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Ozone is restricted to a thin layer in the Earth's atmosphere, the ozonosphere, with a maximum concentration between 20 and 26km above the Earth's surface. It is formed through the combination of atomic oxygen ( 0 ) and molecular oxygen (02) . Atomic oxygen is formed by the photo-dissociation of 0, at around 100 km by solar radiation with A < 175 nm, the process for which may be summarized as:

The free oxygen atoms may then combine with oxygen molecules in a three-body collision (to conserve energy) to form ozone:

where M is any atom or molecule (e.g. 0 2 , N,) capable of absorbing the excess energy liberated in the exothermic chemical reaction.