Park Air Profiles - Yosemite National Park

Air Quality at Yosemite National Park

Most visitors expect clean air and clear views in parks. However, Yosemite National Park (NP), California, experiences some of the worst air pollution of any national park in the U.S. The park is downwind of many air pollution sources, including agriculture, industry, major highways, and urban pollutants from as far away as the San Francisco Bay Area. 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 Yosemite NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Glacier and lake
Visitors come to Yosemite National Park to enjoy scenic views of waterfalls, glaciers, valleys, meadows, and ancient sequoia trees.

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 Yosemite National Park (YOSE) 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 YOSE 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 YOSE ranged from 4.6 to 6.8 kg-N ha-1 yr-1 and total S deposition ranged from 0.4 to 1.4 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). YOSE has been monitoring atmospheric N and S deposition since 1981, see the conditions and trends website for park-specific information.

Sources of N in YOSE include agriculture and vehicle emissions from California’s Central Valley and San Francisco Bay Area.

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 30% of the land area in YOSE 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 100% of the park’s alpine area, but was below the alpine soil critical load throughout the park’s entire alpine area.

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 1436 km2 (47.6%) of the land area of Yosemite National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 100% 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, Yosemite National Park contains:

  • 6 N-sensitive tree species and 22 N-sensitive herbaceous species.
  • 7 S-sensitive tree species and 18 S-sensitive herbaceous species.

Persistent Pollutants

Mountain Yellow-legged frog
The decline of Mountain Yellow-Legged Frogs at Yosemite NP is linked to pesticide deposition.

Airborne mercury, and other toxic air contaminants including pesticides, 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.

Pesticides are heavily used in California's Central Valley and transported by wind to Yosemite NP. Some of the more persistent and toxic pesticides that are now banned in the United States still remain in the park's vegetation and soils, with unknown negative impacts, if any, for park biota. Research also has shown that the breakdown products of some of the current pesticides can be highly toxic to aquatic animals, especially amphibians.

The Sierra Nevada Southern Cascades Contaminants (SNSCC) Workshop's 2009 report (530 KB PDF) summarized the impacts of airborne contaminants on the Sierra Nevada ecosystems. The scientific consensus from the workshop was that high-elevation ecosystems are likely to contain high levels of both pesticides (historic and current-use) and mercury. And additionally, that amphibian declines in these systems might be linked with some of these pesticides coming from agriculture in the nearby San Joaquin and Sacramento valleys.

Mercury and toxics effects:

  • Elevated concentrations of current-use pesticides in park vegetation samples (Landers et al. 2010; Landers et al. 2008);
  • Concentrations of current-use pesticides and other toxics in air and vegetation samples ranked above the median compared to other national parks studied (Landers et al. 2010; Landers et al. 2008);
  • Dramatic population declines of several frog species, including the endangered mountain yellow-legged frogs, are likely linked to pesticides (Sparling et al. 2001; Fellers et al. 2004; Davidson and Knapp 2007);
  • Elevated concentrations of mercury in fish from Hetch Hetchy Reservoir (Davis et al. 2009).


Yosemite falls and trees
Clean, clear air is essential to appreciating the scenic vistas at Yosemite NP.

Many visitors come to Yosemite NP to enjoy world-class views of famous landmarks like El Capitán and Yosemite Falls. Unfortunately, park vistas are often obscured by haze, reducing how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air (see particulate matter below). 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.

At Yosemite NP, most of the haziest days are caused by wildfire smoke. Forests in a dry Mediterranean climate like that of the Sierra Nevada are fire adapted and rely on fire as part of the ecology that sustains their health. Unfortunately, 100 years of fire suppression has left much of the lower elevation areas around Yosemite NP overgrown. As a result, occasional, but sometimes significant, smoke events are likely to occur along with wildland and prescribed fires into the foreseeable future.

It is important to note that significant improvements in park visibility on the clearest days have been documented since the late 1980’s. The clearest and mid-range visibility days in California and around the country are benefiting from clean air regulations and modern pollution control technology. 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 160 miles (without the effects of pollution) to about 110 miles because of pollution at the parks;
  • Reduction of the visual range from about 115 miles to below 55 miles on high pollution days.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Yosemite NP has been monitoring visibility since 1988. View a 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 Yosemite 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, Yosemite NP staff post health advisories cautioning staff and visitors of the potential health risks associated with exposures to elevated levels.

In addition to harming human health, 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. Search ozone-sensitive plant species found at Yosemite NP.

Ozone Effects:

  • Widespread injury to ponderosa pine needles, with up to 30–40% of pines injured at certain survey sites (Peterson et al. 1991; Peterson and Arbaugh 1992; Arbaugh et al. 1998);
  • Reduced growth of ozone-injured pines (Peterson et al. 1991; Peterson and Arbaugh 1992);
  • Greater ozone injury on low elevation ponderosa pines as compared to ponderosa pines on dry, upslope areas in the park, suggesting increased ozone damage on trees in moist areas (Panek and Ustin 2004).

Visit the NPS air quality conditions and trends website for park-specific ozone information. Yosemite NP has been monitoring ozone since 1976. View live ozone and meteorology data and explore air monitoring »

Particulate Matter

Smoke over Yosemite NP
Wildfire smoke is the main source of particulate matter at Yosemite NP

Concentrations of fine particles in the air at Yosemite NP sometimes exceed the National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency to protect human health. Fine particles (smaller than 2.5 microns) originate from either direct emissions by a source, such as construction sites, power plants, and fires, or reactions with gases and aerosols in the atmosphere emitted from pollution sources upwind. An aerosol is a gaseous suspension of fine solid or liquid particles.

Because of their small size, fine particles can get deep into the lungs and cause serious health problems. Numerous scientific studies have linked particle pollution exposure to irritation of the airways, coughing, difficulty breathing, aggravated asthma, chronic bronchitis, heart attacks, and premature death in people with heart or lung disease.

The Yosemite Aerosol Characterization Study (YACS) was a field campaign conducted in 2002 to investigate visibility-impairing aerosols at the park. Determining the content of those particles allows researchers to examine sources of aerosols. A key finding of this study was that fine particles in the park primarily originate from fires. Fine particles can also come from other sources including power plants, vehicle exhaust, agriculture, and construction sites.

Smoke can collect in canyon bottoms like Yosemite Valley, as a result these areas commonly experience the highest fine particle levels when winds are calm or inversion weather patterns occur during smoke events. A stationary air quality monitor at the Yosemite Valley Visitor Center measures fine particles to identify potential health impacts from smoke and fire. As needed, Yosemite NP can also send out three mobile monitors to smoke-sensitive areas. Together, these measurements ensure that the impacts of fire on air quality in Yosemite NP are understood and quantified. Visit Smoke in Yosemite and the park’s Air Quality and Smoke Monitoring web pages to learn more and for current smoke information.

Yosemite NP has been monitoring particulate matter since 2002. Check out the most recent particulate matter levels on our live data site and explore air monitoring »

Arbaugh, M. J., Miller, P. R., Carroll, J. J., Takemoto, B. and Procter, T. 1998. Relationships of ozone exposure to pine injury in the Sierra Nevada and San Bernardino Mountains of California, USA. Environ. Pollut. 101: 291–301.

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

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).

Davidson, C. and Knapp, R. A. 2007. Multiple stressors and amphibian declines: Dual impacts of pesticides and fish on yellow-legged frogs. Ecol Appl 17: 587–597.

Davis, J. A., Melwani, A. R., Bezalel, S. N., Hunt, J. A., Ichikawa, G., Bonnema, A., Heim, W. A., Crane, D., Swenson, S., Lamerdin, C., and Stephenson, M. 2009. Contaminants in Fish from California Lakes and Reservoirs: Technical Report on Year One of a Two-Year Screening Survey. A Report of the Surface Water Ambient Monitoring Program (SWAMP). California State Water Resources Control Board, Sacramento, CA. Available at

Fellers, G. M., McConnell, L. L., Pratt, D., Datta, S. 2004. Pesticides in mountain yellow-legged frogs (Rana muscosa) from the Sierra Nevada mountains of California, USA. Environmental Toxicology and Chemistry 23 (9): 2170–2177.

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.

McClung JJ, Bell MD, Felker-Quinn E. 2021. Extrapolating critical loads of nitrogen for alpine vegetation and assessing exceedance in national parks based on TDep Total N from 2002–2016. Natural Resource Report. NPS/NRSS/ARD/NRR—2021/2240. National Park Service. Fort Collins, Colorado.

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

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Peterson, D. L., Arbaugh, M. J., Robinson, L. J. 1991. Regional growth changes in ozone-stressed ponderosa pine (Pinus ponderosa) in the Sierra Nevada, California, USA. The Holocene 1: 50–61.

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Sparling, D. W., Fellers G. M., McConnell L. L. 2001. Pesticides and amphibian population declines in California, USA. Environmental Toxicology and Chemistry 20: 1591–1595.

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.

Yosemite National Park

Last updated: December 5, 2022