The sulfidation of materials upon exposure to the atmosphere has traditionally been attributed entirely to reactions with sulfur-containing gases and water vapor. New laboratory results demonstrate that the sulfidation of poly-crystalline copper can be markedly enhanced both by solar radiation and by the ubiquitous atmospheric ozone, thus indicating that the high rates of corrosion in urban areas are a result of a complex sequence of multi-component photochemical processes. See also AATA 21-1020.