The dispersion of pollutant gases in the atmosphere from their sources are modeled through plume. It incorporates a lot of factors like wind speed, wind direction, atmospheric temperature, height, type of terrain, overcast or clear skies and so much more. Basically, it can be summarized through unstable or stable conditions.
In unstable conditions, ground level deposition is more dispersed so the concentration is not too high. However, air pollution from elevated smokestacks are readily returned to the ground. In that case, it would still lead to concentrated ground air pollutant concentrations.
In stable conditions, elevated smokestacks are desirable because there is less atmospheric mixing. The concentration of the pollutants is more concentrated in the point sources. Hence, ground level concentration is low.