In the Los Angeles atmosphere photochemical reactions are important in producing eye irritation, plant damage, and ozone. The components of these reactions are regarded as oxygen and organic materials in general, especially hydrocarbons and their oxidation products. A prominent suspect as the sensitizer, that is primary light absorbing material, has been nitrogen dioxide; it is known to absorb strongly over most of the solar spectrum at sea level; mixtures of nitrogen dioxide, air, and hydrocarbons in sunlight have been shown to yield most of the symptoms of smog. Also, large amounts of biacetyl in sunlight produce ozone among the reaction products. Since the photochemical decomposition of biacetyl yields free radicals, it is strongly implied that free radicals and oxygen interact in some way to yield ozone. Extensive literature on thermal and photochemical oxidation of organic materials gives considerable insight into the mechanisms probably involved in these cases.