Photolysis (or photoreaction) can be defined as “any chemical reaction that occurs only in the presence of light.” Environmental photoreactions necessarily take place in the presence of sunlight, which has significant photon fluxes only above 295 nm in the near ultraviolet (UV), extending into the infrared region of the electromagnetic spectrum. Environmental photoreactions occur in surface waters, on soil, and in the atmosphere, sometimes rapidly enough to make them the dominant environmental transformation processes for many organic compounds. In the atmosphere, for example, photooxidation, mediated by

hydroxyl radical (HO), is the dominant loss process for more than 90% of the organic compounds found there (See Atkinson, 1989, 1994, and Chapter 14).