Photochemical smog is a major contributor to air pollution. The word "smog" was originally coined as a portmanteau of "smoke" and "fog." Smog was historically used to describe air pollution produced from the burning of coal, which released smoke and sulfur dioxide. London, during the 19th and 20th centuries, was particularly well-known for this type of air pollution. The "Great Smog of 1952" was identified as the cause of over four thousand deaths in London. While air pollution caused by the burning of coal has become less common, the combustion of fossil fuels continues to affect air quality.
The components of photochemical smog were established during the 1950s. This type of air pollution is formed through the reaction of solar radiation with airborne pollutants like nitrogen oxides and volatile organic compounds. These compounds, which are called primary pollutants, are often introduced into the atmosphere through vehicular emissions and industrial processes. Ultraviolet light can split nitrogen dioxide into nitric oxide and monatomic oxygen. This monatomic oxygen can then react with oxygen gas to form ozone. Products like ozone, aldehydes, and peroxyacetyl nitrate are called secondary pollutants. The mixture of these primary and secondary pollutants forms photochemical smog.
Both the primary and secondary pollutants in photochemical smog are highly reactive. These oxidizing compounds have been linked to a variety of negative health outcomes. For example, ozone is known to irritate the lungs. Smog is a particular health danger in the sunniest and most populated cities like Los Angeles. An image of smog in New York is shown in the figure below.