The Southern Appalachian Mountains are the highest in eastern North America, and combined with predominant weather patterns, tend to trap pollutants such as those produced by power plants, factories, and automobiles. Great Smoky Mountains National Park (GRSM) experiences some of the highest deposition rates of sulfur and nitrogen in North America. Despite declining national trends, sulfur dioxide emissions affecting the Southern Appalachians have increased in the past two decades, with very negative effects on air quality and visibility. These pollutants are deposited in the form of rainfall, dry particles and cloud water. The average annual pH of rainfall in the Smokies is 10 times more acidic than natural rainfall. Clouds with acidity as low as pH 2.0 bathe high elevation forests during much of the growing season. Some high-elevation park streams have the highest nitrate levels of any systems in the country that drain undisturbed watersheds.
Research in the Smokies has shown that some high-elevation soils in the park are receiving so much airborne nitrogen they are suffering from advanced nitrogen saturation. This limits the availability of nutrients to forest plants and increases the presence of toxic ions such as aluminum that can harm vegetation and aquatic biota. It is suspected that similar conditions may exist in the high-elevation portions of Blue Ridge Parkway. In the Appalachian Highlands Network (APHN), Great Smoky Mountain National Park is a designated Class I air quality area, while the other three network parks are Class II air quality areas. Great Smoky Mountain NP has on-site ambient air-quality monitoring; the other parks have nearby monitors. The air pollution issues of most significant concern for the network are ozone, acid deposition and visibility. An ozone injury risk assessment indicates the risk of injury is high in the Smokies and on the Blue Ridge Parkway.
The Appalalchian Highlands Network examines key air quality indicators including ozone, visibility, and atmospheric depostion, which affects ecological health through acidification and fertilization of soils and surface waters. Monitoring is conduted in all four network parks. Specifically, the network:
- Reports on seasonal and annual trends in nitrogen and sulfur deposition at existing monitoring stations in and near network parks.
- Reports on seasonal and annual trends in fine particle concentration at existing monitoring stations in and near network parks.
- Reports on seasonal and annual trends in ozone concentration near APHN parks using metrics that are indicative of human health (e.g., 8-hour average) and plant response (e.g., SUM06).