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Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.
Nature and Sources of the Problem:
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.
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:
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:
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:
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.