The volume of air per square meter of ground depends on how high your ceiling is. As ceilings go, the troposphere/stratosphere boundary is pretty high.

And as far as the higher density goes, it does increase the ground-level partial pressure (and thus the concentration per cubic meter) in equilibrium. But the troposphere is not in equilibrium - there is rather significant convection.

- Jake

Friends come and go. Enemies accumulate.

by dense I mean it weighs a lot per unit volume, and hence tends to sink.

Ading to JakeS: you do mean atomic weight in the end (radon mass per unit volume at a given air pressure depends on concentration). But it is actually true that radon can concentrate in 'poodles' due to stratification of still air (in a basement or mine).

_{*Lunatic*, n.
One whose delusions are out of fashion.}