Causes of Visibility Impairment


Visiblity Impairment

The United States is endowed with many extraordinary natural areas and Congress has taken steps to preserve and protect many of these areas, such as Wildernesses, National Wildlife Refuges, and National Parks. Unfortunately, visitors to these areas are often prohibited from enjoying the spectacular views they expect. Much of the time a white or brown haze hangs in the air and affects the view. This haze is not natural. It is caused by man-made air pollution, often carried by the wind hundreds of miles from where it originated.

Typical visual range in the western United States is 60 to 90 miles, or about one-half of what it would be without manmade air pollution. In the East, the typical visual range is 15 to 30 miles, or about one-third of the visual range under natural conditions.

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Haze is caused by tiny particles that scatter and absorb light before it reaches an observer. As the number of particles increases, more light is absorbed and scattered, resulting in less clarity, color, and visual range.

Contribution of Various Particulates to Haze - Eastern U.S.Five types of particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal material. The importance of each type of particle varies across the United States and from season to season. The typical importance of each particle type in the eastern and western United States is shown in the figures to the right. Details on each particle type are provided below.

  • Contribution of Various Particulates to Haze - Western U.S.Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries. Sulfates are the largest contributor to haze in the eastern United States, due to the regions large number of coal-fired power plants. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light, thereby exacerbating visibility reductions in the East.
  • Nitrate particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those involving power plants, cars, trucks, off-road engines (e.g., construction equipment, lawn mowers, and boats), and other industrial sources. Like sulfates, nitrates scatter more light in humid environments.
  • Organic carbon particles are emitted directly into the air and also form there as a reaction of various gaseous hydrocarbons. Sources of direct and indirect organic carbon particles include vehicle exhaust, vehicle refueling, solvent evaporation (e.g., paints), food cooking, and various commercial and industrial sources. Gaseous hydrocarbons are also emitted naturally from trees and from fires, and are thought to have been the responsible for causing the blue haze (natural) associated with the Blueridge Mountains.
  • Light absorbing carbon (elemental carbon) particles are very similar to soot. They are smaller than most other particles and tend to absorb rather than scatter light. The "brown clouds" often seen in winter over urban areas and mountain valleys are largely attributable to elemental carbon. These particles are emitted directly into the air from virtually all combustion activities, but are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.
  • Crustal (soil) material is very similar to dust. It enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface activities. Whereas other types of particles form from the condensation and growth of microscopic particles and gasses, crustal material results from the crushing and grinding of larger, earth-born material. Because it is difficult to reduce this material to microscopic sizes, crustal material tends to be larger than other particles and tends to fall from the air sooner, contributing less to the overall effect of haze.

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Types of Visibility Impairment

There are generally three types of visibility impairment: uniform haze, layered haze, and plumes.

  • Joyce KilmerThe picture of Joyce Kilmer-Slickrock Wilderness to the right has a uniform haze degrading visibility evenly across the horizon and from the ground to a height well above the highest features of the landscape. Uniform haze often travels long distances and covers large geographic area, in which case it is called a regional haze.

  • LayeredIn a layered haze, you can see the top edge of the pollution layer. This is often the case when pollution is trapped near the ground beneath a temperature inversion.  The photograph of Cold Mountain to the right shows clear skies above the "brown" and "white veil (haze)" caused by air pollution.

  • PlumePlumes result from local sources. Plumes and plume-like layers of elevated pollution take their shape under certain meteorological condition where the air is stable or constrained.  The picture is looking into Linville Gorge Wilderness and a facility (bottom right) is releasing pollutants into the atmosphere and the plume is dispersing to the left.

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Other Concerns

Some of the pollutants which form haze have been linked to serious health effects and environmental damage. Exposure to particles in the air have been linked with increased respiratory illness, decreased lung function, and premature death. In addition, sulfate and nitrate particles contribute to acidic deposition, which can damage forests, reduce fish populations, and erode buildings, historical monuments, and even car paint.

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To reduce haze we must reduce emissions of haze-forming pollutants across broad areas of the country. Cars, trucks, and industries are much cleaner than they were in the past, and several programs are in place to maintain this progress over the next several years. Nonetheless, these programs by themselves are unlikely to restore visibility to its natural conditions in may protected areas.

In April 1999 the United States Environmental Protection Agency (EPA) issued regulations to further reduce haze and protect visibility across the country. The EPA, federal land managers, and other people interested in improving visibility are working with state, local and tribal air regulatory authorities to promote steady improvements in visibility for decades to come.

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