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Download the publication
"Introduction to Visibility."
The above publication
is an excellent introduction to
understand the science of visibility (about 3 MB in size)
Other sites:
FS Images
EPA -
Visibility
VISTAS
VIEWS
IMPROVE |
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Causes of Visibility 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.
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.
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.
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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.
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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.
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Contribution of Various Particulates to Haze
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Eastern U.S.
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Western U.S.
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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.
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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.
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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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There are generally three types of visibility impairment: uniform haze, layered haze, and plumes. |
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The 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.
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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 acid rain, which can damage forests, reduce fish populations, and erode buildings, historical monuments, and even car paint.
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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