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Studies and Monitoring

Yosemite National Park

Yosemite National Park (NP), California, has its own unique environmental concerns based on its particular ecology. Air quality studies and monitoring programs at Yosemite NP focus on ozone, nitrogen deposition, toxic air contaminants, and visibility. Click on the tabs below to review air quality studies and key scientific references at Yosemite NP, as well as to access information on air quality monitoring in the park.

  • Studies & Projects
  • Monitoring & Data
  • Key References

Ongoing research in Yosemite NP, California:

Ground-Level Ozone Impacts

Widespread ozone injury in ponderosa pine has been documented at Yosemite NP in several studies. Chronic, long-term ozone exposure has reduced tree growth in some locations (Peterson et al. 1991; Peterson and Arbaugh 1992; Arbaugh et al. 1998). In addition to harming trees, ozone concentrations in the parks frequently exceed standards set by the U.S. Environmental Protection Agency to protect public health. Park managers continue to work on a variety of programs related to ozone monitoring, research, and emissions reductions.

Sulfur & Nitrogen Impacts

High elevation lakes and streams in the park are extremely dilute. Research is underway to determine the nitrogen (N) critical load for high elevation lakes across the Sierra Nevada, including at Yosemite NP. The nitrogen critical load for lichen communities in the mixed conifer forests of the park has been determined to be 3.1 kg/ha/yr (Fenn et al. 2008). Above this critical load, certain lichen species decline. Current N deposition is now about 3–4 kg/ha/yr (Sickman et al. 2001), suggesting that deposition reductions are needed to protect and restore certain park ecosystems. Because deposition can vary over complex terrain in the park, a study is underway to examine this variation in order to produce a fine-scale map of deposition and exceedances of the critical load. Sources of nitrogen in the park include the Central Valley and San Francisco Bay Area.

Airborne Toxic, including Mercury, Impacts

Air currents transport toxic contaminants such as pesticides, industrial pollutants, and mercury from their sources, and deposit these toxics in rain, snow, and dry deposition (e.g., dust) at Yosemite NP. The Western Airborne Contaminants Assessment Project (WACAP) found airborne contaminants in air and vegetation samples from the park. While no fish samples were collected as part of WACAP at Yosemite NP, pesticide and mercury concentrations in some fish from the nearby Sequoia & Kings Canyon National Parks exceeded human and/or wildlife health thresholds (Landers et al. 2010; Landers et al. 2008). Results from another study at Yosemite NP indicate that a small sample of fish from the park’s Hetch Hetchy reservoir had mercury levels that exceeded human health thresholds (Davis et al. 2009 [pdf, 10.8 MB]). Because mercury can originate locally, regionally, or even globally, a project with the University of Nevada, Reno, seeks to identify potential sources of airborne mercury and its deposition through California and Nevada. Studies regarding another toxic air contaminant, pesticides, suggest these chemicals from the adjacent Central Valley contributed to the disappearance of the foothill yellow-legged frog, and in the ongoing decline of other amphibians in the park and other parks, including the mountain yellow-legged frog species complex (Sparling et al. 2001; Fellers et al. 2004; Davidson and Knapp 2007). The Sierra Nevada—Southern Cascades (SNSC) Contaminants Workshop was held to address regional concerns regarding contaminant distribution and effects, and a research and monitoring strategy is underway to examine conditions and trends in toxic air contaminants in the region. Additional research is examining whether contaminants disrupt reproductive organs in park fish.

Visibility, Fire, and Smoke Impacts

Smoke is primarily responsible for the haze observed at Yosemite NP. The smoke produced from fires (both wildland and contained campfires) impairs visibility of scenic vistas and has the potential to harm human health. Because wildfire in the forests of a dry Mediterranean climate like that of the Sierra Nevada is an inherent and natural part of the ecosystem, fire and smoke are managed at the park to minimize harmful smoke emissions while preserving the fragile ecology that keeps vegetation sparse enough to prevent much larger fires. A stationary monitor at the Yosemite Valley Visitor Center, an area in a deep canyon prone to high fine particle levels during smoky periods, measures fine particles to identify potential health impacts from smoke and fire. As needed, Yosemite NP can also deploy three additional 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 for the latest smoke information.

Air quality monitoring information and data access:

Air Pollutant/Impact

Monitoring Program

Sites and Data Access

Nitrogen & Sulfur Wet deposition NADP/NTN
Dry deposition CASTNet
Toxics & Mercury WACAP
Visibility IMPROVE

Abbreviations in the above table:

    CASTNet: EPA Clean Air Status and Trends Network
    GPMP: Gaseous Pollutant Monitoring Program
    IMPROVE: Interagency Monitoring of Protected Visual Environments
    NADP: National Atmospheric Deposition Program
    NPS: National Park Service
    NTN: National Trends Network
    VIEWS: Visibility Information Exchange Web System
    WACAP: Western Airborne Contaminants Assessment Project

For more information regarding monitoring and data assessments conducted by the National Park Service, link to the NPS Air Quality Monitoring Program or to the NPS Air Quality Monitoring History Database for a history of active and inactive monitoring sites at Yosemite NP.

Key air quality related references from Yosemite NP, California:

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 (pdf, 10.8 MB).

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.

[IMPROVE] Interagency Monitoring of Protected Visual Environments. 2010. Improve Summary Data. Available at

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.

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

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.

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

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 (pdf, 7.4 MB).

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 (pdf, 6.3 MB).

Featured Content

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Pollutants including ozone, nitrogen, mercury, and fine particles affect resources such as forests, streams, and scenic vistas. Find out how on our Yosemite NP Air Pollution Impacts web page.

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Last Updated: January 03, 2017