EE-Unit-II Air Pollution Types and Sources

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 (NO_x), 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 (NO_x) 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.

Sources of Air Pollution

Stationary and Area Sources
A stationary source of air pollution refers to an emission source that does not move, also known as a point source. Stationary sources include factories, power plants, dry cleaners and degreasing operations. The term area source is used to describe many small sources of air pollution located together whose individual emissions may be below thresholds of concern, but whose collective emissions can be significant. Residential wood burners are a good example of a small source, but when combined with many other small sources, they can contribute to local and regional air pollution levels. Area sources can also be thought of as non-point sources, such as construction of housing developments, dry lake beds, and landfills.

Mobile Sources
A mobile source of air pollution refers to a source that is capable of moving under its own power. In general, mobile sources imply “on-road” transportation, which includes vehicles such as cars, sport utility vehicles, and buses. In addition, there is also a “non-road” or “off-road” category that includes gas-powered lawn tools and mowers, farm and construction equipment, recreational vehicles, boats, planes, and trains.

Agricultural Sources
Agricultural operations, those that raise animals and grow crops, can generate emissions of gases and particulate matter. For example, animals confined to a barn or restricted area (rather than field grazing), produce large amounts of manure. Manure emits various gases, particularly ammonia into the air. This ammonia can be emitted from the animal houses, manure storage areas, or from the land after the manure is applied. In crop production, the misapplication of fertilizers, herbicides, and pesticides can potentially result in aerial drift of these materials and harm may be caused.

Natural Sources
Although industrialization and the use of motor vehicles are overwhelmingly the most significant contributors to air pollution, there are important natural sources of “pollution” as well. Wildland fires, dust storms, and volcanic activity also contribute gases and particulates to our atmosphere.

Unlike the above mentioned sources of air pollution, natural “air pollution” is not caused by people or their activities. An erupting volcano emits particulate matter and gases; forest and prairie fires can emit large quantities of “pollutants”; plants and trees naturally emit VOCs which are oxidized and form aerosols that can cause a natural blue haze; and dust storms can create large amounts of particulate matter. Wild animals in their natural habitat are also considered natural sources of “pollution”. The National Park Service recognizes that each of these sources emits gases and particulate matter into the atmosphere but we regard these as constituents resulting from natural processes.