Ozone is a gas heavier than air. Why does it not settle around the Earth's surface?

(N/A) Ozone is continuously formed in the stratosphere by the action of $UV$ radiation on dioxygen $(O_2)$ and is simultaneously decomposed by $UV$ radiation of wavelength $240-360 \ nm$.
$O_3 \xrightarrow{240-360 \ nm} O_2 + O$
The atomic oxygen $(O)$ then reacts with $O_3$ molecules:
$O_3 + O \rightarrow 2O_2$
Thus,a dynamic equilibrium is established between the formation and decomposition of $O_3$,maintaining a constant concentration of ozone in the stratosphere rather than settling at the surface.