Park Air Profiles - Yosemite National Park

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

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.

Visibility

Waterfall 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 »

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 »

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 »

Nitrogen and sulfur

Nitrogen and sulfur compounds deposited from the air may have harmful effects, including acidification, on soils, lakes, ponds, and streams. Excess nitrogen can also lead to nutrient enrichment, a process that changes nutrient cycling and alters plant communities. Ecosystems at Yosemite NP are ranked as very highly sensitive to both acidification and nutrient-enrichment effects relative to other national parks (Sullivan et al. 2011a; Sullivan et al. 2011b; Sullivan et al. 2011c; Sullivan et al. 2011d). Some plants are sensitive to acidification, search for acid-sensitive plant species found at Yosemite NP.

Fortunately, sulfur deposition is generally low in California and unlikely to affect most ecosystems. Nitrogen deposition is higher and its effects are more widespread. Some high elevation ecosystems in the park are very sensitive to nitrogen deposition. These systems receive more nitrogen deposition than lower elevation areas and short growing seasons along with shallow soils limit the capacity of soils and plants to absorb nitrogen.

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 has been increasing in the park since monitoring began in the early 1980’s. Current deposition levels and this concerning trend suggest that significant reductions are needed to protect and restore certain park ecosystems (Sickman et al. 2001). Sources of nitrogen in the parks include agriculture and vehicle emissions from California’s Central Valley and San Francisco Bay Area.

Nitrogen and sulfur effects:

  • Elevated ammonia in lichens from park forests, which may lead to the decline of sensitive lichen species (Jovan and McCune 2006);
  • Nitrogen deposition at levels known to cause declines of certain lichen species (Sickman et al. 2001);
  • Decrease in abundance of certain lichen species important for wildlife food and habitat and replacement by weedy, nitrogen-loving species (Fenn et al. 2008).
  • Nitrogen deposition exceeds the critical load for one or more park ecosystems (NPS ARD 2018).

Visit the NPS air quality conditions and trends website for park-specific nitrogen and sulfur deposition information. Yosemite NP has been monitoring nitrogen and sulfur deposition since 1981, explore air monitoring »

Mercury and toxics

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).

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.

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 https://irma.nps.gov/DataStore/Reference/Profile/2171819.

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.

Fenn, M. E., Jovan, S., Yuan, F., Geiser, L., Meixner, T., Gimeno, B. S. 2008. Empirical and simulated critical loads for nitrogen deposition in California mixed conifer forests. Environmental Pollution 155: 492–511.

Jovan, S. and McCune, B. 2006. Using epiphytic macrolichen communities for biomonitoring ammonia in forests of the greater Sierra Nevada, California. Water, Air and Soil Pollution 170: 69–93.

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

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.

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 https://irma.nps.gov/DataStore/Reference/Profile/660829.

Peterson, D. L. and Arbaugh, M. J. 1992. Mixed conifer forests of the Sierra Nevada. In R. K. Olson, D. Binkley, and M. Böhm (eds.), Response of Western Forests to Air Pollution. Springer-Verlag, New York. pp. 433–459.

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.

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

Sickman, J. O., Leydecker, A., and Melack, J. M. 2001. Nitrogen mass balances and abiotic controls on N retention and yield in high-elevation catchments of the Sierra Nevada, California, United States. Water Resources Research 37: 1445–1461.

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., 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: Sierra Nevada Network (SIEN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/330. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2168734.

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: Sierra Nevada Network (SIEN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/349. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2170608.

Sullivan, T. J., Peterson, D. L., Blanchard, C. L. 2001. Assessment of Air Quality and Air Pollutant Impacts in Class I National Parks of California. National Park Service. 421 pp. Available at https://irma.nps.gov/DataStore/Reference/Profile/561620.

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.

Last updated: September 27, 2018