Free radicals in the atmosphere.
Like the oxidation in a flame, the oxidation in the atmosphere is mediated by free radicals. Unlike a flame, however, atmospheric oxidation needs an external source of energy: the sun light. In fact the most important radical acting in the lower atmosphere, the hydroxyl radical, OH, is produced following the UV-photolysis of ozone, O3, which yields an excited oxygen atom, O1D: O3 + h nu----.O1D + O2; lambda less than or equal to 320 nm .O1D + H2O----.OH + .OH OH reacts with most atmospheric trace gases, in many cases as the first and rate determining step in the reaction chain leading to oxidation. In this way a host of various other radicals (e.g. peroxy radicals), most of them very short lived, are generated. Usually these oxidation reactions form chains which regenerate OH, thus maintaining OH at a relatively high concentration level on the order of 10(6) cm-3 during the day. The reactions which control the OH concentration will be discussed in detail. During the night radical formation is greatly diminished. It proceeds, for example, through the reaction of olefines with O3, and, in dry air, through reaction of olefines and aldehydes with the nitrate radical, NO3.