photochemical smog

View more Science videos

A noxious smog produced by the reaction of nitrogen oxides with hydrocarbons in the presence of ultraviolet light from the sun. The reaction is very complex and one of the products is ozone.

Nitrogen dioxide (NO₂) is emitted from petrol engines. Ultraviolet light splits this into nitric oxide (NO) and monatomic oxygen (O). This oxygen reacts with free atmospheric oxygen (O₂) to form ozone (O₃) which is irritating to the lungs. The ozone also reacts with the nitric oxide to make further nitrogen dioxide in a dangerous feedback loop. Furthermore, the hydro-carbons emitted from the burning of fossil fuels react with some of the monatomic oxygen to form photochemical smog. Photochemical smog is most common in areas like Mexico City or Los Angeles where the sunshine is strong and long-lasting and where car use is high. While it is less visible than ordinary smog (although sunsets may be tinged with purple and green), photochemical smog can irritate eyes and lungs and damage plants.

Air pollution produced by the action of sunlight on hydrocarbons, nitrogen oxides, and other pollutants.