Park Air Profiles - Grand Canyon National Park

Desert Viewpoint, Grand Canyon National Park
Visitors come to Grand Canyon National Park to enjoy scenic views of unique geology at one of the largest canyons in the world.

Air Quality at Grand Canyon National Park

Most visitors expect clean air and clear views in parks. Grand Canyon National Park (NP), Arizona, world-renowned for its breathtakingly iconic views, is downwind of air pollution from coal-fired power plants in the Four Corners region, nearby mining, and urban and industrial pollutants from Mexico and California. Air pollutants carried into the park can harm natural and scenic resources such as forests, soils, streams, fish, and visibility. The National Park Service works to address air pollution effects at Grand Canyon 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 Grand Canyon National Park (GRCA) relative to other national parks is very 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 GRCA 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 GRCA ranged from 1.8 to 5.7 kg-N ha-1 yr-1 and total S deposition ranged from 0.3 to 1.0 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). GRCA has been monitoring atmospheric N and S deposition since 1981, see the conditions and trends website for park-specific information.

Arid ecosystems have shown variable responses to excess N. Vegetation communities in GRCA have evolved under low N conditions and are likely to be very sensitive to nutrient enrichment. Excess N may allow more weedy, invasive plants to out-compete native species, reducing biodiversity (Fenn et al. 2003). 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).

About half of the N and a third of the S deposited in GRCA ecosystems comes down in rain and snow as “wet deposition.” The rest is “dry deposition” of particles, dust, and droplets. Water chemistry data for GRCA indicates that surface waters are well buffered and not likely to be acidified by atmospheric deposition. Soils are also 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 927 km2 (19%) of the land area of Grand Canyon National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 45.2% 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, Grand Canyon National Park contains:

  • 4 N-sensitive tree species and 26 N-sensitive herbaceous species.
  • 10 S-sensitive tree species and 22 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 exceeded the threshold for bird toxicity in 10% of fish sampled from tributaries of the Colorado River, but did not exceed the thresholds for fish toxicity or human consumption. Fish (rainbow and brown trout) were sampled from three sites at the park (Havasu Creek, Shinumo Creek, and Bright Angel Creek) and compared to data across 21 western parks. The average fish mercury concentration (76.0 ng/g ww) was slightly lower than the study-wide mean (77.7 ng/g ww). However, the data may not reflect the risk at other locations in the park (Eagles-Smith et al. 2014).
  • Mercury and selenium concentrations in the mainstem of the Colorado River are variable. Walters et al. 2015 reported that fish regularly exceeded exposure risk thresholds for wildlife and humans (20-100% of fish), while follow-up data from Eagles-Smith 2016 found only 1 of 73 fish sampled from the Colorado River exceeded the consumption threshold for human health. Further analysis of mercury in Colorado River food webs revealed that flood events can impact the transfer of mercury (Walters et al. 2020).
  • Some dragonfly larvae sampled from the park had mercury concentrations at moderate impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from 19 sites in the park; 5% of the data fall into the moderate (100-300 ng/g dw) impairment category 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).
  • Microplastics, thought to be distributed by atmospheric transport, were found in precipitation samples taken from the park. Grand Canyon NP is estimated to have an annual deposition rate of 10.7-11.9 metric tons of plastic per year (Brahney et al. 2020).


Grand Canyon National Park
Clean, clear air is essential to appreciating the scenic vistas at Grand Canyon NP.

At Grand Canyon 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. Most of the fine particles affecting the Grand Canyon NP travel long distances from urban and industrial areas, mixing en route to form a uniform “regional haze” that obscures scenic vistas.

Visibility effects:

  • Reduction of the average natural visual range from about 175 miles (without the effects of pollution) to about 140 miles because of pollution
  • Reduction of the visual range from about 120 miles to below 90 miles on high pollution days

There has been extensive visibility research at Grand Canyon NP, especially from the late 1970s to the late 1990s. Projects like the Winter Haze Intensive Tracer Experiment (WHITEX) and Measurement of Haze and Visual Effects (Project MOHAVE) focused on determining sources of visibility impairment at the park (Malm et al., 1989; Pitchford et al., 1999). Such research identified coal-fired power plants, copper smelters, urban areas like southern California and Las Vegas, and wildland fire as the most significant sources of haze for the park. As reductions in human-caused air pollution have been achieved, the significance of smoke on visibility impairment has increased.

Additionally, research at Grand Canyon NP has improved the general understanding of visibility processes. Investigations centered on the primary visibility impairing compounds and the role of water vapor in reducing visibility (Wilson and McMurry, 1982; Pitchford and McMurry, 1994; Malm and Day, 2001).

The State of Arizona and Western Regional Air Partnership (WRAP) work together to address regional sources of haze affecting Grand Canyon NP. WRAP is a voluntary organization of Western states, tribes, and federal agencies that works to develop new technical and policy tools that help Western states meet Environmental Protection Agency haze regulations. Other federal agencies involved include the Bureau of Land Management, Fish & Wildlife Service, and U.S. Forest Service.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Grand Canyon NP has been monitoring visibility since 2000. Check out the live air quality webcam and explore air monitoring »

Ground-Level Ozone

Ponderosa Pine Tree
Ponderosa Pine trees are one of the ozone sensitive species found at Grand Canyon 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.

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. Ozone concentrations sometimes reach levels known to be harmful to plants at Grand Canyon NP. Pinus ponderosa (ponderosa pine) is known to be sensitive to ozone. However field surveys of Ponderosa pine in 1992–93 and 2008 did not document any ozone injury (Binkley et al. 1997; NPCA 2010). Dry conditions typical of the park cause plants to close their stomata to conserve water. This limits ozone uptake and injury. Plants in riparian areas however, are adequately watered and may uptake enough ozone to cause injury. Also, unlike urban areas, where ozone concentrations typically fall at night, summer ozone levels often remain elevated at Grand Canyon NP. Higher nighttime ozone levels may increase damage potential for drought-adapted plant species that respire at night. Some plants are more sensitive to ozone than others, search ozone-sensitive plant species found at Grand Canyon NP.

Visit the NPS air quality conditions and trends website for park-specific ozone information. Grand Canyon NP has monitored ozone concentrations on the South Rim continuously since 1989. Check out the live ozone and meteorology data from Grand Canyon NP and explore air monitoring »

Binkley, D., C. Giardina, I. Dockersmith, D. Morse, M. Scruggs, K. Tonnessen. 1997. Status of Air Quality and Related Values in Class I National Parks and Monuments of the Colorado Plateau. National Park Service, Air Resources Division, Denver, Colorado. Chapter 4: Bandelier National Monument. Available at

Brahney, J., M. Hallerud, E. Heim, M. Hahnenberger, and S. Sukumaran. 2020. Plastic rain in protected areas of the United States. Science 368(6496): 1257-1260.

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

Eagles-Smith, Collin. 2016. Total Mercury Concentrations in Fish Muscle from the Colorado River and Bright Angel Creek, AZ, USA (2015): U.S. Geological Survey data release.

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.

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.

Eagles-Smith, C.A., J.J. Willacker, and C.M.Flanagan Pritz. 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. Available at:

Fenn, M. E., Baron, J. S., Allen, E. B., Rueth, H. M., Nydick, K. R., Geiser, L., Bowman, W. D., Sickman, J. O., Meixner, T., Johnson, D. W., & Neitlich, P. (2003). Ecological effects of nitrogen deposition in the western United States. BioScience, 53(4), 404-420.[0404:EEONDI]2.0.CO;2

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.

Malm, W., K. Gebhart, D. Latimer, T. Cahill, R. Eldred, A. Pielke, R. Stocker, and J. Watson. 1989. Winter Haze Intensive Tracer Experiment (WHITEX). Final report for the National Park Service. Fort Collins, CO.

Malm, W.C. and D.E. Day. 2001. Estimates of aerosol species scattering characteristics as a function of relative humidity. Atmospheric Environment 35: 2845-2860.

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.

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

[NPCA] National Parks Conservation Association. 2010. State of the Parks: Grand Canyon National Park. Resource Challenges and Future Directions. Washington, D.C. 84 pp.

[NPS] National Park Service. 1996. Baseline Water Quality Data Inventory and Analysis Grand Canyon National Park. NPS Technical Report NPS/NRWRD/NRTR—96/84. Washington, D.C. Available at

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.

Pitchford, M.L. and P.H. McMurry. 1994. Relationship between measured water vapor growth and chemistry of atmospheric aerosol for Grand Canyon, Arizona, in winter 1990. Atmospheric Environment. 28(5): 827-839.

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.

Walters, D.M., Rosi-Marshall, E., Kennedy, T.A., Cross, W.F. and Baxter, C.V. 2015. Mercury and selenium accumulation in the Colorado River food web, Grand Canyon, USA. Environ Toxicol Chem. 34: 2385-2394.

Walters, D.M., W.F. Cross, T.A. Kennedy, C.V. Baxter, R.O. Hall Jr., E.J. Rosi. 2020. Food web controls on mercury fluxes and fate in the Colorado River, Grand Canyon. Science Advances 6(20)

Part of a series of articles titled Park Air Profiles.

Grand Canyon National Park

Last updated: January 3, 2023