AP® Environmental Science · Unit 9 · Topic 9.1
The Ozone Layer and Ultraviolet Radiation
The ozone layer is a thin band of O3 in the stratosphere that absorbs most of the sun's ultraviolet radiation, shielding DNA in surface life from damage it could not otherwise survive.
Ozone, O3, sits concentrated in the stratosphere roughly 15 to 35 km up. It works as a continuous chemical cycle. A high-energy ultraviolet photon hits an O3 molecule and breaks it apart into O2 and a free oxygen atom. That free atom then collides with another O2 molecule and reforms O3, releasing heat. Each split-and-reform event absorbs a photon's energy, so by the time sunlight reaches the ground, the most damaging wavelengths have already been removed. The cycle runs constantly, so the layer stays roughly in balance between destruction and reformation.
Ultraviolet light comes in three bands that differ by wavelength and energy. UVC has the shortest wavelength and highest energy, and it is almost entirely absorbed by oxygen and ozone high in the atmosphere, never reaching the surface. UVB is intermediate: most is absorbed by ozone, but some gets through, and it is the main cause of sunburn and a major driver of skin cancer. UVA has the longest wavelength and lowest energy, passes through the ozone layer largely unaffected, and reaches the surface in full force, contributing to skin aging and some cancer risk. Stratospheric ozone is beneficial and protective, absorbing incoming UV before it does harm. Ground-level ozone is a pollutant formed by sunlight reacting with vehicle and industrial emissions, and it damages lungs and plant tissue on contact. Same molecule, opposite story depending on altitude. This distinction sets up why the ozone hole and smog are treated as separate environmental problems in this unit, even though both involve ozone.
Key terms
- Stratosphere
- The atmospheric layer roughly 15 to 35 km up where most ozone is concentrated.
- UVC
- The shortest-wavelength, highest-energy ultraviolet radiation, almost completely absorbed before reaching the surface.
- UVB
- Mid-energy ultraviolet radiation, mostly absorbed by ozone but partly reaching the surface, linked to sunburn and skin cancer.
- UVA
- The longest-wavelength, lowest-energy ultraviolet radiation, which passes through the ozone layer largely unabsorbed.
- Ozone-oxygen cycle
- The continuous process where UV photons split O3 into O2 and O, which then recombines into O3.
Watch it explained
- How it worksWalk me through an ozone molecule absorbing an ultraviolet photon, splitting, and reforming.
- How it worksHow do UVA, UVB, and UVC differ in energy and in what actually stops them?
- Common mistakeWhy isn't stratospheric ozone the same problem as ozone at ground level?
- Going furtherWhat happens to skin cancer rates as the ozone column thins by 10 percent?
Check yourself
End of topic.