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.
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.
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 »
Nitrogen and sulfur
Nitrogen and sulfur compounds deposited from the air may have harmful effects, including nutrient imbalances and loss of biodiversity. Arid ecosystems and grasslands are vulnerable to changes caused by nitrogen deposition. Invasive grasses tend to thrive in areas with high nitrogen deposition, displacing native vegetation adapted to natural low nitrogen conditions. Increases in nitrogen 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). Increased cover of exotic grasses can increase fire risk (Rao et al. 2010; Balch et al. 2013) and can lead to a complete conversion from woodland vegetation to large areas dominated by exotic grasses. Healthy ecosystems can naturally buffer a certain amount of pollution, but as nitrogen and sulfur accumulate, a threshold is passed where the ecosystem is harmed. “Critical load” is a term used to describe the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). Nitrogen deposition exceeds the critical load for forests, herbaceous plants, and lichens at Zion NP (NPS ARD 2018).
Nitrogen, together with sulfur, can also acidify surface waters and soils. Given the abundance of base cations in underlying park soils and rocks, surface waters in Zion NP are generally well-buffered from acidification (Binkley et al. 1997). Some plants are sensitive to acidification, search for acid-sensitive plant species found at Zion NP.
Visit the NPS air quality conditions and trends website for park-specific nitrogen and sulfur deposition information.
Mercury and toxics
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.
A study of mercury in fish from western national parks (Eagles-Smith et al. 2014) indicates that mercury levels in fish from the Virgin River at Zion NP are elevated. At both sites along the Virgin River, mean mercury concentrations in fish were more than 3 times higher than the study-wide mean concentration. Mercury concentrations in 20% of speckled dace exceed the threshold for reproductive impairment, and 90% exceed the health threshold for fish-eating birds (Eagles-Smith et al. 2014). The source of the mercury in fish at the park is unknown, but similar results were found nearby at Capitol Reef NP.
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.
Allen, E. B. and L. H. Geiser. 2011. North American Deserts. In L.H. Pardo, M.J. Robin-Abbott and C.T. Driscoll (Eds.). Assessment of Nitrogen Deposition Effects and Empirical Critical Loads of Nitrogen for Ecoregions of the United States. General Technical Report NRS–80. U.S. Forest Service, Newtown Square, PA. pp. 133–142. Available at: http://nrs.fs.fed.us/pubs/38109.
Balch, J. K., Bradley, B. A., D’Antonio, C. M., Gomez-Dans, J. 2013. Introduced annual grass increases regional fire activity across the arid western USA (1980–2009). Global Change Biology 19: 173–183.
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
Bowman, W. D., J. S. Baron, L. H. Geiser, M. E. Fenn, E. A. Lilleskov. 2011. Northwestern Forested Mountains. In L.H. Pardo, M.J. Robin-Abbott and C.T. Driscoll (Eds.). Assessment of Nitrogen Deposition Effects and Empirical Critical Loads of Nitrogen for Ecoregions of the United States. General Technical Report NRS–80. U.S. Forest Service, Newtown Square, PA. pp. 133–142. Available at: http://nrs.fs.fed.us/pubs/38109.
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.
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.
Inouye, R.S. 2006. Effects of shrub removal and nitrogen addition on soil moisture in sagebrush steppe. Journal of Arid Environments. 65: 604–618.
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 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
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.
[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.
Pardo, L.H., M. J. Robin-Abbott, C. T. Driscoll, eds. 2011. Assessment of Nitrogen deposition effects and empirical critical loads of Nitrogen for ecoregions of the United States. Gen. Tech. Rep. NRS-80. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern ReseCANY Station. 291 p. Available at: http://nrs.fs.fed.us/pubs/38109.
Rao, L. E., E. B. Allen, and T. Meixner. 2010. Risk-based determination of critical nitrogen deposition loads for fire spread in southern California deserts. Ecological Applications 20:1320–1335.
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., McDonnell, T. C., McPherson, G. T., Mackey, S. D., Moore, D. 2011a. Evaluation of the sensitivity of inventory and monitoring national parks to nutrient enrichment effects from atmospheric nitrogen deposition: main report. Natural Resource Report NPS/NRPC/ARD/NRR—2011/313. National Park Service, Denver, Colorado. Available at https://www.nps.gov/articles/nitrogen-risk-assessment.htm
Sullivan, T. J., McDonnell, T. C., McPherson, G. T., Mackey, S. D., Moore, D. 2011b. Evaluation of the sensitivity of inventory and monitoring national parks to nutrient enrichment effects from atmospheric nitrogen deposition: Northern Colorado Plateau Network (NCPN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/330. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2168722
Sullivan, T. J., McPherson, G. T., McDonnell, T. C., Mackey, S. D., Moore, D. 2011c. Evaluation of the sensitivity of inventory and monitoring national parks to acidification effects from atmospheric sulfur and nitrogen deposition: main report. Natural Resource Report NPS/NRPC/ARD/NRR—2011/349. National Park Service, Denver, Colorado. Available at https://www.nps.gov/articles/acidification-risk-assessment.htm
Sullivan, T. J., McPherson, G. T., McDonnell, T. C., Mackey, S. D., Moore, D. 2011d. Evaluation of the sensitivity of inventory and monitoring national parks to acidification effects from atmospheric sulfur and nitrogen deposition: Northern Colorado Plateau Network (NCPN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/360. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181489
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, Colorado. Available at https://www.nps.gov/articles/aqrv-assessment.htm