[Anaraxes]

(1) Anderson got it wrong.

The Earth's atmosphere has a pretty consistent composition. It doesn't sort itself out by molecular weight. (The biggest exceptions are more water vapor lower down, and more ozone up in the stratosphere.) Some of the mixing mechanics seem pretty basic to me -- Hadley cells and lesser convection, prevailing winds from Coriolis force -- so I'd find it less plausible that a few quirks of weather or geography would made a difference on an alien planet.

You'd want some sort of process that will preferentially deplete CO2 higher up and/or produce it lower down. CO2 is more stable than ozone, so the latter in particular seems tricky. Maybe you could manage with weathering or some sort of biological process. But I'm no planetary scientist.