Visibility

For visitors and residents, spectacular views are major attractions in the Southern Appalachians. However, visibility has deteriorated considerably since the 1940s, visibility is poorest in the summer, which is the major tourist season. The degradation in visibility is linked to sulfur emissions from the combustion of fossil fuels, such as coal and oil (SAMAB 1996c).

What is visibility?

Visibility describes air-quality conditions that allow appreciation of the texture, color, form, and clarity of landscape features. Although several components interact to determine visibility, including the object being viewed, lighting conditions, and the viewer, the focus of this discussion is the influence of air-quality on visibility (SAMAB 1996c).

In addition to being an important component of the recreational experience, visibility is protected by federal law. The Clean Air Act (CAA) Amendments of 1977 call for the prevention of impaired visibility due to manmade air pollution in Class I federal areas (wilderness areas >5,000 acres and national parks >6,000 acres). In the southern Appalachians, there are seven Class I areas: the James River Face, Linville Gorge, Shining Rock, Joyce Kilmer-Slickrock, and Cohutta Wilderness areas and the Shenandoah and Great Smoky Mountains National Parks (SAMAB 1996c).

Visibility impairment is caused by aerosols (solid or liquid particles dispersed in the air) or gases in the atmosphere. These particles scatter or absorb light in a processcalled light extinction. Scattering efficiency for visible light is greatest for particles and aerosols with diameters in the 0.1-1.0 micron range. The significant chemical components in fine aerosols are sulfates, nitrates, organic carbon, soot (light-absorbing carbon), and soil dust. A wide variety of these pollutants result from daily activities such as driving cars to work, generating electricity to light homes and businesses, and producing consumer goods (SAMAB 1996c).

Visibility impairment is usually a regional problem, rather than a local one. Most visibility impairment results from the transport of emissions and secondary particles by wind, often over hundreds of miles (EPA 1995). Regional haze is caused by the combined effects of emissions from many sources distributed over a large area, rather thanby individual plumes from a few sources at specific sites. Stable atmosphere conditionscause air stagnation,which inhibits movement of pollutants. For a given level of pollution, visibility decreases with increasing relative humidity (Sisler and others 1993). In a humid atmosphere, certain kinds of particles, especially sulfates, attract water and grow to a size that makes them more efficient light scatterers(SAMAB 1996c).

Measuring visibility

Several different parameters are used to express visibility. Standard visual range (SVR), derived from photographs, has been the most commonly used measure of visibility by the Forest Service. SVR, usually expressed in kilometers, is the greatest distance at which an observer can barely see a black object viewed against the horizon sky. The higher the SVR value, the better the visibility conditions(SAMAB 1996c).

The light-extinction coefficient represents the ability of the atmosphere to absorb and scatter light. As the light-extinction coefficient increases, visibility decreases. The transmissometer and nephelometer are used to measures the light-extinction coefficient and the scattering coefficient, respectively. Concentrations of particles in the air directly contribute to the extinction coefficient. The percentage of light extinction attributed to different particle types is often presented in an annual extinction-budget plot, an important method for assessing the causes of visibility impairment(SAMAB 1996c).

The deciview (dv), is a visibility index designed to describe changes in visibility perception across locations with all types of baseline conditions (Pitchford and Malm 1994). The dv value increases as haze increases, so it is known as a haziness index. A change in any given dv should appear to have approximately the same magnitude of visual change regardless of baseline visibility conditions. In contrast, neither SVR nor extinction coefficients have a consistent relationship to perceived visual changes. For example, an improvement of 10 miles in SVR may be quite perceptible at an eastern location with an annual average visibility of 40 miles, but a 10-mile change in SVR may not be perceptible at a western location with an annual average visibility of 150 miles. A 1 deciview change is about a 10 percentchange in the extinction coefficient—a small but perceptible scenic change regardless of location (SAMAB 1996c).

Decline of visibility in the Southern Appalachians

In the Eastern United States, annual average natural background visibility is considered to be 93 ? 30 miles (150 ? 45 kilometers) (Trijonis and others 1991), which corresponds to an average range of 7 to 13 dv. Natural background visibility is defined as the visibility condition without the addition of anthropogenic (human-caused) pollution. Currently, the annual average visibility in the Southern Appalachians is 20 miles (32 kilometers) (Sisler and others 1993), which corresponds to 24 dv(SAMAB 1996c).

Sulfates, which result from sulfur dioxide transformation in the atmosphere, are the largest single human-caused contributor to haziness in the Southern Appalachians. They account for 60 percent of haze on average days. On days with the worst visibility, sulfates account for nearly 80 percent of the fine particulate mass. Analysis of fine particulate data from the Shenandoah and Great Smoky Mountains National Parks shows an annual increase in sulfate of 2 to 3 percent each year between 1982 and 1992. This increasing trend was even more pronounced in the summer, when sulfate concentrations increased 4 percent each year. Poor visibility in the summer is also a function of weather in the Southern Appalachians. In the summer, stagnant air masses often hang over the Southeast, trapping pollution and allowing concentrations to increase. Also, high relative humidity allows sulfate aerosols to collect water and grow to the sizes most likely to cause haziness (SAMAB 1996c).

Visibility in the Southeast degraded between the 1950s and 1960s, improved between the 1970s and 1980s, and has not improved since the 1980s.

Scientists have suggested that the changes from a winter maximum in haze in the 1960s to a summer maximum in the 1980s can be attributed in part to increased sulfate from increased sulfur dioxide emissions at coal-fired electricity generating plants. Another cause may be more complete conversion of precursors (nitrogen oxides, sulfur dioxides, and organics) to particulate matter during the summer(SAMAB 1996c).

Nationally, the 1990 CAA Amendments should reduce sulfur dioxide emissions by 10 million tons below the 1980 level, and there will be a cap on emissions from utilities and industrial sources. Once they are fully implemented, the CAA Amendments of 1990 should lead to visibilityimprovement of2 to 3 deciviews (roughly 3-7 miles) in the summer in the Southern Appalachians (SAMAB 1996c).