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Air Pollution Impacts

Yosemite National Park

Natural and scenic resources in Yosemite National Park (NP) are susceptible to the harmful effects of air pollution. Ozone, nitrogen, mercury, and fine particles impact natural resources such as lakes, streams, fish, and vegetation, and scenic resources such as visibility. Click on the tabs below to learn more about air pollutants and their impacts on natural and scenic resources at Yosemite NP.

  • Ozone
  • Nitrogen & Sulfur
  • Toxics & Mercury
  • Visibility

Ozone and Public Health Concerns

Ground-level ozone concentrations at the park are among the highest recorded in national parks and 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.

If ozone concentrations are predicted to be high at Yosemite NP, the park educates employees and park visitors by posting alerts about the risks of exposure to high ozone levels and precautions to reduce exposure.

Naturally-occurring ozone in the upper atmosphere absorbs the sun’s harmful ultraviolet rays and helps to protect all life on earth. However, in the lower atmosphere, ozone is an air pollutant, forming when nitrogen oxides from vehicles, power plants, and other sources combine with volatile organic compounds from gasoline, solvents, and vegetation in the presence of sunlight. In addition to causing respiratory problems in people, ozone can injure plants. Ozone enters leaves through pores (stomata), where it can kill plant tissues, causing visible injury, or reduce photosynthesis, growth, and reproduction.

Photo of healthy (top) and ozone-injured (bottom) ponderosa pine needles in Yosemite NP, California.
Photo of healthy (top) and ozone-injured (bottom) ponderosa pine needles in Yosemite NP, California.

Effects of ozone on vegetation at Yosemite NP include:

  • 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, indicative of stomatal opening and ozone uptake on trees in moist areas (Panek and Ustin 2004).

Search the list of ozone-sensitive plant species (pdf, 184 KB) found at each national park.

Get Ozone Data »


How much nitrogen is too much?

Nitrogen is a fertilizer and some nitrogen is necessary for plants to grow. However, in natural ecosystems, too much nitrogen disrupts the balance of plant communities, allowing weed species to grow faster. Nitrogen deposited from air pollution may upset the balance of some high elevation lakes at Yosemite NP. Nitrogen deposition is now about 3–4 kg/ha/yr (Sickman et al. 2001). Projects are underway to examine the effect of excess nitrogen to other resources in the park and throughout the Sierra Nevada, and determine whether the critical load has been exceeded. In Sierra Nevada forests, research suggests that the critical load for some lichen species is about 3.1 kg/ha/yr (Fenn et al. 2008). Above that level of nitrogen deposition, lichen communities begin to change, with sensitive species gradually replaced by pollution-tolerant species. Critical loads for lake and forest resources can be used to establish goals for ecosystem recovery.

Nitrogen and sulfur compounds deposited from air pollution may harm lakes, streams, soils, and vegetation in Yosemite NP. While sulfur deposition is generally very 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 particularly sensitive to nitrogen deposition. Not only do these systems receive more nitrogen deposition than lower elevation areas, but short growing seasons and shallow soils limit the capacity of soils and plants to absorb nitrogen, and dilute surface waters cannot buffer the effects of acid deposition.

Image of a bright yellow crustose lichen on a rock at Yosemite NP in California.
Lichens at Yosemite NP, California, are sensitive to nitrogen deposition.

Effects of nitrogen and sulfur deposition at Yosemite NP include:

  • Elevated ammonia in lichens from park forests, which may lead to the decline of sensitive lichen species (Jovan and McCune 2006);
  • Decrease in abundance of certain lichen species important for wildlife food and habitat and replacement by weedy, nitrogen-loving species (Fenn et al. 2008).

Get Sulfur & Nitrogen Data »


Photo of a yellow-legged frog swimming in water at Yosemite NP in California
Yellow-legged frog population declines at Yosemite NP, California, are thought to be associated with pesticides.

Toxic air pollutants include pesticides, industrial by-products, heavy metals like mercury, and flame retardants for fabrics. Certain toxic contaminants are known or suspected to cause cancer or other serious health effects in humans and wildlife, including reproductive problems, impaired growth and development, behavioral abnormalities, and reduced immune response.

Effects of mercury and airborne toxics on ecosystems at Yosemite NP include:

  • Elevated concentrations of current-use pesticides (chlorpyrifos, endosulfans, dacthal, and lindane) in park vegetation samples (Landers et al. 2010; Landers et al. 2008);
  • Concentrations of current use pesticides and other toxic air contaminants in air and vegetation samples ranked above the median as 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, 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 [pdf, 10.8 MB]).

Get Toxics & Mercury Data »


Three images depicting air quality impacts on visibility at Yosemite National Park, California (clear to hazy from top to bottom).
Air pollutants can affect visibility at Yosemite NP, California (clear to hazy from top to bottom).

Many visitors come to Yosemite NP to enjoy world-class views of famous landmarks like El Capitan and Yosemite Falls. Unfortunately, these vistas are often obscured by haze caused by fine particles in the air. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze and visibility impairment. Organic compounds, soot, and dust reduce visibility as well. Some of the park’s worst visibility is caused by smoke from nearby forest fires.

Visibility effects at Yosemite NP include:

  • Reduction of the average natural visual range from about 160 miles (without the effects of pollution) to about 75 miles because of pollution at the parks;
  • Reduction of the visual range from about 110 miles to below 35 miles on high pollution days.

(Source: IMPROVE 2010)

Additional Information:

  • Explore scenic vistas through a live webcam at Yosemite National Park!
  • Learn about the Yosemite Aerosol Characterization Study (YACS), a field campaign conducted in 2002 to investigate visibility-impairing aerosols at the park. An aerosol is a gaseous suspension of fine solid or liquid particles. Determining the content of those particles allows researchers to examine sources of aerosols, either natural or human-influenced. more »

Fire and Smoke at Yosemite NP

Concentrations of fine particles in the park’s air sometimes exceed the National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency to protect public health. These episodes are most often due to fires, including both wildland and contained campfires. Fine particles (smaller than 2.5 microns) are also a result of direct emissions from other sources including power plants, mobile sources (like cars), agriculture, and construction sites, or are formed downwind from sources by reactions with gases and aerosols that react in the atmosphere. For example, power plants, industries, and automobiles emit gases such as sulfur dioxides and nitrogen oxides, which form particles of sulfate and nitrate in the atmosphere.

Fires and the resultant smoke are issues of particular concern at Yosemite NP. Not only do park visitors enjoy a fire while camping in the park, but wildland fire is a natural part of the Sierra Nevada ecosystem. However, the smoke a given fire (big or small) produces impairs visibility and can harm human health. Canyon bottoms like Yosemite Valley are areas that commonly experience the highest fine particle levels during calm, smoky periods.

Smoke particles come in all sizes, but the small or “fine” smoke particles pose the greatest health concern because they can get deepest into the lungs and can cause serious health problems. Temporary symptoms from short-term exposures can include burning eyes, runny nose, coughing, shortness of breath, and illnesses like bronchitis. Longer-term exposures to particle pollution may aggravate asthma, increase risk heart attacks and premature death in people with heart or lung disease. Visit Smoke in Yosemite and the park’s Air Quality and Smoke Monitoring web pages for more information about current smoke impacts in Yosemite NP.

Get Visibility Data »


Featured Content

Studies and Monitoring icon

Studies and monitoring help the NPS understand the environmental impacts of air pollution. Access air quality data and see what is happening with Studies and Monitoring at Yosemite NP.

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Last Updated: December 30, 2016