Park Air Profiles - North Cascades National Park

Air Quality at North Cascades National Park

Most visitors expect clean air and clear views in parks. North Cascades National Park (NP), Washington, is in close proximity to the fast-growing Seattle and Vancouver metropolitan areaslies. The park is downwind of air pollution from the Puget Sound lowlands and Fraser River Valley of British Columbia and sometimes is affected by air masses originating in Asia. 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 North Cascades NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Park visitor and dog in Ross Lake Recreation Area
Visitors come to North Cascades NP to enjoy scenic views of the Cascades mountain range, lakes, and alpine forests.

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 North Cascades National Park (NOCA) 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 NOCA 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 NOCA was 10.7 kg-N ha-1 yr-1 and total S deposition was 2.5 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). NOCA has been monitoring atmospheric N and S deposition since 1999, see the conditions and trends website for park-specific information.

High elevation lakes and streams in NOCA have shown variable response to atmospheric deposition of N and S due to a limited ability to neutralize acid deposition (Sullivan et al. 2016).

Alpine ecosystem effects

Alpine environments are particularly vulnerable to large inputs of reactive nitrogen because of the sparse cover of vegetation, short growing seasons, large areas of exposed bedrock and talus, and snowmelt nutrient releases (Williams et al., 1996; Nanus et al., 2012). Approximately 16% of the land area in NOCA is alpine (~328 km2 above 1550 m). McClung et al. (2021) compared the 2015 estimated total N deposition (TDep; NADP, 2018) to the critical load of N for an increase in alpine sedge growth (alpine plant critical load = 3 kg-N ha-1yr-1) and the critical load of N for alpine soil nitrate leaching (alpine soil critical load = 10 kg-N ha-1yr-1; Bowman et al., 2012). They found that deposition exceeded the alpine plant critical load in 60% of the park’s alpine area, but was below the alpine soil critical load throughout the park’s entire alpine area.

Additional N and S Research:
  • Spring snowmelt, late summer storms, or rain-on-snow can release acids accumulated in snow that are harmful to aquatic life and amphibians (Clow and Campbell, 2008).
  • Increased nitrate concentrations in alpine lakes as elevation increases, suggest that atmospheric deposition contributes to the increasing levels of N in high elevation lakes (Larson et al. 1999).
  • In the Columbia River Gorge and the Willamette Valley, sensitive lichen species important to wildlife have declined and been replaced by pollution-tolerant species (Geiser and Neitlich 2007; Geiser et al. 2010).

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 1045 km2 (51.5%) of the land area of North Cascades National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 11.6% 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, North Cascades National Park contains:

  • 9 N-sensitive tree species and 35 N-sensitive herbaceous species.
  • 10 S-sensitive tree species and 27 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 North Cascades NP exceeded the threshold for bird toxicity, but not fish toxicity or human health. Fish were sampled and analyzed for mercury from three sites sampled at the park and compared to data across 21 western parks. The average fish mercury concentration (54.9 ng/g ww) was lower than the study-wide mean (77.7 ng/g ww). Mercury concentrations exceeded the threshold for bird toxicity in 5% of fish sampled (Eagles-Smith et al. 2014). The data may not reflect the risk at other unsampled locations in the park.
  • Some dragonfly larvae sampled from the park had mercury concentrations at moderate impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from five sites in the park; 23% 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. 2018, Eagles-Smith et al. 2020).
  • Related studies also found toxic contaminants in park air, fish, and vegetation. Current-use pesticides in particular had elevated concentrations in air and vegetation samples (Landers et al. 2010; Landers et al. 2008). Fish from lakes with elevated mercury and toxics contaminant levels display changes in metabolic, endocrine, and immune response-related genes (Moran et al. 2007). Evidence suggests nearshore tree cover and fish diet account for the most variance in fish mercury at North Cascades NP (Chiapella et al. 2021).
  • No fish sampled from the park were found to be intersex (Schreck and Kent 2013). Reproductive abnormalities such as intersex, the presence of both male and female reproductive structures in the same fish, can signify exposure to contaminants.
  • Microplastics have been detected in glacial snowpack from two sites in North Cascades NP. Concentrations ranged from 9.4 to 31.4 particles/liter, which is three to four orders of magnitude greater than concentrations typically measured in U.S. rivers (Baldwin et al. unpublished).

Explore an interactive map of contaminant sampling in the Pacific Northwest.


Cascade Pass and Pelton Basin
Clean, clear air is essential to appreciating the scenic vistas at North Cascades NP.

Many visitors come to North Cascades NP to enjoy views of the “American Alps,” including jagged spires, sheer cliffs, and glaciers. Unfortunately, these vistas are sometimes obscured by haze. 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. Significant improvements in park visibility have been documented since the 2000’s. Overall, visibility in the park still needs improvement to reach the Clean Air Act goal of no human caused impairment.

Visibility effects:

  • Reduction of the average natural visual range from about 155 miles (without the effects of pollution) to about 140 miles because of pollution at the park;
  • Reduction of the visual range from about 110 miles to below 75 miles on high pollution days.
  • A 1990 study called PREVENT (Pacific Northwest Regional Visibility Experiment Using Natural Tracers) found that sulfur (largely from nearby power plants and urban sources) is the largest contributor to reduced visibility at North Cascades NP. Nitrates also contribute to visibility reduction at the park and are mostly from emissions from pulp and paper mills, fires, power plants, and transportation (Malm et al. 1994).

Visit the NPS air quality conditions and trends website for park-specific visibility information. North Cascades NP has been monitoring visibility since 1997. View a live air quality webcam, and explore air monitoring »

Ground-Level Ozone

Quaking Aspen Trees showing fall colors
Quaking Aspen trees are one of the ozone sensitive species found at North Cascades 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 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 in the region are generally low, and ozone injury to plants in the park has not been evaluated. Still, some plants are more sensitive to ozone than others. Several species in the park, including Pinus ponderosa (ponderosa pine) and Populus tremuloides (quaking aspen), are known to be sensitive to ozone. Search for more ozone-sensitive plant species found at North Cascades NP.

Visit the NPS air quality conditions and trends website for park-specific ozone information.


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Bowman W, Murgel J, Blett T, Porter E. 2012. Nitrogen critical loads for alpine vegetation and soils in Rocky Mountain National Park. Journal of Environmental Management. 103:165-171

Chiapella, A.M., Eagles-Smith, C.A. and Strecker, A.L. (2021), From forests to fish: Mercury in mountain lake food webs influenced by factors at multiple scales. Limnol Oceanogr, 66: 1021-1035.

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).

Clow, D. W. and Campbell, D. H. 2008. Atmospheric deposition and surface-water chemistry in Mount Rainier and North Cascades National Parks, U.S.A., water years 2000 and 2005–2006: U.S. Geological Survey Scientific Investigations Report 2008—5152, 37 pp. Available at

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:

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.

Geiser, L. and Neitlich, P. 2007. Air pollution and climate gradients in western Oregon and Washington indicated by epiphytic macrolichens. Environmental Pollution 145: 203–218.

Geiser, L. H., Jovan, S. E., Glavich, D. A., Porter, M. K. 2010. Lichen-based critical loads for atmospheric nitrogen deposition in Western Oregon and Washington Forests, USA. Environmental Pollution 158: 2412–2421.

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.

Landers, D. H., S. L. Simonich, D. A. Jaffe, L. H. Geiser, D. H. Campbell, A. R. Schwindt, C. B. Schreck, M. L. Kent, W. D. Hafner, H. E. Taylor, K. J. Hageman, S. Usenko, L. K. Ackerman, J. E. Schrlau, N. L. Rose, T. F. Blett, and M. M. Erway. 2008. The Fate, Transport, and Ecological Impacts of Airborne Contaminants in Western National Parks (USA). EPA/600/R—07/138. U.S. Environmental Protection Agency, Office of Research and Development, NHEERL, Western Ecology Division, Corvallis, Oregon. Available at

Landers, D. H., Simonich, S. M., Jaffe, D., Geiser, L., Campbell, D. H., Schwindt, A., Schreck, C., Kent, M., Hafner, W., Taylor, H. E., Hageman, K., Usenko, S., Ackerman, L., Schrlau, J., Rose, N., Blett, T., Erway, M. M. 2010. The Western Airborne Contaminant Assessment Project (WACAP): An Interdisciplinary Evaluation of the Impacts of Airborne Contaminants in Western U.S. National Parks. Environmental Science and Technology 44: 855–859. Available at

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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.

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Schreck, C.B. and M. Kent. 2013. Extent of Endocrine Disruption in Fish of Western and Alaskan National Parks. NPS-OSU Task Agreement J8W07080024. NPS Final Report, 72 pp.

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.

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Part of a series of articles titled Park Air Profiles.

North Cascades National Park

Last updated: February 7, 2023