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Visibility


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 air pollution, including particles and gases. In addition to limiting the distance that we can see, the scattering and absorption of light caused by air pollution can also degrade the color, clarity, and contrast of scenes. The same particles which are linked to serious health effects can also significantly affect our ability to see.

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

Both primary releases and secondary formation of particles contribute to visibility impairment. "Primary" particles, such as dust from roads or elemental carbon from wood combustion, are emitted directly into the atmosphere. "Secondary" particles are formed in the atmosphere from primary gaseous emissions and include sulfate formed from SO2. In the eastern United States, reduced visibility is mainly attributable to secondarily formed particles. While these secondarily formed particles still dominate in the West, primary emissions from sources such as wood smoke contribute a larger percentage of the total particulate load than in the East.

Good Visibility
Poor Visibility
Great Smokies National Park under a range of visibility conditions.

Humidity can significantly increase the effect of pollution on visibility. Some particles, such as sulfates, accumulate water and grow in size, becoming more efficient at scattering light and causing visibility impairment. Annual average relative humidity levels are 70-80 percent in the East as compared to 50-60 percent in the West. Poor summer visibility in the eastern United States is primarily the result of high sulfate concentrations combined with 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 (ie., 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 g/m3 of fine particles in the atmosphere. The two lower photographs illustrate the change in visual range that occurs by adding 10g/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 visual range 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 SOx during these periods.

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 the 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 Act's national goal of remedying the 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.

Visibility impairment is usually greater in the rural East compared to most of the West. In the rural East, sulfates account for about 50-70 percent of annual average light extinction. Nitrates and organic and elemental carbon all account for between 10-15 percent of total light extinction in most Eastern locations. In the rural West, sulfates also play a significant role, accounting for about 25-40 percent of total light extinction in most regions. Sulfates account for more than 50 percent of annual average light extinction in the Cascades of Oregon. Organic carbon typically is responsible for 15-35 percent of total light extinction in the rural West, elemental carbon accounts for about 15-25 percent, and soil dust accounts for about 10-20 percent. Nitrates typically account for less than 10 percent of total light extinction in western locations, except in the southern California region, where it accounts for almost 40 percent.

Programs to Improve Visibility: EPA is planning to issue a new regional haze program in 1998 to address visibility impairment in national parks and wilderness areas caused by numerous sources located over broad regions. The program will lay out a framework within which States develop implementation plans to achieve "reasonable progress" toward the national visibility goal of remedying any existing and preventing any future human-caused impairment. These plans would include goals for improving visibility over time, as well as emission management strategies to achieve these goals. States will also be required to periodically track progress and revise any strategies as necessary. Because fine particles are frequently transported hundreds of miles, pollution that occurs in one State may contribute to the visibility impairment of another State. Thus, to effectively address the regional haze problem, States are encouraged to coordinate their goals and strategies with each other.

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 is designed to achieve significant reductions in SOx emissions, which is expected to reduce sulfate haze particularly in the eastern United States. Additional control programs on sources of NOx to reduce formation of ozone can also improve regional visibility conditions. In addition, the national ambient air quality standards, programs to remove diesel and NOx from mobile sources, and wood stove programs to reduce fuel combustion and soot emissions can benefit areas adversely impacted by visibility impairment due to sources of organic and elemental carbon.

 


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