The pollution-laden brown cloud long associated with Denver, Colorado, can also appear in the smaller city of Boulder (as seen here). 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. Scientists from the National Center for Atmospheric Research, universities, and state agencies have investigated both the chemical composition of the Denver area's brown cloud and its daily movements.
