At the large scale of the atmosphere and in the lower layers, the relative weight of gases doesn't matter (otherwise Oxygen and Nitrogen would seperate out and

At the large scale of the atmosphere and in the lower layers, the relative weight of gases doesn't matter (otherwise Oxygen and Nitrogen would seperate out and there wouldn't be a mix of other gases in it either). The makeup down here is largely controller by convection and turbulences, only farther up the relative weight starts to matter.