A cycle of reactions, known as the Chapman cycle is responsible for the generation of O3, and its importance peaks in a region known as the stratosphere.
The cycle is:
O2 + hv = O + O (reaction 1)
O2 + O + M = O3 + M (reaction 2)
O3 + hv = O2 + O (reaction 3)
O3 + O = 2O2 (reaction 4)
Ozone is a vital filter of UV radiation, and as the cycle shows, it is produced by the reaction of radicals with O2.
At lower altitudes, the O2 concentration is high but there is not enough hv. At higher altitudes there is plenty of hv but not enough O2 and so the cycle works at optimum levels at about 25 km, the level of the stratosphere. If Professor Moore's predictions are correct and the stratosphere were to sink to levels of the same height as the summit of Everest, then the already low levels of O2 could potentially fall even further - fatal to climbers in the aptly named "Death Zone."