Air quality at Chiricahua National Monument
Most visitors expect clean air and clear views in parks. Chiricahua National Monument (NM), Arizona, is a wonderland of rock spires, pinnacles, columns, and balanced rocks. However, upwind urban and industrial sources—including the Tucson and Phoenix metropolitan areas, oil and gas development and production, and power plants—can degrade air quality. 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 Chiricahua NM, and in parks across the U.S., through science, policy and planning, and by doing our part.
Many visitors come to Chiricahua NM to enjoy views of striking rock pinnacles against the Sonoran Desert landscape. 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. Significant improvements in park visibility have been documented since the 1990’s. Still, visibility in the park is a long way from the Clean Air Act goal of no human caused impairment.Visibility effects:
- Reduced visibility, at times, due to human-caused haze and fine particles of air pollution, including dust;
- Reduction of the average natural visual range from about 165 miles (without pollution) to about 115 miles because of pollution at the park;
- Reduction of the visual range to below 75 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 plant tissues making it harder for plants to produce and store food. It also weakens plants making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Chiricahua NM were at low risk of damage to plant leaves (see network report: Kohut 2004). Generally, dry conditions in the park during peak ozone concentrations are likely to limit ozone uptake by plants. However along streams and seeps, where conditions are wetter, plants may have higher ozone uptake and injury (Kohut et al. 2012). Surveys in the early 1990s found slight ozone injury on ponderosa pines at nearby Saguaro National Park (Miller et al. 1996). Ozone sensitive plant species at the park include Apocynum androsaemifolium (spreading dogbane) and Rudbeckia laciniata (cut-leaf coneflower). Search for more ozone-sensitive plant species found at Chiricahua NM.
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 Chiricahua NM relative to other national parks is moderate (Sullivan et al. 2011c; Sullivan et al. 2011d). The park’s seeps and springs may be sensitive to incoming acid inputs. However there is no evidence that acidification has occurred, and many areas of the park are thought to be well-buffered from acidification. Some plants are sensitive to acidification, search for acid-sensitive plant species found at Chiricahua NM.
Excess nitrogen can also lead to nutrient enrichment, a process that changes nutrient cycling and alters plant communities. Plants in arid ecosystems are particularly vulnerable to changes caused by nitrogen deposition, as they are often nitrogen-limited. Ecosystem sensitivity to nutrient enrichment at Chiricahua NM relative to other national parks is high (Sullivan et al. 2011a; Sullivan et al. 2011b). 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). Based on current research, estimated deposition levels at Chiricahua NM are low enough that no known critical-loads for park ecosystems are currently exceeded. Since other desert ecosystems are known to be very sensitive to nutrient additions, it is important to continue to monitor for changes across the monument.
Invasive grasses tend to thrive in areas with elevated nitrogen deposition, displacing native vegetation adapted to low nitrogen conditions. In nearby desert ecosystems, an increase in nitrogen has been found to promote invasions of fast-growing exotic annual grasses and forbs at the expense of native species (Brooks 2003; Allen et al. 2009; Schwinning et al. 2005). Lehman’s Love Grass and Russian thistle are invasive species of particular concern at Chiricahua NM and the Sonoran Desert ecosystems. Greater cover of exotic grasses has been shown to increase fire risk in arid areas (Rao et al. 2010). Interactions between nitrogen, invasive annual grasses, and fire have profound implications for changes to biodiversity in the Sonoran Desert.
Visit the NPS air quality conditions and trends website for park-specific nitrogen and sulfur deposition information. Chiricahua NM has been monitoring nitrogen and sulfur since 1999. Explore air monitoring »
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.
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.
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
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.
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
[NPS] National Park Service, Air Resources Division. 2013. Air quality in national parks: trends (2000–2009) and conditions (2005–2009). Natural Resource Report NPS/NRSS/ARD/NRR–2013/683. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/2197275.
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
Rao, L. E., E. B. Allen, and T. Meixner. 2010. Risk-based determination of critical nitrogen deposition loads for fire spread in southern California deserts. Ecological Applications 20: 1320–1335.
Schwinning, S., B. I. Starr, N. J. Wojcik, M. E. Miller, J. E. Ehleringer, R. L. Sanford. 2005. Effects of nitrogen deposition on an arid grassland in the Colorado plateau cold desert. Rangeland Ecology and Management. 58: 565–574.
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. 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