Natural and artificially emitted hydrocarbons in the presence of oxides of nitrogen undergo photochemical reactions which produce a cloud of toxic chemicals including ozone and a variety of caustic agents. This process is powered by sunlight and some of the products, such as ozone, reach a peak soon after photon flux from the sun reaches a maximum, around midday. The thermal inversions often associated with some cities can lead to a dangerous buildup of smog in urban areas. Human deaths have been attributed to photochemical smog since the Industrial Revolution in cities such as London and New York. [Science; v.241; p.1473; 1988.] [Atmospheric Environment. Part A, General Topics; v.26A; p.625; 1992.]

This definition is in context to Atmospheric Chemistry. See more contextual defintions for Photochemical Smog.