SOURCES AND EFFECT OF AIR POLLUTANTS
Air pollution occurs in many forms but can generally be thought of as gaseous and particulate contaminants that are present in the earth's atmosphere. Gaseous pollutants include sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), carbon monoxide (CO), volatile organic compounds (VOC), hydrogen sulfide (H2S), hydrogen fluoride (HF), and various gaseous forms of metals. These pollutants are emitted from large stationary sources such as fossil fuel fired power plants, smelters, industrial boilers, petroleum refineries, and manufacturing facilities as well as from area and mobile sources. They are corrosive to various materials which causes damage to cultural resources, can cause injury to ecosystems and organisms, aggravate respiratory diseases, and reduce visibility.
Particulates come in both large and small or "fine" solid forms. Large particulates include substances such as dust, asbestos fibers, and lead. Fine particulates include sulfates (SO4) and nitrates (NO3). Important sources of particulates are power plants, smelters, mining operations, and automobiles. Asbestos and lead affect organisms, while sulfates and nitrates not only cause health problems, but also contribute to acid rain or acid deposition and a reduction in visibility. Particulate matter, a term sometimes used instead of particulates, refers to the mixture of solid particles and liquid droplets found in the air.
Toxic air pollutants are a class of chemicals which may potentially cause health problems in a significant way. The sources of toxic air pollutants include power plants, industries, pesticide application, and contaminated windblown dust. Persistent toxic pollutants, such as mercury, are of particular concern because of their global mobility and ability to accumulate in the food chain. More research is needed to fully understand the fate and effects of mercury and the many other toxic pollutants.
Primary pollutants are those that are emitted directly into the air from pollution sources. Secondary pollutants are formed when primary pollutants undergo chemical changes in the atmosphere. Ozone is an example of a secondary pollutant. It is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) are mixed and warmed by sunlight. Ozone (O3) is a major component of what is often referred to as smog. The ozone which is present in the troposphere, or the atmosphere that is close to the ground, should not be confused with beneficial ozone that is located in the stratosphere or upper atmosphere. This beneficial ozone in the stratosphere helps protect the earth from harmful ultraviolet light from the sun.