It discolors paints and tarnishes many metals. This gas is produced largely at sewage treatment plants and at oil refineries as a by-product in refining crude oil. Concentrations of H2S are limited by District regulations. Photochemical SMOG Ozone Photochemical air pollution or photochemical smog results from a chemical reaction which takes place in the atmosphere between NO2 and reactive organic gases under the photochemical influence of sunlight. Various factors affect this process, including the quantity of gases present, the volume of air available for dilution, the temperature, and the intensity of the ultraviolet light.
Ideal conditions occur in the summer and early fall on warm, windless, sunny days.
The major effects of photochemical smog are aggravation of respiratory diseases, eye irritation, visibility reduction and vegetation damage. The largest fraction of photochemical smog is ozone. The greatest source of smog-producing gases is the automobile. In the Bay Area, more than 50 percent of the reactive organic gases and nitrogen oxides comes from cars and trucks. California's automobile control program together with the District's regulatory controls have reduced excesses of the national ozone standard from a high of 65 days in 1969.