Temperature increases with elevation. Where most of the ozone is. Ozone is O 3 . Energy is required to split an O 2 molecule. Most ozone on earth is in the stratosphere where it is created by absorption of UV light.
Ozone is highly reactive and highly undesirable to breathe, so ozone in the troposphere is bad.
Before the evolution of photosynthesis, there was no oxygen in the atmosphere. The intensity of the sun's UV radiation was hardly diminished by the atmosphere. UVB radiation, the higher-energy UV light, damages living organisms. DNA is especially susceptible to UVB damage. Most skin cancer is caused by exposure to UVB radiation.