Atmospheric trace gases are primarily removed by reactions with radicals and therefore radicals play an important role in the chemistry of the atmosphere. Radicals in the atmosphere are produced by the photodissociation of molecules, or by the reaction of a primary radical with another molecule. In Table 1 a short overview of several important atmospheric radical sources is given. From this table it can also be seen that many of the follow-up reactions produce the source molecules, for example the NO_2 produces O(~3P) which in turn can produce O_3, which may lead to OH radicals. The OH radical reacts with many different compounds, cleaning the atmosphere of various trace gases (Ehhalt, 1999).
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