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Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.

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. Shenandoah National Park image

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.

Visibility Maps
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 in good and bad visibility conditions
Grand Canyon National Park under a range of visibility conditions.

This document is provided for historical purposes only. The most recent version can be found at AIRTrends

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