Nature and Sources of the Problem:
Visibility impairment occurs as a result of the scattering and absorption
of light by particles and gases in the atmosphere. It is most simply described
as the haze which obscures the clarity, color, texture, and form of what
we see. The same particles which are linked to serious health effects [sulfates,
nitrates, organic carbon, soot (elemental carbon), and soil dust] can significantly
affect our ability to see.
High relative humidity can significantly increase the effect of pollution
on visibility. Some particles, such as sulfates, accumulate water and grow
to sizes at which they are more efficient at scattering light and creating
haze. Poor summer visibility in the eastern U.S. is primarily the result
of high sulfate concentrations exposed to high humidity levels.
Shenandoah National Park under a range of pollution
conditions. Visibility in cleaner areas is more sensitive to increases
in pollution than degraded environments. |
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The same amount of pollution can have dramatically different effects
on visibility, depending on existing conditions. This is illustrated by
the photographs above which characterize visibility in Shenandoah National
Park under a range of conditions. The top left photograph represents a
"clear" day at Shenandoah (80 miles visual range). These conditions are
close to naturally-occurring visibility (i.e., without human-made pollution).
An average day at Shenandoah is represented by the top right photograph
(18 miles visual range), and is the result of an additional 10 µg/m3
of fine particles in the atmosphere. The two lower photographs illustrate
the change in visual range that occurs by adding 10 µ/m3
of fine particles to the area when the air is already degraded. It shows
that small amounts of air pollution in cleaner areas can have dramatic
effects on visibility impairment. It also implies that more emission reductions
may be needed in heavily degraded environments to make noticeable differences.
Long Term Trends:
Visibility impairment has been analyzed using data collected since 1960
at 280 monitoring stations located at airports across the country. At these
stations, measurements of visual range (the maximum distance at which an
observer can discern the outline of an object) were recorded. The following
maps show the amount of haze during the summer months of 1970, 1980, and
1990. The dark blue color represents the best visibility and red represents
the worst visibility. Overall, the maps show that visibility impairment
in the eastern U.S. increased greatly between 1970 and 1980 and decreased
slightly between 1980 and 1990. This follows the overall trend in emissions
of sulfur oxides, which are a major source of fine particles and reduced
visibility.
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Maps of haze from airport visual data (July-September) show the
amount of summertime haze (visibility impairment) during 1970, 1980,
and 1990. Haze in the eastern U.S. increased significantly between 1970
and 1980, and decreased slightly between 1980 and 1990. |
Visibility Monitoring Network:
In 1987, a visibility monitoring network was established as a cooperative
effort between EPA, States, National Park Service, U.S. Forest Service,
Bureau of Land Management, and U.S. Fish and Wildlife Service. The network
is designed to track progress toward the Clean Air Act¹s national
goal of remedying existing and preventing future visibility impairment
in national parks and wilderness areas. The network is the largest in the
country devoted to fully characterizing visibility. It also provides information
for determining the types of pollutants and sources primarily responsible
for reduced visibility.
In many parts of the U.S., sulfates are the largest single contributor
to haze. Data from this monitoring network reveal that sulfates account
for approximately twothirds of the visibility reduction in the Appalachian
Mountains in the East. Organic carbon, the next-largest contributor, causes
about 15 percent of visibility reduction. In most areas of the western
U.S. and Alaska, sulfates and organic particles contribute equally to haze.
In southern California, nitrate particles are the greatest contributor
to haze.
Programs to Improve Visibility:
In April 1994, EPA began developing a new regional haze program to address
visibility impairment in national parks and wilderness areas. This program
will introduce new approaches to monitoring and modeling regional haze
as well as define a policy for achieving "reasonable progress"
toward the reduction of visibility impairment.
The program will build on efforts of the Grand Canyon Visibility Transport
Commission, which was established to assess options for improving visibility
impairment, particularly for protection of national parks and wilderness
areas on the Colorado Plateau. In June 1996, this Commission issued its
report, Recommendations for Improving Western Vistas. Some of the recommendations
in its report include:
- Continuing implementation of Clean Air Act requirements for reducing
SO2 emissions from stationary sources through the year 2000.
After 2000, establish SO2 emission targets and a plan for an
emissions cap and trading program.
- Decreasing mobile source emissions through a mix of national, regional,
and local strategies.
- Minimizing visibility impairment caused by controlled burning.
- Improving regional monitoring and emissions tracking capabilities.
Other air quality programs are expected to lead to emission reductions
that will improve visibility in certain regions of the country. The Acid
Rain Program has achieved significant reductions in SO2 emissions,
which are expected to lead to improvements in visibility impairment caused
by sulfate haze, particularly in the eastern U.S. Better controls on NOx
sources also can improve regional visibility conditions. Other programs,
such as EPA's NAAQS, mobile source and woodstove programs to reduce particulate
emissions, can benefit areas impacted by visibility impairment.
Grand Canyon National Park under a range of visibility
conditions.
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