Air Quality at Great Smoky Mountains National Park
Most visitors expect clean air and clear views in parks. Great Smoky Mountains National Park (NP) in North Carolina and Tennessee, experiences some of the highest measured air pollution of any national park in the U.S. Research and monitoring conducted in the park has shown that airborne pollutants emitted from mostly outside the Smokies are degrading park resources and visitor enjoyment. The burning of fossil fuels—coal, oil, and gas—causes most of the pollution.
Wind currents moving toward the southern Appalachians transport pollutants from urban areas, industrial sites, and power plants located both near and far. The height and physical structure of the mountains, combined with predominant weather patterns, tend to trap and concentrate human-made pollutants in and around the national park.
Air pollutants carried into the park can harm natural and scenic resources including streams, soils, forests, fish and wildlife, and visibility. The National Park Service works to address air pollution effects at Great Smoky Mountains NP, and in parks across the U.S., through science, policy and planning, and by doing our part.
Nitrogen and Sulfur
Nitrogen (N) and sulfur (S) compounds deposited from the air may have harmful effects on ecosystem processes. Healthy ecosystems can naturally buffer a certain amount of pollution, but once a threshold is passed the ecosystem may respond negatively. This threshold is the critical load, or the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). N and S deposition change ecosystems through eutrophication (N deposition) and acidification (N + S deposition). Eutrophication increases soil and water nutrients which causes some species to grow more quickly and changes community composition. Ecosystem sensitivity to nutrient N enrichment at Great Smoky Mountains National Park (GRSM) relative to other national parks is very low (Sullivan et al. 2016); for a full list of N sensitive ecosystem components, see: NPS ARD 2019. Acidification leaches important cations from soils, lakes, ponds, and streams which decreases habitat quality. Ecosystem sensitivity to acidification at GRSM relative to other national parks is very high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in GRSM ranged from 5.7 to 8.1 kg-N ha-1 yr-1 and total S deposition ranged from 1.7 to 2.6 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). GRSM has been monitoring atmospheric N and S deposition since 1980, see the conditions and trends website for park-specific information.
GRSM receives the highest level of acid deposition of any monitored national park. Although the Acid Rain Program has significantly reduced acid deposition throughout the East, problems remain. High ridge top ecosystems at GRSM, especially soils, are particularly vulnerable to acid deposition from high concentrations of N and S compounds. These systems receive more deposition from rain, fog, and clouds than lower elevation areas. Additionally, low buffering capacity, short growing seasons, and shallow soils make higher elevation areas more sensitive to acid inputs.
Additional N and S research:
- Acid rain with an average acidity (pH) as low as 4.6, is 3–8 times more acidic than normal rainfall (NADP 2018)
- Acidic clouds and fog (pH 2.0) that cover high elevation forests at times, contribute to the decline of old growth red spruce forests (MADPro 2007; Cole 1992; Li & Aneja 1992; Lovett et al. 1982)
- Acidification of forest soils, promotes loss of plant nutrients and release of toxic aluminum harmful to vegetation and stream life (Eagar & Adams 1992; Johnson et al. 1991)
- Acidification of high elevation streams contributes to declines in aquatic diversity and native brook trout (SAMI 2002; Herlihy 1996)
- Some high elevation park streams that drain undisturbed watersheds are the highest nitrate levels of any systems in the U.S. Nitrate levels in these streams approach the public health standard for drinking water (Stoddard 1994)
- Several streams in the park have been designated as "impaired" by the State of Tennessee because of acidification
Epiphytic macrolichen community responses
Epiphytic macrolichens grow on tree trunks, branches, and boles. Since these lichens grow above the ground, they obtain all their nutrients directly from precipitation and the air. Many epiphytic lichen species have narrow environmental niches and are extremely sensitive to changes in air pollution. Geiser et al. (2019) used a U.S. Forest Service national survey to develop critical loads of nitrogen (N) and critical loads of sulfur (S) to prevent more than a 20% decline in four lichen community metrics: total species richness, pollution sensitive species richness, forage lichen abundance, and cyanolichen abundance.
McCoy et al. (2021) used forested area from the National Land Cover Database to estimate the impact of air pollution on epiphytic lichen communities. Forested area makes up 2023 km2 (96%) of the land area of Smoky Mountains National Park.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 98.5% of the forested area.
- S deposition exceeded the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in 8.3% of the forested area.
For exceedances of other lichen metrics and the predicted decline of lichen communities see Appendices A and B of McCoy et al. (2021).
Additional modeling was done on 459 lichen species to test the combined effects of air pollution and climate gradients (Geiser et al. 2021). A critical load indicative of initial shifts from pollution-sensitive toward pollution-tolerant species occurred at 1.5 kg-N ha-1 yr-1 and 2.7 kg-S ha-1 yr-1 even under changing climate regimes.
Plant species response
Plants vary in their tolerance of eutrophication and acidification, and some plant species respond to nitrogen (N) or sulfur (S) pollution with declines in growth, survival, or abundance on the landscape. Horn et al. (2018) used the U.S. Forest Service national forest survey to develop critical loads of N and critical loads of S to prevent declines in growth or survival of sensitive tree species. Clark et al. (2019) used a database of plant community surveys to develop critical loads of N and critical loads of S to prevent a decline in abundance of sensitive herbaceous plant species. According to NPSpecies, Smoky Mountains National Park contains:
- 35 N-sensitive tree species and 117 N-sensitive herbaceous species.
- 40 S-sensitive tree species and 90 S-sensitive herbaceous species.
At Great Smoky Mountains NP, scenic views are often affected by haze that reduces how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air, and these particles can also affect human health. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze. Organic compounds, soot, and dust reduce visibility as well. Pollution-caused haze typically appears as a uniform whitish haze, different from the natural blue mist-like clouds for which the Smokies were named. Significant improvements in park visibility have been documented since the 1990’s. Still, visibility in the park is a long way from the Clean Air Act goal of no human caused impairment.
- Reduction of the average natural visual range from about 110 miles (without the effects of pollution) to about 60 miles because of pollution at the park
- Reduction of the visual range in the summer from about 80 miles to below 35 miles on high pollution days
- Severe haze episodes can reduce visibility to 5 miles or less
Visit the NPS air quality conditions and trends website for park-specific visibility information. Great Smoky Mountains NP has been monitoring visibility since 1988. Check out the live air quality webcams at Look Rock, Purchase Knob, and Clingmans Dome; and explore air monitoring »
At ground level, ozone is harmful to human health and the environment. Ground-level ozone does not come directly from smokestacks or vehicles, but instead is formed when other pollutants, mainly nitrogen oxides and volatile organic compounds, react in the presence of sunlight.
Ozone levels in the park have come down over the past 10 years but still sometimes exceed the National Ambient Air Quality Standard set by the U.S. Environmental Protection Agency to protect public health. Ozone is a respiratory irritant, causing coughing, sinus inflammation, chest pains, scratchy throat, lung damage, and reduced immune system functions. Children, the elderly, people with existing health problems, and active adults are most vulnerable. When ozone levels exceed, or are predicted to exceed, health standards, Great Smoky Mountain NP staff post health advisories cautioning visitors of the potential health risks associated with exposures to elevated levels.
Over the course of a growing season, ozone can damage plant tissues making it harder for plants to produce and store food. It also weakens plants making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. Search ozone-sensitive plant species found at Great Smoky Mountains NP.
Ozone effects on vegetation:
- Visible injury to leaves of trees and understory plants, including black cherry, tulip tree (yellow poplar), sassafras, winged-sumac, blackberry, tall milkweed and cutleaf coneflower (Neufeld et al. 1991)
- Up to 90% of black cherry trees and milkweed plants in numerous park locations show symptoms of ozone damage
- Decline of growth in forest trees (Somers et al. 1998; McLaughlin et al. 2007a)
Visit the NPS air quality conditions and trends website for park-specific ozone information. Great Smoky Mountains NP has been monitoring ozone since 1993. Check out the live ozone and meteorology data from Great Smoky Mountains, NP and explore air monitoring »
Concentrations of fine particles in the air at Great Smoky Mountains NP sometimes exceed the National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency to protect human health. Fine particles (smaller than 2.5 micrometers) originate from either direct emissions by a source, such as construction sites, power plants, and fires, or reactions with gases and aerosols in the atmosphere emitted from pollution sources upwind.
Because of their small size, fine particles can get deep into the lungs and cause serious health problems. Numerous scientific studies have linked particle pollution exposure to irritation of the airways, coughing, difficulty breathing, aggravated asthma, chronic bronchitis, heart attacks, and premature death in people with heart or lung disease.
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[EPA] U.S. Environmental Protection Agency. 2003. Guidance for Tracking Progress Under the Regional Haze Rule. EPA-454/B-03-004. U.S. EPA Office of Air Quality Planning and Standards, Research Triangle Park, NC.
Geiser, Linda & Nelson, Peter & Jovan, Sarah & Root, Heather & Clark, Christopher. (2019). Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens. Diversity. 11. 87. 10.3390/d11060087.
Geiser, Linda & Root, Heather & Smith, Robert & Jovan, Sarah & Clair, Larry & Dillman, Karen. (2021). Lichen-based critical loads for deposition of nitrogen and sulfur in US forests. Environmental Pollution. 291. 118187. 10.1016/j.envpol.2021.118187.
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Kohut R.J. 2007. Ozone Risk Assessment for Vital Signs Monitoring Networks, Appalachian National Scenic Trail, and Natchez Trace National Scenic Trail. NPS/NRPC/ARD/NRTR—2007/001. National Park Service. Fort Collins, Colorado. Available at https://www.nps.gov/articles/ozone-risk-assessment.htm
Li, Z., and Aneja, V.P. 1992. Regional analysis of cloud chemistry at high elevations in the eastern United States. Atmospheric Environment 26A(11): 2001–2017.
Lovett, G.M., Reiners, W.A., and Olson, R.K. 1982. Cloud droplet deposition in subalpine balsam fir forest: Hydrological and chemical inputs. Science 218: 1303–1304.
[MADPro] Mountain Acid Deposition Program. 2007. Cloud deposition monitoring, Clingmans Dome, TN, Great Smoky Mountains National Park. U.S. Environmental Protection Agency, Clean Air Markets Division, Office of Air and Radiation, Washington, D.C.
McCoy K., M. D. Bell, and E. Felker-Quinn. 2021. Risk to epiphytic lichen communities in NPS units from atmospheric nitrogen and sulfur pollution: Changes in critical load exceedances from 2001‒2016. Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2287254.
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[NPCA] National Parks Conservation Association. 2006. Recommendations for a Smokies Mercury Study. Letter to the State of Tennessee Department of Environment and Conservation.
[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/
Pardo, L. and Duarte, N. 2007. Assessment of Effects of Acidic Deposition on Forested Ecosystems in Great Smoky Mountains National Park using Critical Loads for Sulfur and Nitrogen. NPS Final Report. Available at https://irma.nps.gov/DataStore/Reference/Profile/2166643
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[SAMI] Southern Appalachians Mountains Initiative. 2002. Final Report. 145 pp. Available at https://irma.nps.gov/DataStore/Reference/Profile/596269
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Somers, G.L., Chappelka, A.H., Rosseau, P., and Renfro, J.R. 1998. Empirical evidence of growth decline related to visible ozone injury. Forest Ecology and Management 104:129–137.
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