Ozone

In the stratosphere about 30km up is a layer of a gas called ozone. It is dispersed among other gases in an extremely tiny quantity. If all the ozone in the atmosphere were collected and brought to the earth;'s surface at atmospheric pressure it would from a layer 3mm thick.

Ozone is the earth's natural sunscreen. it absorbs ultra violet light. When ultra violet light of the right frequency hits oxygen molecules in the stratosphere the oxygen atoms dissociate forming two oxygen radicals.
O2 → O + O

Ozone is formed when an oxygen molecule reacts with an oxygen atom. Ozone is not the only possible product. Oxygen atoms can also react with other oxygen atoms or ozone molecules to form oxygen molecules.
O2 + O → O3 ΔH = -100kJ mol-1
O + O → O2 ΔH = -498kJ mol-1
O3 + O → O2 + O2 ΔH = -390kJ mol-1

Ozone absorbs ultra violet light in the following reaction. This reaction is predominantly responsible for the sun screening properties of ozone.
O3 + ultraviolet light → O2 + O

Ozone is thus being created and destroyed at the same time in the ozone layer. The rate of production should equal the rate of destruction meaning the concentration of ozone should be stable.