Oh please, the ideal gas law states nothing of the sort. Ideal gas law is pV=nRT - the volume and the amount of the gas also enter into the equation. Since an atmosphere has no hard upper bound, assumption of constant volume is incorrect, if you're adding or removing gas then the assumption of constant amount of gas is incorrect. So adding helium will not necessarily do anything to the temperature, as the volume of the atmosphere and the number of particles will change. Time to do a statistical physics and thermodynamics course maybe?     