Chemicals such as non-methane hydrocarbons and oxides of nitrogen, occurring either naturally or as a result of human activities, which contribute to the formation of ozone, a major component of smog
California Air Resources Board ARB (California Environmental Protection Agency), glossary of air pollution terms, Sacramento, CA, USA 2013. http://www.arb.ca.gov/html/gloss.htm, (Air Pollut - Amer) {594}