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Park Air Profiles - Zion National Park

Air Quality at Zion National Park

Most visitors expect clean air and clear views in parks. Zion National Park (NP) is home to a scenic canyon country full of high plateaus, a maze of slot canyons, and a diverse ecosystem. The park enjoys relatively good air quality, but it is upwind of urban and industrial sources of air pollution. 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 Zion NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Rainbow and towering rocks in Zion NP
Visitors come to Zion National Park to enjoy scenic views of sandstone cliffs and canyons.

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 Zion National Park (ZION) relative to other national parks is high (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 ZION relative to other national parks is moderate (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.

From 2017-2019 total N deposition in ZION ranged from 3.0 to 4.2 kg-N ha-1 yr-1 and total S deposition ranged from 0.6 to 0.8 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). See the conditions and trends website for park-specific information on N and S deposition at ZION.

Arid ecosystems have shown variable responses to excess N. Invasive grasses tend to thrive in areas with high N deposition, displacing native vegetation adapted to natural low N conditions. Increases in N have been found to promote invasions of fast-growing exotic annual grasses (e.g., cheatgrass) and forbs (e.g., Russian thistle) at the expense of native species (Brooks 2003; Schwinning et al. 2005; Allen et al. 2009). In contrast, a recent study showed little vegetation response to fertilization, but did see a decline in the stability of the soil crust community (Phillips et al. 2021).

Given the abundance of base cations in underlying park soils and rocks, surface waters in ZION are generally well-buffered from acidification (Binkley et al. 1997).

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. Epiphytic lichen communities are less diverse in arid areas, but are still impacted by 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 269 km2 (44.6%) of the land area of Zion National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 75.3% 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, Zion National Park contains:

  • 5 N-sensitive tree species and 22 N-sensitive herbaceous species.
  • 10 S-sensitive tree species and 18 S-sensitive herbaceous species.

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 Zion NP exceeded the thresholds for fish and bird toxicity. Prey fish were sampled and analyzed for mercury from two sites at the park and compared to data across 21 western parks. The average fish mercury concentration (241.5 ng/g ww) was higher than the study-wide mean (77.7 ng/g ww). Mercury concentrations exceeded the thresholds for fish toxicity and bird toxicity in 20% and 90% of fish sampled, respectively (Eagles-Smith et al. 2014). The data may not reflect the risk at other locations in the park.
  • Some dragonfly larvae sampled from the park had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae were sampled and analyzed for mercury from three sites in the park; 33% of the data fall into the moderate (100-300 ng/g dw) and 33% fall into the high/severe (>300 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. 2020, Eagles-Smith et al. 2018).

Visibility

Scenic view in Zion NP
Clean, clear air is essential to appreciating the scenic vistas at Zion NP.

Visitors come to Zion NP to experience the canyon country, gaze up at massive sandstone cliffs, and peer into narrow, deep sandstone canyons. When compared to many parts of the U.S., visibility on the Colorado Plateau is outstanding. Park vistas are sometimes obscured by haze, reducing how well and how far people can see, even affecting the clarity and vibrant colors of rock formations viewed from relatively short distances. 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. Since the mid 2000s, significant improvements in park visibility have been documented. Still, visibility improvements are needed to reach the Clean Air Act goal of no human caused impairment.

Visibility effects:

  • Reduction of the average natural visual range from about 160 miles (without pollution) to about 125 miles because of pollution at the park
  • Reduction of the visual range to below 90 miles on high pollution days.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Zion NP has been monitoring visibility since 2003. Explore air monitoring »

Ground-Level Ozone

Snowberry
Snowberry is one of the ozone sensitive species found at Zion 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.

Over the course of a growing season, ozone can damage the leaves of plants, reducing their growth rate and making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. There are a few ozone-sensitive plants in Zion NP including Amelanchier alnifolia (serviceberry), Populus tremuloides (quaking aspen), and Acer negundo (Box Elder). A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Zion NP were at low risk of damage to plant leaves (see network report: Kohut 2004). Generally, dry conditions in the park during peak ozone concentrations are likely to limit ozone uptake by plants. However, along the Virgin River and streams, where conditions are wetter, plants may have higher ozone uptake and injury (Kohut et al. 2012). Past surveys at the park located probable ozone injury on Symphoricarpos oreophilus (snowberry) and Rhus trilobata (skunkbush) (NPS 2000). Search ozone-sensitive plant species found at Zion NP.

Visit the NPS air quality conditions and trends website for park-specific ozone information. Zion NP has been monitoring ozone since 2004. Check out the live ozone and meteorology data from Zion NP and explore air monitoring »

Allen, E. B., L. E. Rao, R. J. Steers, A. Bytnerowicz, and M. E. Fenn. 2009. Impacts of atmospheric nitrogen deposition on vegetation and soils in Joshua Tree National Park. Pages 78–100 in R. H. Webb, L. F. Fenstermaker, J. S. Heaton, D. L. Hughson, E. V. McDonald, and D. M. Miller, editors. The Mojave Desert: ecosystem processes and sustainability. University of Nevada Press, Las Vegas, Nevada, USA.

Binkley et al. 1997. Status of Air Quality and Related Values in Class I National Parks and Monuments of the Colorado Plateau. Chapter 13. Zion National Park. National Park Service, Air Resources Division, Denver, CO. Available at https://irma.nps.gov/DataStore/Reference/Profile/585485

Brooks, M.L. 2003. Effects of increased soil nitrogen on the dominance of alien annual plants in the Mojave Desert. Journal of Applied Ecology. 40:344–353.

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). https://doi.org/10.1038/s41477-019-0442-8

Eagles-Smith, C.A., Willacker, J.J., and Flanagan Pritz, C.M., 2014, Mercury in fishes from 21 national parks in the Western United States—Inter and intra-park variation in concentrations and ecological risk: U.S. Geological Survey Open-File Report 2014-1051, 54 p. http://dx.doi.org/10.3133/ofr20141051.

Eagles-Smith, C.A., S.J. Nelson., C.M. Flanagan Pritz, J.J. Willacker Jr., and A. Klemmer. 2018. Total Mercury Concentrations in Dragonfly Larvae from U.S. National Parks (ver. 6.0, June 2021): U.S. Geological Survey data release. https://doi.org/10.5066/P9TK6NPT

Eagles-Smith, C.A., J.J. Willacker, S.J. Nelson, C.M. Flanagan Pritz, D.P. Krabbenhoft, C.Y. Chen, J.T. Ackerman, E.H. Campbell Grant, and D.S. Pilliod. 2020. Dragonflies as biosentinels of mercury availability in aquatic food webs of national parks throughout the United States. Environmental Science and Technology 54(14):8779-8790. https://doi.org/10.1021/acs.est.0c01255

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.

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. https://doi.org/10.1371/journal.pone.0205296

Kohut, B. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation in Parks in the Northern Colorado Plateau Network. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181489

Kohut, B., C. Flanagan, E. Porter, J. Cheatham. 2012. Foliar Ozone Injury on Cutleaf Coneflower at Rocky Mountain National Park, Utah. Western North American Naturalist 72(1): 32–42.

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.

[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/

[NPS] National Park Service. 2000. Results of 1999 ozone injury surveys at Bryce Canyon NP, Cedar Breaks NM and Zion NP. Memorandum. Available at https://irma.nps.gov/App/Reference/Profile/581126.

Phillips, M. L., D. E. Winkler, R. H. Reibold, B. B. Osborne, and S. C. Reed. 2021. Muted responses to chronic experimental nitrogen deposition on the Colorado Plateau. Oecologia 195:513-524.

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. https://doi.org/10.1641/0006-3568(2005)055[0603:PROFLI]2.0.CO;2

Schwinning, S., B. I. Starr, N. J. Wojcik, M. E. Miller, J. E. Ehleringer, R. L. Sanford. 2005. Effects of nitrogen deposition on an arid grassland in the Colorado plateau cold desert. Rangeland Ecology and Management. 58: 565–574.

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.

Part of a series of articles titled Park Air Profiles.

Zion National Park

Last updated: January 3, 2023