The hydroxyl radical, OH, is responsible for the oxidation of most of the compounds that are released into, or formed in, the atmosphere. It is ubiquitous in the atmosphere, although its mean concentration is about 106cm−3 and the mixing ratio seldom exceeds 1 ppt (1 part in 1012). In the lower atmosphere it is formed by the reaction of excited oxygen atoms O(1D) with water or by the reaction of hydroperoxyl radicals with nitric oxide. In the stratosphere it is involved in ozone-destroying catalytic cycles. The capacity of the atmosphere to clean itself via the occurrence of oxidation reactions (oxidizing capacity) is usually related to the level of OH present.