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Scenic views and native vegetation images from parks within Sonoran Desert Network

Air Pollution Impacts

Chiricahua National Monument

Natural and scenic resources in Chiricahua National Monument (NM) are susceptible to the harmful effects of air pollution. Fine particles, nitrogen, sulfur, and ozone impact scenic resources such as visibility, and natural resources such as soils and vegetation. Click on the tabs below to learn more about air pollutants and their impacts on natural and scenic resources at Chiricahua NM.

  • Visibility
  • Ozone
  • Nitrogen & Sulfur

Many visitors come to Chiricahua NM to enjoy views of striking rock pinnacles against the Sonoran Desert landscape. Unfortunately, these vistas are sometimes obscured by haze caused by fine particles in the air. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze and visibility impairment. Additionally, organic compounds, soot, fires, and dust storms reduce visibility.

Visibility effects at Chiricahua NM include:

  • 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 160 miles (without pollution) to about 120 miles because of pollution at the park;
  • Reduction of the visual range to below 75 miles on high pollution days.

(Source: IMPROVE 2013)

Images of good and poor visibility at Chiricahua National Monument, Arizona
Air pollutants can affect visibility at Chiricahua NM, Arizona (clear to hazy from left to right).

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Photograph of Spreading Dogbane.
Elevated ground-level ozone concentrations may affect ozone-sensitive plants, such as spreading dogbane, at Chiricahua NM, Arizona.

Naturally-occurring ozone in the upper atmosphere absorbs the sun’s harmful ultraviolet rays and helps to protect all life on earth. However, in the lower atmosphere, ozone is an air pollutant, forming when nitrogen oxides from vehicles, power plants, and other sources combine with volatile organic compounds from gasoline, solvents, and vegetation in the presence of sunlight. In addition to causing respiratory problems in people, ozone can injure plants. Ozone enters leaves through pores (stomata), where it can kill plant tissues, causing visible injury, or reduce photosynthesis, growth, and reproduction.

A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Chiricahua NM were at low risk of foliar ozone injury (Kohut 2004 [pdf, 180 KB]). However, ozone concentrations and cumulative doses at the park may be high enough to induce foliar injury to sensitive vegetation under certain conditions. Surveys in the early 1990s found slight ozone injury on ponderosa pines at the nearby Saguaro National Park (Miller et al. 1996), but the typical dry conditions in the park cause plant stomates to close, limiting ozone uptake. In other parks, scientists have found that in moist areas along streams and seeps, plants may keep stomates open more often, allowing ozone uptake and subsequent injury (Kohut et al. 2012). Ozone sensitive plant species at the park include Apocynum androsaemifolium (spreading dogbane) and Rudbeckia laciniata (cut-leaf coneflower).

Additional Information:

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Photograph of Rock lichen from Chiricahua NM.
Rock lichen, pictured here at Chiricahua NM, are very responsive to nitrogen inputs. Excess N can shift lichen community composition from N-sensitive to N-loving species, with the potential for greater ecosystem impacts.

Nitrogen (N) and sulfur (S) compounds deposited from air pollution can harm vegetation, soils, lichens, and surface waters throughout Chiricahua NM. N levels are considerably higher than estimated natural background levels (NPS 2013 [pdf 2.1 MB]).

Nitrogen acts as a fertilizer, disrupting soil nutrient cycling, altering plant communities, and contributing to overenrichment and eutrophication. Plants in arid ecosystems are particularly vulnerable to changes caused by nitrogen deposition, as they are often nitrogen-limited. Ecosystem sensitivity to nutrient N enrichment at Chiricahua NM relative to other national parks is high (Sullivan et al. 2011a; Sullivan et al. 2011b [pdf, 6.4 MB]).

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 N has been found to promote invasions of fast-growing exotic annual grasses (e.g., cheatgrass) and forbs (e.g., Russian thistle) 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 grasses 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 N, invasive annual grasses, and fire have profound implications for changes to biodiversity in non-fire adapted ecosystems (e.g., Sonoran Desert).

N, together with S, can also acidify surface waters and soils. Ecosystem sensitivity to N & S acidification at Chiricahua NM relative to other national parks is moderate (Sullivan et al. 2011c; Sullivan et al. 2011d [pdf, 2.2 MB]). 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.

Additional Information:

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Studies and Monitoring icon

Studies and monitoring help the NPS understand the environmental impacts of air pollution. Access air quality data and see what is happening with Studies and Monitoring at Chiricahua NM.

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Last Updated: December 30, 2016