Air is a mixture of gases, including nitrogen (79%), oxygen (20%), carbon dioxide (0,03%), and several inert gases: argon (almost 1%), helium, xenon, neon, and crypton. Water vapor exists in varying amounts. Air is a finite resource capable of cleansing itself of many, but not all pollutants. There are six major pollutants: carbon monoxide, sulfur oxides, nitrogen oxides, particulates, hydrocarbons, and photochemical oxidants. The major air pollutants come from three principal sources: transportation, stationary sources (factories and power plants), and industrial processes. Air pollutants are released from vaporization (or evaporation), attrition (or friction), and combustion. Combustion is by far the major producer.
The cities can be generally divided into two categories, depending (based) on climate and the type of air pollution. Gray-air cities are generally located in cold, moist climates. The major pollutants are sulfur oxides and particulates. These pollutants combine with atmospheric moisture to form the grayish haze called smog, a term coined in 1905 to describe the mixture of smoke and fog that plagued industrial England. The gray-air cities depend greatly on coal and oil and are usually heavily industrialized. The air in these cities is especially bad during cold, wet winters, when the demand for home heating oil and electricity is heavy and atmospheric moisture content is high.
Brown-air cities are typically located in warm, dry, and sunny climates and are generally newer cities with few polluting industries. The major sources of pollution in these cities are the automobile and the electric power plant, the primary pollutants are carbon monoxide, hydrocarbons, and nitrogen oxides. In brown-air cities atmospheric hydrocarbons and nitrogen oxides from automobiles and power plants react in the presence of sunlight. A number of secondary pollutants such as ozone, formaldehyde, and peroxyacylnitrate (PAN) are formed. The reactions are called photochemical reactions because they involve both sunlight and chemical pollutants. The resulting brownish-orange shroud of air pollution is called photochemical smog. Ozone (O3) is the major photochemical oxidant, a highly reactive chemical, it erodes rubber, irritates the respiratory system, and damages trees.
Because the air laden with photochemical smog often drifts out of the city, the suburbs and surrounding rural areas usually have higher levels of photochemical smog than the city itself.