The ozone layer in the stratosphere protects earth from the sun's ultraviolet radiation. where does the ozone come from?

Ozone is created when energetic photons from the sun such as the UV strikes a molecule of oxygen splitting the two atoms. The radical atom then bombards with another oxygen molecule and join to create an O3 complex called ozone. The rate at which the ozone is created is balanced by the rate at which it is destroyed by the same energetic photons.

when it is at an intermediate population density