Forest Trail Explorer

US Forest Service and Southern Research Station

Air Quality

Natural Background (1860)
Worst Baseline (2000–2004)
Worst (estimated progress for 2018)
Natural Background (2064)

Clean Air Act of 1970

Air pollution in Western North Carolina, specifically total sulfate and nitrate deposition, has been intensely studied on 44 sites for over a century. Study sites demonstrate a significant increase in total sulfate deposition from 1860–1970 followed by a significant decline after the Clean Air Act of 1970 (CAA). The CAA mandates pollution control devices and other techniques be adopted to meet targeted reduction levels. In 1977, an amendment to the CAA set a national goal to prevent any future, and remedy any existing, impairment of visibility in mandatory Class I areas which are a result of human-caused air pollution.

Visibility Conditions

There are four federally mandated Class I areas in Western North Carolina: Joyce Kilmer-Slickrock Wilderness, Linville Gorge Wilderness, Shining Rock Wilderness, and the Great Smoky Mountains National Park. To achieve the visibility goal set by the CAA, a Regional Haze Rule has been implemented to achieve natural background visibility (following reasonable progress) by the year 2064.

The first year to determine if reasonable progress is being achieved is 2018. The initial emissions reductions will focus on reducing sulfur dioxide emissions in order to improve visibility by 2018 in comparison to the 2000–2004 baseline conditions. Information collected from study sites predict a decrease in total sulfate deposition between 2010 and 2018, and it is reasonable to assume that further sulfur dioxide emission reductions will be required to achieve natural background visibility at the four Class I areas by 2064.

Man-made emissions of sulfur dioxide, nitrogen oxides, and ammonia are converted in the atmosphere and deposited on the ground. The deposition of sulfates, nitrates, and ammonia can occur in three forms: wet, dry, and cloud water. Before dry deposition can occur, sulfates and nitrates, along with other pollutants, contribute to a uniform haze that obscures scenic views in Western North Carolina.

At most low elevation locations in the eastern United States, the annual amount of wet and dry deposition is similar. Fog may be an additional source of deposition below 3,500 feet elevation, but is considered a minor component, contributing less than 10 percent of the annual total. Cloud water, however, is a significant contributor at elevations above 3,500 feet, and the region’s forests may be immersed in clouds 30–50 percent of the year. Moreover, elevations above 3,500 feet may have twice as much deposition as lower elevation sites as a result of the larger amount of acid compounds deposited from the cloud water.

The above content is a part of the Western North Carolina Vitality Index. To view the full report, visit

Forest Inventory and Analysis Resources

For more information on FIA, please click here.


North Carolina Department of Environment and Natural Resources, Division of Air Quality. “North Carolina Division of Air Quality, Regional Haze State Implementation Plan for North Carolina Class I Areas.” Accessed from:

USDA Forest Service, National Forests in North Carolina. Jackson, W., personal communication, 2010.

Photo Credits

Simulated visibility conditions in Joyce Kilmer-Slickrock Wilderness Area (1860, 2000-2004, 2018, 2064), Winhaze model (

Visibility Conditions at Shining Rock Wilderness Area: