In our nation's scenic areas, the visual range has been substantially reduced by air pollution. In eastern parks, average visual range has decreased from 90 miles to 15-25 miles. In the West, visual range has decreased from 140 miles to 35-90 miles.

Haze is caused when sunlight encounters tiny pollution particles in the air. Some light is absorbed by particles. Other light is scattered away before it reaches an observer. More pollutants mean more absorption and scattering of light, which reduce the clarity and color of what we see. Some types of particles such as sulfates, scatter more light, particularly during humid conditions.

Air pollutants come from a variety of natural and manmade sources. Natural sources can include windblown dust, and soot from wildfires. Manmade sources can include motor vehicles, electric utility and industrial fuel burning, and manufacturing operations.

Some haze-causing particles are directly emitted to the air. Others are formed when gases emitted to the air form particles as they are carried many miles from the source of the pollutants.

Some of the pollutants which form haze have also been linked to serious health problems and environmental damage. Exposure to very small particles in the air have been linked with increased respiratory illness, decreased lung function, and even premature death. In addition, particles such as nitrates and sulfates contribute to acid rain formation which makes lakes, rivers, and streams unsuitable for many fish, and erodes buildings, historical monuments, and paint on cars.

Sulfates and nitrates that form in the atmosphere from sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions contribute to visibility impairment, meaning we can't see as far or as clearly through the air. Sulfate particles account for 50 to 70 percent of the visibility reduction in the eastern part of the United States, affecting our enjoyment of national parks, such as the Shenandoah and the Great Smoky Mountains. The Acid Rain Program is expected to improve the visual range in the eastern U.S. by 30 percent. Based on a study of the value national park visitors place on visibility, the visual range improvements expected at national parks of the eastern United States due to the Acid Rain Program's SO2 reductions will be worth over a billion dollars annually by the year 2010. In the western part of the United States, nitrates and carbon also play roles, but sulfates have been implicated as an important source of visibility impairment in many of the Colorado River Plateau national parks, including the Grand Canyon, Canyonlands, and Bryce Canyon.

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 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.

Haze is relatively uniform at the horizon but tends to diminish slightly at higher elevations. Look at the levels of man-made pollutants including fine particles, black carbon and ozone. Also, note the relative humidity. Haze often occurs on hot, humid summer days with medium or high levels of fine particles, ozone and sometimes black carbon. Relative humidity tends to be medium to high.

A brown cloud appears to envelop the scene but quickly thins out at higher elevations. Brown clouds tend to occur on calm winter mornings during rush hour traffic. Look at the particle and black carbon levels -- they are usually high. Ozone will be low and relative humidity may vary.

Air pollution, including particulates (soot) and related gases (nitrogen dioxide and sulfur dioxide) can scatter and absorb light, limiting the distance that one can see and obscuring color and clarity. Visibility can often be reduced over large regions, and is therefore called Regional Haze. Haze makes the outline of a skyline or a natural vista difficult to see.

The Grand Canyon Visibility Transport Commission conducted a study on the economic impact of visibility impairment on national parks and wilderness areas and the cost of controls. The estimated value for the prevention or elimination of haze ranged from $3 to $6 dollars per day per visitor-party at the park. In 2004, 4,308,549 people visited the Grand Canyon.

Impacts of Haze:

• What is Visibility Impairment?
• Causes of Poor Visibility
• Links to all National Parks (click on "Facts" for visitor information)
• "Hazy Days in Our Parks" - Smithsonian Magazine, June 2005

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