A large confined volume in which sunlight or simulated sunlight is allowed to irradiate air mixtures of atmospheric trace gases (@[email protected], nitrogen oxides, sulfur dioxide, etc.) which undergo @[email protected] In theory these chambers allow the controlled study of complex reactions which occur in the atmosphere. However, ill-defined wall reactions which generate some molecular and radical species (e.g. HONO, CH2O, HO-radicals, etc.) and remove certain products (H2O2, HNO3, etc.), the use of reactant concentrations well above those in the atmosphere, ill-defined light intensities and @[email protected] distribution within the chamber, and other factors peculiar to chamber experiments require that caution be exercised in the extrapolation of results obtained from them to atmospheric systems.