Nitrogen dioxide (NO2) is emitted from petrol engines. Ultraviolet light splits this into nitric oxide (NO) and monatomic oxygen (O). The hydrocarbons emitted from the burning of fossil fuels react with some of the monatomic oxygen to form photochemical smog. See Hart et al. (2004) 13th Joint Conference on the Applications of Air Pollution Meteorology. Photochemical smog is most common where the sunshine is strong and long-lasting and where car use is high; see Zhang and Oanh (2002) Atmos. Env. 36, 26 on photochemical smog in Bangkok. Photochemical smog damages plants (Arbaugh et al. (2003) Env. Int. 29, 2–6), and irritates eyes and lungs (Fukuoka (1997) Int. J. Biometeorol. 40, 1).