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This is also called the ozone layer, where ozone concentrations are as high as 10 parts per million, and is a vitally important region of the atmosphere. This layer of ozone is located approximately 20-50 kilometers above the earth's surface. Stratospheric ozone is important because it prevents most of the high-energy ultraviolet solar radiation from reaching the earth's surface. Photodissociation, a photochemical process, is responsible for the formation of the protective ozone layer in the stratosphere. In the upper atmosphere, diatomic oxygen absorbs high-energy ultraviolet radiation. The absorption of radiation causes the diatomic oxygen molecule to break forming two oxygen radicals. The oxygen radical can then recombine with other diatomic oxygen molecules to form triatomic oxygen, or ozone. In the middle regions of the stratosphere, ozone is found in concentrations as high as 10 parts per million. Ozone can also form in the lower portions of the troposphere, due to anthropogenic activity and by a completely different mechanism. Without the protective stratospheric ozone layer in the upper atmosphere, life (as we know it) on earth would not be possible.

[Science; v263; 71-5; 1994.] [Lancet (North American edition); v342; 1156-8, 1993.]

Source: Atmospheric Chemistry Glossary