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When we visit a wilderness or national
park, or look at the skyline of a city, often we do not
enjoy a clear vista -- 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 eastern U.S.
is 15 to 30 miles, or about one-third of what it would be
without human caused air pollution. In the West, the typical
visual range is 60 to 90 miles, or about one-half of the
visual range under natural conditions. Haze diminishes the
natural visual range.
Haze is caused by fine particles that
scatter and absorb light before it reaches the observer.
As the number of fine particles increases, more light is
absorbed and scattered, resulting in less clarity, color,
and visual range.
Five types of fine particles contribute
to haze: sulfates, nitrates, organic carbon, elemental carbon,
and crustal (soil) material. The importance of each type
of particle varies across the U.S. and from season to season.
The typical importance of each particle type in the eastern
and western U.S. is shown in the figure to the right. Details
on each particle type are provided below.
Contribution
of Various Particulates to Haze
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Eastern
U.S.
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Western
U.S.
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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
U.S., due to the region's large number of coal-fired power
plants. In humid environments, sulfate particles grow
rapidly to a size that are very efficient at scattering
light, thereby exacerbating the problem in the East.
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,
but these sources have only a small effect on overall
visibility.
Nitrate Particles form in the air from nitrogen oxide
gas. This gas is released from virtually all combustion
activities, especially those involving cars, trucks, off-road
engines (e.g., construction equipment, lawn mowers, and
boats), power plants, and other industrial sources. Like
sulfates, nitrates scatter more light in humid environments.
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 in mountain
valleys can be largely attributed 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 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.
Haze generally
appears either as uniform haze, layered haze, or plumes. |
A
uniform haze degrades 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 areas, in which case
it is called a regional haze.
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In
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.
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Plumes
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.
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Some of the pollutants that form
haze have been linked to serious health effects and environmental
damage. Exposure to fine 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 U.S. Environmental
Protection Agency (EPA) issued regulations to further
reduce haze and protect visibility across the country.
The EPA and federal land managers from other agencies
are working with state, local and tribal authorities to
promote steady improvements in visibility for decades
to come.
We are challenged to do our part
to help reduce air pollution. Click Here to learn more
about what you can do to reduce
air pollution.
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