Air quality at Saguaro National Park
Most visitors expect clean air and clear views in parks. Saguaro National Park (NP), in Arizona, is a desert and mountainous landscape and home to North America's largest cacti—the giant saguaro. Air quality in the park is affected at times by, upwind urban and industrial sources, including the Tucson and Phoenix metropolitan areas. Pollution sources in Mexico can also affect the park. 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 Saguaro NP, and in parks across the U.S., through science, policy and planning, and by doing our part.
Visitors come to Saguaro NP to stand amidst giant saguaro cacti and to enjoy a scenic backdrop of the Tucson and Rincon Mountain ranges. Park vistas are sometimes obscured by haze, reducing how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air, and these particles can also affect human health. 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. Modest improvements in park visibility on the haziest days have been documented since the mid 2000’s, but visibility in the park requires significant improvement to reach the Clean Air Act goal of no human caused impairment.Visibility effects:
- Reduced air clarity, at times, due to human-caused haze and fine particles of air pollution, including dust;
- Reduction of the average natural visual range from about 160 miles (without pollution) to about 100 miles because of pollution at the park;
- Reduction of the visual range to below 70 miles on high pollution days.
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.
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. A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Saguaro NP are at low risk of ozone injury (Kohut 2007; Kohut 2004). However, ozone concentrations and cumulative doses at the park are high enough to damage plant leaves on sensitive vegetation under certain conditions (NPS 2010). Surveys in the early 1990s found slight ozone injury on ponderosa pines (Miller et al. 1996), but the typical dry conditions in the park limit ozone uptake by plants. Some plants are more sensitive to ozone than others. Ozone sensitive plant species at the park include Apocynum androsaemifolium (Spreading dogbane), Rudbeckia laciniata (cut-leaf coneflower), and Pinus ponderosa (ponderosa pine). Search for more ozone-sensitive plant species found at Saguaro NP.
Nitrogen and sulfur
Nitrogen and sulfur compounds deposited from the air may have harmful effects, including acidification, on soils, lakes, ponds, and streams. Ecosystem sensitivity to acidification at Saguaro NP relative to other national parks is high (Sullivan et al. 2011c; Sullivan et al. 2011d). Small streams with steep-sided canyon walls in higher elevations of the park have little ability to retain nutrients and water, or to buffer potentially acidic run-off. Perennial pools in lush desert oases in the park may be sensitive to acid deposition. Fortunately, there is no evidence that acidification has occurred in park streams or pools, and many areas of the park are well-buffered from acidification. Still, some plants are sensitive, search for acid-sensitive plant species found at Saguaro NP.
Excess nitrogen can also lead to nutrient enrichment, a process that changes nutrient cycling and alters plant communities, including arid, semi-arid, and grassland communities. Plants in arid ecosystems, like that of Saguaro NP, are often nitrogen-limited and especially vulnerable to changes caused by nitrogen deposition. Ecosystem sensitivity to nutrient enrichment at Saguaro NP relative to other national parks is very high (Sullivan et al. 2011a; Sullivan et al. 2011b).
In nearby desert ecosystems, increased nitrogen has been found to promote the spread of fast-growing exotic annual grasses (like cheatgrass) and forbs (like Russian thistle) at the expense of native species (Brooks 2003; Allen et al. 2009; Schwinning et al. 2005). Buffelgrass is an invasive grass of particular concern in southern Arizona and the Sonoran Desert ecosystems. Nitrogen favors buffelgrass over native species in similar ecosystems (Lyons et al. 2013). Greater cover of non-native grasses has been shown to increase fire risk in arid areas (Rao et al. 2010). Interactions between nitrogen, non-native annual grasses, and fire have profound implications for changes to biodiversity in non-fire adapted ecosystems like the Sonoran Desert. Increased nitrogen may also exacerbate water use in plants like big sagebrush (Inouye 2006).
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 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.
Allen, E. B., L. E. Rao, R. J. Steers, A. Bytnerowicz, and M. E. Fenn. 2009. Impacts of atmospheric nitrogen deposition on vegetation and soils in Joshua Tree National Park. Pages 78–100 in R. H. Webb, L. F. Fenstermaker, J. S. Heaton, D. L. Hughson, E. V. McDonald, and D. M. Miller, editors. The Mojave Desert: ecosystem processes and sustainability. University of Nevada Press, Las Vegas, Nevada, USA.
Allen, E. B. and L. H. Geiser. 2011. North American Deserts. In L.H. Pardo, M.J. Robin-Abbott and C.T. Driscoll (Eds.). Assessment of Nitrogen Deposition Effects and Empirical Critical Loads of Nitrogen for Ecoregions of the United States. General Technical Report NRS–80. U.S. Forest Service, Newtown Square, PA. pp. 133–142. Available at: http://nrs.fs.fed.us/pubs/38109.
Brooks, M.L. 2003. Effects of increased soil nitrogen on the dominance of alien annual plants in the Mojave Desert. Journal of Applied Ecology. 40:344–353.
Inouye, R.S. 2006. Effects of shrub removal and nitrogen addition on soil moisture in sagebrush steppe. Journal of Arid Environments. 65: 604–618.
Lyons, K.G., Maldonado-Leal, B.G., Owen, G. 2013. Community and Ecosystem Effects of Buffelgrass (Pennisetum ciliare) and Nitrogen Deposition in the Sonoran Desert. Invasive Plant Science and Management 6: 6(1): 65–78. In Press.
Kohut, B., C. Flanagan, E. Porter, J. Cheatham. 2012. Foliar Ozone Injury on Cutleaf Coneflower at Rocky Mountain National Park, Colorado. Western North American Naturalist 72(1): 32–42.
Kohut, B. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation in Parks in the Sonoran Desert Network. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181558.
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
Miller, P.R. 1996. Extent of Ozone Injury to Trees in the Western United States. U.S. Forest Service Pacific Southwest Research Station. General Technical Report PSW–GTR–155–Web. Available at http://www.fs.fed.us/psw/publications/documents/gtr-155/01-miller.html
National Park Service [NPS], Air Resources Division. 2010. Air quality in national parks: 2009 annual performance and progress report. Natural Resource Report NPS/NRPC/ARD/NRR—2010/266. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/662783.
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
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: Sonoran Desert Network (SODN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/330. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2168736.
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: Sonoran Desert Network (SODN). Natural Resource Report NPS/NRPC/ARD/NRR–2011/379. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2170609.
Sullivan, T. J. and T.C. McDonnell. 2014. Mapping of nutrient-nitrogen critical loads for selected national parks in the intermountain west and Great Lakes regions. Natural Resource Technical Report NPS/ARD/NRTR—2014/895. National Park Service, Fort Collins, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2214130.
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