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Triatomic oxygen, a very reactive, irritant gas, mainly formed in the atmosphere by various photochemical reactions. Its concentration is measured in Dobson units. It is found primarily at three distinct levels: in the ozone layer at 10–50 km, where the major portion is created through the absorption by oxygen molecules of ultraviolet radiation from the Sun; in the upper troposphere (9–13 km) where nitrogen oxides from lightning and aircraft exhaust gases are significant sources; and at ground level, where it is produced by reactions involving various pollutants (particularly nitrogen oxides and hydrocarbons). Its lifetime at this level is c.25 days. In the upper troposphere it acts as a powerful greenhouse gas, and its increasing concentration at this altitude is a cause for concern (see also solar wind.)

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