Air Quality at Rocky Mountain National Park
Most visitors expect clean air and clear views in parks. Rocky Mountain National Park (NP), Colorado, is impacted by many sources of air pollution, including vehicles, power plants, agriculture, fire, oil and gas, and other industry. 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 Rocky Mountain NP, 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 Rocky Mountain National Park (ROMO) relative to other national parks is very 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 ROMO relative to other national parks is very high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in ROMO ranged from 2.6 to 6.0 kg-N ha-1 yr-1 and total S deposition ranged from 0.5 to 1.1 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). ROMO has been monitoring atmospheric N and S deposition since 1983, see the conditions and trends website for park-specific information.
Reducing N deposition to below the critical load is a park resource management goal, and is a goal for the Rocky Mountain National Park Initiative to protect and restore natural resources in the park (Porter and Johnson 2007).
Alpine ecosystem effects
Alpine environments are particularly vulnerable to large inputs of reactive nitrogen because of the sparse cover of vegetation, short growing seasons, large areas of exposed bedrock and talus, and snowmelt nutrient releases (Williams et al., 1996; Nanus et al., 2012). Approximately 15% of the land area in ROMO is alpine (~328 km2 above 1550 m). McClung et al. (2021) compared the 2015 estimated total N deposition (TDep; NADP, 2018) to the critical load of N for an increase in alpine sedge growth (alpine plant critical load = 3 kg-N ha-1yr-1) and the critical load of N for alpine soil nitrate leaching (alpine soil critical load = 10 kg-N ha-1yr-1; Bowman et al., 2012). They found that deposition exceeded the alpine plant critical load in 81% of the park’s alpine area, but was below the alpine soil critical load throughout the park’s entire alpine area.
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. 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 595 km2 (55%) of the land area of Rocky Mountain National Park.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 80.3% 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, Rocky Mountain National Park contains:
- 4 N-sensitive tree species and 43 N-sensitive herbaceous species.
- 6 S-sensitive tree species and 38 S-sensitive herbaceous species.
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.
- Mercury concentrations in some fish sampled at Rocky Mountain NP exceeded the toxicity thresholds for fish, bird, and human health. Fish were sampled and analyzed for mercury from 19 sites sampled at the park and compared to data across 21 western parks. The average fish mercury concentration (66.1 ng/g ww) was slightly lower than the study-wide mean (77.7 ng/g ww). Mercury concentrations exceeded the thresholds for fish toxicity, bird toxicity, and human consumption in 2%, 15%, and 3% of fish sampled, respectively (Eagles-Smith et al. 2014).
- Some dragonfly larvae sampled from the park had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from seven sites in the park; 52% of the data fall into the moderate (100-300 ng/g dw) and 15% fall into the high (300-700 ng/g dw) impairment categories for potential mercury risk. An index of moderate impairment or higher suggest some fish may exceed the US EPA benchmark for protection of human health (Eagles-Smith et al. 2020, Eagles-Smith et al. 2018). The data may not reflect the risk at other unsampled locations in the park.
- Some fish sampled from the park were found to be intersex. Reproductive abnormalities such as intersex, the presence of both male and female reproductive structures in the same fish, can signify exposure to contaminants. Two out of 52 male fish sampled from the park were found to be intersex (Schwindt et al. 2009; Schreck and Kent 2013).
- Airborne pesticides were found in park fish, frog, water, and sediment samples. Most of the pesticides and other bioactive contaminants were observed in concentrations that did not exceed any known benchmarks for aquatic life (Keteles 2011; Battaglin et al. 2018). However, pesticide concentrations in fish exceeded the subsistence consumption threshold for human health and the threshold for kingfisher health at 71% and 29% of sample sites, respectively (Flanagan Pritz et al. 2014).
- Related studies similarly found persistent pollutants in park snow, rain, sediment, and fish. Contaminants found included mercury, flame retardants, PBDEs, atrazine, dacthal, and carbaryl; concentrations were generally higher on Rocky Mountain NP’s east side than the west side (Landers et al. 2008; Landers et al. 2010; Mast et al. 2003). In 2021, the average mercury concentration found in snowpack was 3.17 ng/L at Lake Irene, 4.21 ng/L at Loch Vale Forest, and 2.6 ng/L at Loch Vale Meadow (USGS 2021).
- Microplastics were found in park precipitation samples. These microplastics, thought to be distributed by atmospheric transport, consisted of mostly clothing fibers like cotton, polyester, and nylon. Rocky Mountain NP is estimated to have an annual deposition rate of 9.4-9.8 metric tons of plastic per year (Brahney et al. 2020; Wetherbee et al. 2019).
Park vistas are sometimes obscured by haze, reducing 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 needs improvement to reach the Clean Air Act goal of no human caused impairment.
- Reduction of the average natural visual range from about 175 miles (without the effects of pollution) to about 135 miles because of pollution at the park.
- Reduction of the visual range in the summer from about 120 miles to below 75 miles on high pollution days.
Explore scenic vistas through three live webcams at Rocky Mountain National Park!
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.
Especially during the summer months, ozone levels in the park sometimes exceed the National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency to protect public health. Ozone is a respiratory irritant, causing coughing, sinus inflammation, chest pains, scratchy throat, lung damage, and reduced immune system functions. Children, the elderly, people with existing health problems, and active adults are most vulnerable. When ozone levels exceed, or are predicted to exceed, health standards, Rocky Mountain NP staff post health advisories cautioning staff and visitors of the potential health risks associated with exposures to elevated levels.
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. There are at least 15 ozone-sensitive plants in the park, including: cutleaf coneflowers (Rudbeckia laciniata), quaking aspen (Populus tremuloides), Scouler’s willow (Salix scouleriana), white sagebrush (Artemisia ludoviciana), and Canadian goldenrod (Solidago canadensis). Surveys at the park, reveal visible injury to cut-leaf coneflower leaves (Kohut 2012). Search ozone-sensitive plant species found at Rocky Mountain NP.
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