Although smog originally denoted a blend of smoke and fog, it now refers more generally to dense, visible air pollution, as seen here in Denver. In older industrial cities, such as London and New York, pollution traditionally arose from large-scale burning of coal and fuel oil that released tons of ashes, soot, and sulfur compounds. More recently--especially in lower-latitude cities such as Los Angeles and Denver--automobiles have been a primary culprit for pollution and smog. Nitric oxide from automobile exhaust combines with oxygen in the air to form the brown gas nitrogen dioxide. Also, when hydrocarbons and nitrous oxides from auto emissions are exposed to sunlight, a photochemical reaction helps lead to ozone and other irritating compounds. Smog is especially likely during an inversion (a reversal in the atmosphere's usual temperature structure in which a shallow layer of relatively cool air rests near the ground and traps pollution beneath relatively warm air).
