Photochemical smog, an air pollution phenomenon discussed in Chapter 13, is a significant contributor to the brown cloud discussed above. It occurs in urban areas where the combination of conditions are right for its formation. emissions and appropriate atmospheric pollution-forming In order for high levels of smog to form, relatively stagnant air must be subjected to sunlight under low humidity conditions in the presence of pollutant nitrogen oxides and hydrocarbons. The auto- mobile is a major source of these pollutants, but hydrocarbons may come from biogenic sources, of which OL-pinene and isoprene from trees are the most abundant (see Section 12.2). Under smog- forming conditions, the urban atmosphere acts as a huge chemical reactor in which hydrocarbons, nitrogen oxides, sulfur oxides, and oxygen undergo reactions driven by sunlight to produce ozone, organic oxidants, aldehydes, organic particles, nitrates, sulfates, and other noxious products. Smog does pose significant hazards to living things and materials in local urban areas in which millions of people are exposed and the oxidants generated by it have detrimental effects on