THE OZONE LAYER
When O2 absorbs sunlight it splits into two oxygen atoms:
O2 = O + O
Ozone is produced when one oxygen atom and one O2 collide with sufficient energy:
O2 + O = O3
This second reaction only occurs when both sufficient O2 concentration and sufficient sunlight are available. Therefore ozone is formed in a layer between 15 and 35km above earth, where both O2 and high energy UV concentrations are ideal. This is the ozone layer. Above 35km altitude there is more free oxygen than O2 because of the large amount of sunlight reaching this region, so most free oxygen atoms collide with other free oxygen atoms to reform O2 rather than forming ozone. Below 15km altitude most of the high energy UV has been filtered from the sunlight when breaking O2 at higher altitudes, so at low altitudes O2 is not split to form the free oxygen atoms needed for ozone formation.