Estimated total nitrogen and sulfur deposition levels from 2000-2002 (top) compared to the 2019-2021 (bottom) average at CHIR. Estimated values were developed using the National Atmospheric Deposition Program - Total Deposition (TDep) approach that combines measured and modeled data. Estimated values are valuable for analyzing gradients of deposition and the resulting ecosystem risks where monitors are not present.
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.
Nitrogen and Sulfur
Nitrogen (N) and sulfur (S) compounds deposited from the air may have harmful effects on ecosystem processes. Healthy ecosystems can naturally buffer a certain amount of pollution, but once a threshold is passed the ecosystem may respond negatively. This threshold is the critical load, or the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). N and S deposition change ecosystems through eutrophication (N deposition) and acidification (N + S deposition). Eutrophication increases soil and water nutrients which causes some species to grow more quickly and changes community composition. Ecosystem sensitivity to nutrient N enrichment at Chiricachua National Monument (CHIR) relative to other national parks is high (Sullivan et al. 2016); for a full list of N sensitive ecosystem components, see: NPS ARD 2019. Acidification leaches important cations from soils, lakes, ponds, and streams which decreases habitat quality. Ecosystem sensitivity to acidification at CHIR relative to other national parks is high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in CHIR ranged from 3.5 to 3.8 kg-N ha-1 yr-1 and total S deposition ranged from 0.6 to 0.7 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). CHIR has been monitoring atmospheric N and S deposition since 1999, see the conditions and trends website for park-specific information.
Invasive grasses tend to thrive in areas with elevated N deposition, displacing native vegetation adapted to low N conditions. In nearby desert ecosystems, an increase in N 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 CHIR and the Sonoran Desert ecosystems.
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.
Epiphytic macrolichen community responses
Epiphytic macrolichens grow on tree trunks, branches, and boles. Since these lichens grow above the ground, they obtain all their nutrients directly from precipitation and the air. Many epiphytic lichen species have narrow environmental niches and are extremely sensitive to changes in air pollution. Epiphytic lichen communities are less diverse in arid areas, but are still impacted by air pollution. Geiser et al. (2019) used a U.S. Forest Service national survey to develop critical loads of nitrogen (N) and critical loads of sulfur (S) to prevent more than a 20% decline in four lichen community metrics: total species richness, pollution sensitive species richness, forage lichen abundance, and cyanolichen abundance.
McCoy et al. (2021) used forested area from the National Land Cover Database to estimate the impact of air pollution on epiphytic lichen communities. Forested area makes up 13.5 km2 (27.4%) of the land area of Chiricachua National Monument.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 98.3% of the forested area.
- S deposition was below the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in every part of the forested area.
For exceedances of other lichen metrics and the predicted decline of lichen communities see Appendices A and B of McCoy et al. (2021).
Additional modeling was done on 459 lichen species to test the combined effects of air pollution and climate gradients (Geiser et al. 2021). A critical load indicative of initial shifts from pollution-sensitive toward pollution-tolerant species occurred at 1.5 kg-N ha-1 yr-1 and 2.7 kg-S ha-1 yr-1 even under changing climate regimes.
Plant species response
Plants vary in their tolerance of eutrophication and acidification, and some plant species respond to nitrogen (N) or sulfur (S) pollution with declines in growth, survival, or abundance on the landscape. Horn et al. (2018) used the U.S. Forest Service national forest survey to develop critical loads of N and critical loads of S to prevent declines in growth or survival of sensitive tree species. Clark et al. (2019) used a database of plant community surveys to develop critical loads of N and critical loads of S to prevent a decline in abundance of sensitive herbaceous plant species. According to NPSpecies, Chiricachua National Monument contains:
- 5 N-sensitive tree species and 13 N-sensitive herbaceous species.
- 9 S-sensitive tree species and 10 S-sensitive herbaceous species.
Change in N and S deposition from 2000 to 2021
The maps below show how the spatial distribution of estimated Total N and Total S deposition in CHIR has changed from 2000-2002 to 2019-2021 (TDep MMF version 2022.02). Slide the arrows in the middle of the image up and down to compare N and S deposition between the two years (Yearly Data).
- Minimum N deposition decreased from 4.3 to 3.5 kg-N ha-1 yr-1 and maximum N deposition decreased from 6.3 to 5.5 kg-N ha-1 yr-1.
- Minimum S deposition decreased from 1.6 to 0.6 kg-S ha-1 yr-1 and maximum S deposition decreased from 2.1 to 0.8 kg-S ha-1 yr-1.
Pollutants like mercury and pesticides are concerning because they are persistent and toxic in the environment. These contaminants can travel in the air thousands of miles away from the source of pollution, even depositing in protected places like national parks. In addition, while some of these harmful pollutants may be banned from use, historically contaminated sites continue to endure negative environmental consequences.
When deposited, airborne mercury and other toxic air contaminants are known to harm wildlife like birds and fish, and cause human health concerns. Many of these substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.
The NPS Air Resources Division reports on park conditions and trends for mercury. Visit the webpage to learn more. Fish consumption advisories may be in effect for mercury and other contaminants (NPS 2022).
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.
- 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.
Explore Other Park Air Profiles
There are 47 other Park Air Profiles covering parks across the United States and its territories.
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.
Clark, C.M., Simkin, S.M., Allen, E.B. et al. Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States. Nat. Plants 5, 697–705 (2019). https://doi.org/10.1038/s41477-019-0442-8
Geiser, Linda & Nelson, Peter & Jovan, Sarah & Root, Heather & Clark, Christopher. (2019). Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens. Diversity. 11. 87. 10.3390/d11060087.
Geiser, Linda & Root, Heather & Smith, Robert & Jovan, Sarah & Clair, Larry & Dillman, Karen. (2021). Lichen-based critical loads for deposition of nitrogen and sulfur in US forests. Environmental Pollution. 291. 118187. 10.1016/j.envpol.2021.118187.
Horn KJ, Thomas RQ, Clark CM, Pardo LH, Fenn ME, Lawrence GB, et al. (2018) Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.. PLoS ONE 13(10): e0205296. https://doi.org/10.1371/journal.pone.0205296
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, 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.
McCoy K., M. D. Bell, and E. Felker-Quinn. 2021. Risk to epiphytic lichen communities in NPS units from atmospheric nitrogen and sulfur pollution: Changes in critical load exceedances from 2001‒2016. Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2287254.
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
[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/
[NPS] National Park Service. 2022. Fish Consumption Advisories. https://www.nps.gov/subjects/fishing/fish-consumption-advisories.htm
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
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. 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, CO.
Part of a series of articles titled Park Air Profiles.
Last updated: August 17, 2023