A chemical reaction that involves either the absorption or emission of radiation. The absorption of an ultraviolet photon often provides the energy required to break chemical bonds and initiate a reaction sequence. Examples of photochemical reactions are the photolysis of nitrogen dioxide, NO₂ → NO + O, or ozone, O₃ → O₂ + O. The latter reaction leads to the initiation of chain reactions that cause the breakdown of hydrocarbons and other pollutants in the troposphere. See also photolysis, photodecomposition.