Why do all heavier gases in the atmosphere not settle down to the surface?

Question

Some pollutants heavier than air that are released into the atmosphere include $\text{CO}_2,\text{SO}_2,\text{SO}_3,\text{NO}_2$ etc. Since these gases are heavier than air, Why do they not settle down closer to the surface? By that, I mean why is the concentration of these gases not maximum at sea level?

This article has been used as a reference: it is evident that the $\text{CO}_2$ concentration around the 200 HPa band is more than the 800 HPa band in the month of July. Note that this article also does not provide clarity on surface $\text{CO}_2$ levels, it gives data for the 5 km-25 km altitude range.

Possible duplicates: https://physics.stackexchange.com/q/247637/2451 , https://physics.stackexchange.com/q/278154/2451 , https://physics.stackexchange.com/q/34733/2451 and links therein. — Qmechanic, May 01 '20 at 09:00
Does this answer your question? $\ CO_2$ is heavier than oxygen and nitrogen still it does not form the lower layer of atmosphere — John Rennie, May 01 '20 at 11:19

score 2 · Answer 1 · answered May 01 '20 at 06:43

The lower portion of the atmosphere (where weather occurs) experiences a lot of vertical mixing. For example, a cumulus cloud that develops into a thunderhead will transport air from near ground-level to heights of 40,000 feet or more. This tends to keep the atmosphere fairly well-mixed.

Why do all heavier gases in the atmosphere not settle down to the surface?

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