. At -78° carbon dioxide would form into liquid state, but in a small amount. Between about -70°C and -180°C the noble gasses (neon, argon, krypton and xenon) should condense into drops, but the atmosphere only contains a very small amount of these elements, so you wouldn't even notice them. At -209°C nitrogen would turn liquid, at -218°C oxygen, at -259°C hydrogen and finally at -273°C helium. But since these temperatures are very difficult to reach, every element has different procedures of making them.