Park Air Profiles - Shenandoah National Park

Air Quality at Shenandoah National Park

Most visitors expect clean air and clear views in parks. Shenandoah National Park (NP) in Virginia experiences some of the highest measured air pollution of any national park in the U.S. The park is downwind of many sources of air pollution, including power plants, factories, vehicles, and agriculture from the mid-Atlantic region and Ohio River Valley. Air pollutants blown into the park can harm natural and scenic resources such as soils, surface waters, plants, wildlife, and visibility. The National Park Service works to address air pollution effects at Shenandoah NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Visitor looking out at scenic hills and fall colors
Visitors come to Shenandoah National Park to enjoy the spectacular vistas, beautiful fall colors, and waterfalls.

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 Shenandoah National Park (SHEN) 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 SHEN 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 SHEN ranged from 7.3 to 11.3 kg-N ha-1 yr-1 and total S deposition ranged from 1.6 to 2.2 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). SHEN has been monitoring atmospheric N and S deposition since 1981, see the conditions and trends website for park-specific information.

SHEN receives among the highest measured deposition of both sulfur and nitrogen of all monitored national parks. Although the Acid Rain Program has significantly reduced acid deposition throughout the Eastern U.S., problems remain. High ridgetop ecosystems at SHEN, especially soils, are particularly vulnerable to acid deposition. 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 acidic inputs.

Acidification of streamwaters and changes in condition of sensitive species, such as native brook trout, have been documented in the park as a result of acid deposition (Webb et al. 2004). Sulfur deposition needs to drop substantially below current levels in order to prevent further acidification and associated biological impacts in acid-sensitive streams within the park according to a study that analyzed critical loads for sulfur deposition (Sullivan et al. 2016). Plants sensitive to the effects of acidification in the park include Acer saccharum (sugar maple) and Picea rubens (red spruce).

SHEN is a leader among the national parks with respect to park-specific knowledge of acidic deposition effects and watershed ecosystem conditions. Extensive monitoring, research, and assessment of acidic deposition effects on sensitive surface waters in the park dates back to 1979 (Shenandoah Watershed Study—Virginia Trout Stream Sensitivity Study). Of note, the 2014 Appalachian Trail Atmospheric Deposition Effects Study assessed ecosystem response to acid deposition in sensitive ridgetop areas. This research, conducted at numerous sites within SHEN, determined critical loads for forests, soils, and streams.

Additional N and S Research:

  • Acid rain with an average acidity (pH) as low as 4.6, ten times more acidic than normal rainfall (NADP 2018)
  • Many streams have pH measurements as low as 5, ten times more acidic than the pH of park streams prior to human-caused pollution (NAPAP 1998)
  • Streams that have been highly damaged by acidification in the park support fewer fish species than park streams with better acid neutralizing capacity. (Bulger et al. 1999)
  • Sensitive fish species including brook trout, dace, chub, sculpin, darter, and bass have been affected by stream acidification (Webb et al. 2004; Bulger et al. 1999)
  • Aquatic insect communities, an important food source for trout, have been harmed by acidification in some park streams (Moeykens and Voshell 2002)
  • Decreased capacity of soils to buffer sulfur, with subsequent declines of essential nutrients calcium and magnesium, creating the potential for toxic aluminum to leach into streams (Welsch et al. 2001; Sullivan et al. 2016)

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 728 km2 (93%) of the land area of Shenandoah National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 94.7% of the forested area.
  • S deposition was below the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in every part 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, Shenandoah National Park contains:

  • 33 N-sensitive tree species and 118 N-sensitive herbaceous species.
  • 40 S-sensitive tree species and 94 S-sensitive herbaceous species.

Persistent Pollutants

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

Toxic airborne mercury deposits into ecosystems at Shenandoah NP and may affect fish and wildlife in the park. Research reported that a population of tree swallows at the headwaters of the Shenandoah River had elevated mercury levels (Brasso and Cristol 2008), with possible reduced reproductive success. Most human-caused airborne mercury is a result of burning coal for power production. While wet mercury deposition is monitored at the park, research is needed to evaluate the effects of mercury on fish, birds, and other organisms at the park.

Shenandoah NP has been monitoring mercury since 2002. Explore air monitoring »


Scenic vista at Shenandoah NP
Clean, clear air is essential to appreciating the scenic vistas at Shenandoah NP.

Many visitors come to enjoy the spectacular vistas found at Shenandoah NP. From the vistas along Skyline Drive to the rocky peaks of Hawksbill and Old Rag mountains, clean air is critical for inspirational views of the Blue Ridge Mountains, flanked by the beautiful Shenandoah Valley to the west and Virginia Piedmont to the east. Unfortunately, park vistas are sometimes obscured by haze, reducing 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, dust, and wood smoke reduce visibility as well.

Pollution-caused haze typically appears as a uniform whitish haze, different from the natural haze caused by organic compounds released by trees over the Blue Ridge Mountains of the eastern United States. Significant improvements in park visibility have been documented since the late 1980’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 120 miles (without the effects of pollution) to about 75 miles because of pollution;
  • Reduction of the visual range from about 80 miles to below 35 miles on high pollution days;
  • Human-caused haze frequently impairs scenic vistas at the park.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Shenandoah NP has been monitoring visibility since 1988. View a live air quality webcam, and explore air monitoring »

Ground-Level Ozone

Butterfly on milkweed plant
Milkweed is one of the ozone sensitive species found at Shenandoah 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.

During the summer months, ozone levels in the park sometimes exceed the National Ambient Air Quality Standards 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, Shenandoah NP staff post health advisories cautioning staff and 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 Shenandoah NP.

Ozone effects:

  • Visible injury to leaves of trees, tree seedlings, and understory plants including black cherry, tulip tree (yellow poplar), white ash, green ash, sweetgum, milkweed, virgin’s bower, black locust, and wild grape (Sullivan et al. 2003; Hildebrand et al. 1996; Winner et al. 1989; Duchelle et al. 1982);
  • Reduced average height of yellow poplar, green ash, white ash, black locust, Virginia pine, Eastern white pine, table mountain pine, and Eastern hemlock (Duchelle et al. 1982);
  • Reduced above-ground biomass production of native vegetation (Duchelle et al. 1983);
  • Increased ozone injury on plant leaves with increased elevation on virgin’s bower, black locust, and wild grape (Winner et al. 1989);
  • Increased ozone injury on plant leaves with increased ambient ozone exposures on black cherry and white ash (Hildebrand et al. 1996).

Visit the NPS air quality conditions and trends website for park-specific ozone information. Shenandoah NP has been monitoring ozone since 1983. View live ozone and meteorology data and explore air monitoring »

Brasso, R. L. and Cristol, D. A. 2008. Effects of mercury exposure on the reproductive success of tree swallows (Tachycineta bicolor). Ecotoxicology 17:133–141.

Bulger, A. J., Cosby, B. J. Dolloff, C. A., Eshleman, K. N., Webb, J. R., and Galloway, J. N. 1999. The “Shenandoah National Park: Fish in Sensitive Habitats (SNP: FISH)” An Integrated Assessment of Fish Community Responses to Stream Acidification. National Park Service Final Report. 570 pp. Available at

Clark, C.M., Simkin, S.M., Allen, E.B. et al. Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States. Nat. Plants 5, 697–705 (2019).

Duchelle, S. F., J. M. Skelly, and B. I. Chevone. 1982. Oxidant effects on forest tree seedling growth in the Appalachian Mountains. Water Air Soil Pollut. 18: 363–373.

Duchelle, S. F., J. M. Skelly, T. L. Sharick, B. I. Chevone, Y-S. Yang, and J. E. Nellessen. 1983. Effects of ozone on the productivity of natural vegetation in a high meadow of the Shenandoah National Park of Virginia. J. Environ. Manage. 17:299–308.

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.

Hildebrand, E., Skelly, J. M., and Fredericksen, T. S. 1996. Foliar response of ozone-sensitive hardwood trees species from 1991 to 1993 in Shenandoah National Park, Virginia. Can. J. For. Res. 26: 658–669.

Horn KJ, Thomas RQ, Clark CM, Pardo LH, Fenn ME, Lawrence GB, et al. (2018) Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.. PLoS ONE 13(10): e0205296.

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.

Moeykens, M. D. and Voshell, J. R. 2002. Studies of Benthic Macroinvertebrates for the Shenandoah National Park Long-Term Ecological Monitoring System: Statistical Analysis of LTEMS Aquatic Dataset from 1986 to 2000 on Water Chemistry, Habitat and Macroinvertebrates. Report to Shenandoah National Park from the Dept. of Entomology, Virginia Polytechnic and State University, Blacksburg, VA. 49 pp.

[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at

[NAPAP] National Acid Precipitation Assessment Program. 1998. Biennial Report to Congress: An Integrated Assessment. National Acid Precipitation Assessment Program: Silver Spring, MD.

Porter, E., Blett, T., Potter, D.U., Huber, C. 2005. Protecting resources on federal lands: Implications of critical loads for atmospheric deposition of nitrogen and sulfur. BioScience 55(7): 603–612.[0603:PROFLI]2.0.CO;2

Sullivan, T. J. 2016. Air quality related values (AQRVs) in national parks: Effects from ozone; visibility reducing particles; and atmospheric deposition of acids, nutrients and toxics. Natural Resource Report NPS/NRSS/ARD/NRR—2016/1196. National Park Service, Fort Collins, CO.

Sullivan, T. J., Cosby, B. J., Laurence, J. A., Dennis, R. L., Savig, K., Webb, J. R., Bulger, A. J., Scruggs, M., Gordon, C., Ray, J., Lee, E. H., Hogsett, W. E., Wayne, H., Miller, D., and Kern, J. S. 2003. Assessment of Air Quality and Related Values in Shenandoah National Park. National Park Service Technical Report NPS/NERCHAL/NRTR—03/090. NPS Northeast Region: Philadelphia, PA. Available at

Webb, J. R., Cosby, B. J., Deviney, Jr., F. A., Galloway, J. N., Maben, S. W., and Bulger, A. J. 2004. Are brook trout streams in western Virginia and Shenandoah National Park recovering from acidification? Environmental Science and Technology 38: 4091–4096.

Welsch, D. L., Webb, J. R., and Cosby, B. J. 2001. Description of Summer 2000 Field Work: Collection of Soil Samples and Tree Corps in the Shenandoah National Park with Summary Soils Data. Dept. of Environ. Sciences: Univ. of Virginia.

Winner, W. E., Lefohn, A. S., Cotter, I. S., Greitner, C. S., Nellessen, J., McEvoy, Jr., L. R., Olson, R. L., Atkinson, C. J., and Moore, L. D. 1989. Plant responses to elevational gradients of ozone exposures in Virginia. Proceedings National Academy of Sciences 86: 8828–8832.

Part of a series of articles titled Park Air Profiles.

Shenandoah National Park

Last updated: December 5, 2022