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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.

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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.

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