A photochemical reaction requires light energy to take place. Certain pollutants in the atmosphere, nitrogen oxides and hydrocarbons, undergo photochemical reactions. These reactions produce new pollutants, including ozone, aldehydes, and exotic organic compounds. The new pollutants are referred to, in sum, as “photochemical air pollution” because they arise from photochemical reactions. The sources of photochemical air pollution then are the sources of nitrogen oxides and of hydrocarbons. By the same reasoning, the control of photochemical air pollution consists of controlling emissions of nitrogen oxides and of hydrocarbons. The dark spots are the result of elevated ozone levels. Such levels result when excessive use of the automobile leads to high concentration of hydrocarbons and nitrogen oxides.