Volatile organic compounds (VOCs) are emitted into the atmosphere from anthropogenic and biogenic sources1-5 and may also be formed in situ in the atmosphere as products of the atmospheric transformations of other VOCs. On a worldwide basis, emission of VOCs from biogenic sources (mainly vegetation) dominates, with estimated emissions being 1150Tg (C) per year from vegetation2 and ~100 Tg (C)per year from anthropogenic sources.1 Although on a worldwide basis VOCs from biogenic sources dominate over those from anthropogenic sources by a factor of ~10, in urban areas anthropogenic VOCs often dominate. The major classes of emitted VOCs are alkanes, alkenes, aromatic hydrocarbons, and oxygenated compounds, with vegetative emissions typically being composed of alkenes (isoprene, monoterpenes, and sesquiterpenes) and oxygenated VOCs (including 2-methyl-3-buten-2-ol, acetone, methanol, cis-3-hexen-1-ol, cis-3-hexenyl acetate, and camphor)2.3.6 and anthropogenic emissions resulting from vehicle emissions (comprising mainly gasoline constituents, including, in the U.S., the ethanol and methyl tert-butyl ether oxygenates added to the fuel) and emissions from, for example, landfills, refineries, petrochemical sources, solvent usage, and industrial facilities. In urban air in cities such as Los Angeles, CA and Boston, MA, the nonmethane VOCs are typically made up of the following: alkanes, 40-45%; alkenes,10%; aromatic hydrocarbons, 20%; and oxygenates,10-15%; plus unidentified VOCs 7,8 As noted, the alkane and aromatic hydrocarbon fractions of these urban air nonmethane VOCs often have a composition generally similar to that of gasoline. In the atmosphere, VOCs from both anthropogenic and biogenic sources undergo a number of physical and chemical processes leading to their removal from the atmosphere or transformation in the atmosphere.9 Physical removal of VOCs to the earth's surface may occur by dry deposition and wet deposition. These processes have been discussed in detail elsewhere10,11 and are not dealt with here. The chemical processes leading to the atmospheric degradation of VOCs are the subject of this article.