Park Air Profiles - Great Smoky Mountains National Park

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

Autumn in Cataloochee Creek at Great Smoky Mountains NP
Visitors come to Great Smoky Mountains National Park to enjoy scenic views, including beautiful fall colors.

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.

Change in N and S deposition from 2000 to 2021

The maps below show how the spatial distribution of estimated Total N and Total S deposition in GRSM has changed from 2000-2002 to 2019-2021 (TDep MMF version 2022.02). Slide the arrows in the middle of the image up and down to compare N and S deposition between the two years (Yearly Data).

  • Minimum N deposition decreased from 8.9 to 6.5 kg-N ha-1 yr-1 and maximum N deposition decreased from 15.3 to 9.7 kg-N ha-1 yr-1.
  • Minimum S deposition decreased from 8.5 to 1.7 kg-S ha-1 yr-1 and maximum S deposition decreased from 15.9 to 2.3 kg-S ha-1 yr-1.
Two maps showing GRSM boundaries. The left map shows the spatial distribution of estimated total nitrogen deposition levels from 2000-2002. The right map shows the spatial distribution of estimated total sulfur deposition levels from 2000-2002. Two maps showing GRSM boundaries. The left map shows the spatial distribution of estimated total nitrogen deposition levels from 2000-2002. The right map shows the spatial distribution of estimated total sulfur deposition levels from 2000-2002.

Estimated total nitrogen and sulfur deposition levels from 2000-2002 (top) compared to the 2019-2021 (bottom) average at GRSM. Estimated values were developed using the National Atmospheric Deposition Program - Total Deposition (TDep) approach that combines measured and modeled data. Estimated values are valuable for analyzing gradients of deposition and the resulting ecosystem risks where monitors are not present.

Persistent Pollutants

Pollutants like mercury and pesticides are concerning because they are persistent and toxic in the environment. These contaminants can travel in the air thousands of miles away from the source of pollution, even depositing in protected places like national parks. In addition, while some of these harmful pollutants may be banned from use, historically contaminated sites continue to endure negative environmental consequences.

When deposited, airborne mercury and other toxic air contaminants are known to harm wildlife like birds and fish, and cause human health concerns. Many of these substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.

  • Mercury concentrations in some fish sampled at Great Smoky Mountains NP exceeded the threshold for human consumption. Preliminary data from 18 sites in the park indicate an average fish mercury concentration of 0.108 ppm ww. Mercury concentrations in 10% of the fish sampled (n=344) exceeded the US EPA threshold established for human consumption (0.3 ppm ww) (Eagles-Smith et al. 2019). However, the data may not reflect the risk at other unsampled locations in the park. Fish consumption advisories may be in effect for mercury and other contaminants (NPS 2022).
  • Some dragonfly larvae sampled at Great Smoky Mountains NP had mercury concentrations at moderate or higher impairment levels.Dragonfly larvae have been sampled and analyzed for mercury from 12 sites in the park; 81% of the data fall into the moderate (100-300 ng/g dw) and 5% fall into the high (300-700 ng/g dw) impairment categories for potential mercury risk. An index of moderate impairment or higher suggests some fish may exceed the US EPA benchmark for protection of human health (Eagles-Smith et al. 2018; Eagles-Smith et al. 2020).
  • Other studies also found mercury in bird feathers (Keller et al. 2014) and soil, sediment, and macroinvertebrate (Buchwalter 2009) samples from the park.
  • Great Smoky Mountains NP has been monitoring litterfall mercury dry deposition since 2007 (NADP 2023). Studying litterfall mercury serves to fill gaps in our knowledge of total deposition inputs of atmospheric mercury to an ecosystem (Risch et al. 2012).
  • Microplastics, thought to be deposited by rainfall or wind, were found in 96% of samples collected in Great Smoky Mountains NP(Leffler 2021). The most abundant microplastics were blue and gray strands

The NPS Air Resources Division reports on park conditions and trends for mercury. Visit the webpage to learn more.


Great Smoky Mountains NP Webcam View
Clean, clear air is essential to appreciating the scenic vistas at Great Smoky Mountains NP.

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.

Visibility effects:

  • 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 »

Ground-Level Ozone

Butterfly on milkweed plant
Milkweed is one of the ozone sensitive species found at Great Smoky Mountains NP.

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 »

Particulate Matter

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.

Great Smoky Mountains NP has been monitoring particulate matter since 2002. Check out the most recent particulate matter levels on our live data site and explore air monitoring »

Explore Other Park Air Profiles

There are 47 other Park Air Profiles covering parks across the United States and its territories.


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Part of a series of articles titled Park Air Profiles.

Great Smoky Mountains National Park

Last updated: August 17, 2023