While California’s air pollution regulations are among the strictest in the country, topography and geography combine to trap air pollutants in its Central Valley. Westerly winds often blow that pollution into Yosemite, especially during the summer. Wildfire smoke also often affects Yosemite’s air quality during summer and fall. Yosemite National Park, in cooperation with state and other federal agencies, monitors the damage caused by air pollution with a comprehensive, science-based air resources program. This program targets major air pollutants and impacts potentially affecting Yosemite's visitors and ecology. Yosemite’s air quality monitoring can be grouped into four measurable indicators: visibility, particulate matter, ground level ozone, and atmospheric deposition.
Smoke and Particulate Matter
Yosemite’s worst days of visibility and air quality are due to smoke. Yosemite is a fire-adapted ecosystem and fire is a natural and beneficial agent of change. Yosemite fire managers work hard to minimize impacts of smoke on public health while maximizing benefits of fire to forest resources.
Smoke contains a mixture of gasses and particulate matter. The smallest particulate matter, particulates less than 2.5 micrometers (PM2.5) in diameter, is of greatest concern to air quality specialists due to its health impacts. Yosemite continuously samples PM2.5 and PM10 (particulates less than 10 micrometers in diameter) in Yosemite Village. While the vast majority of fine particulate matterin the park is from smoke, on seemingly clear days Yosemite Valley still does report PM2.5 from local vehicle traffic and infrastructure. During smoke events, Yosemite deploys additional portable PM2.5 samplers throughout the park to inform park management and public health decisions. Visit current air quality and smoke in Yosemite to learn more.
High up in the stratosphere, the ozone layer provides protection from the sun’s ultraviolet radiation. At ground level, ozone is a harmful pollutant, often referred to as smog. Ozone forms when nitrous oxide gasses (NOx) chemically react with volatile organic compounds (VOCs) in the presence of heat and sunlight. California’s Central Valley has high NOx and VOC emissions from agriculture, oil drilling operations, industrial facilities, and major traffic corridors. Because these pollutants are carried by westerly winds to Yosemite, days of elevated ozone levels in Yosemite often correspond with days of poor air quality in the Central Valley. Some level of NOx and VOCs are also emitted in the park by local traffic and operations
For human beings, ozone is a caustic gas that is a respiratory irritant, producing symptoms analogous to a sunburn inside the lungs. Ozone injury can be cumulative and cause coughing, sinus inflammation, chest pains, scratchy throat—even permanent lung damage and reduced immune function. Children, the elderly, people with existing health problems, and active adults (including hikers) are most vulnerable. These “sensitive groups” on the Air Quality Index (AQI) scale should follow the guidelines for avoiding exposure to ozone above the levels set forth by the AQI. Check current ozone levels at Turtleback Dome, situated just above the landmark Tunnel View.
Even during the most severe ozone pollution events, ozone levels are often lower in Yosemite than in the Central Valley, which has some of the most unhealthy ozone levels in the country. Most ozone pollution events occur on hot, stagnant summer days, when prevailing winds aren't strong enough to push the pollution east over the Sierra Nevada into the Great Basin. The park's highest ozone levels occur on exposed, west-facing slopes at elevations between 4,000 and 6,000 feet (Turtleback Dome and Crane Flat, for example). However, monitoring in Yosemite Valley and east, including high country locations, shows that ozone levels are significantly lower elsewhere in the park, and usually do not exceed human health standards even during the worst ozone pollution events.
For plants, ozone's effects become cumulative because they can't move themselves indoors to avoid the ozone damage. Repairing ozone damage saps the energy and nutrients that plants use for defense against other kinds of environmental stress, like drought or pests. Ponderosa pine trees are an ozone-sensitive species prevalent throughout Yosemite and surrounding forests. A study throughout Sierra Nevada parks found up to 40% of ponderosa pine trees surveyed sustained ozone injury to their needles, and ozone injured pines suffered reduced growth.
Airborne pollutants are deposited in the park via either wet (rain, snow, etc) or dry (dust, particles, gas) deposition. Yosemite operates various sites that monitor deposition of nitrogen, sulfur, mercury, ammonia, and other toxics including pesticides. These pollutants are released into the atmosphere from agriculture, combustion, and industry, and once deposited, adversely affect ecosystem health.
Generally known as a fertilizer, nitrogen in excess can cause many issues. Yosemite’s ecosystems are adapted to naturally low levels of nitrogen. High-elevation ecosystems in Yosemite are particularly sensitive to nitrogen deposition. Higher elevations receive more precipitation, thus, more wet deposition of pollutants such as nitrogen, and the short growing season provides a narrow window for plants to absorb nitrogen from the soil. Excess nitrogen in the soil affects the cycling of nutrients and alters plant communities, the over-fertilization often favoring non-native or invasive plants, subsequently reducing biodiversity. Nitrogen and sulfur accumulation in lakes, rivers, and other bodies of water contributes to the acidification of these water systems, in turn negatively impacting aquatic life and forest health.
Mercury, pesticides, and other toxic compounds such as coolants and fire retardants accompany ozone and haze in air pollution and deposit onto the landscape. The presence of pesticides in Yosemite’s ecosystem has been linked to their historic and current use on agricultural lands of the Central Valley. These accumulate in Yosemite’s water systems, aquatic life, soils, and vegetation, negatively affecting organisms’ health and working their way up the food chain. Pesticides are likely linked to the decline of several amphibian species in the park, including the endangered Sierra Nevada yellow-legged frog.
Despite its special designation and stringent protection under the Clean Air Act, Yosemite National Park currently records air quality issues of significant concern, with implications for ecological and human health, scenic views, and visitor enjoyment. While updated environmental regulations have taken steps to improve air pollution, data analysis indicates Yosemite’s overall air quality is in poor condition and demonstrates a deteriorating trend.
Air is a globally shared resource, and emissions anywhere contribute to air pollution and ultimately, climate change. Responsible action at home helps protect Yosemite’s air quality. Action you can take at home to promote cleaner air everywhere include: educate yourself about environmental issues, choose a cleaner commute (walk, bike, public transportation, carpool, electric vehicles), use energy-efficient appliances and earth-friendly cleaning products, and support local and organic agriculture. While in Yosemite, do your part to protect the park’s air quality. Carpool to Yosemite, bring bikes or use the free shuttle system, and respect campfire hour regulations.
Sierra Nevada - Southern Cascades Contaminants Workshop Report (2009) summarized the impacts of airborne contaminants on the Sierra Nevada ecosystems. The workshop involved participants from Yosemite, Sequoia and Kings Canyon, and Lassen Volcanic National Parks along with other federal, state, county and tribal agencies.
Visit lichens in Yosemite to learn how these rootless organisms that cover Yosemite’s granite cliffs serve as indicators of air quality.