Park Air Profiles - Rocky Mountain National Park

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

Hallett Peak reflected in the water of Dream Lake in Rocky Mountain NP
Visitors come to Rocky Mountain NP to enjoy scenic views of alpine lakes, forests, and wildlife in the Rocky Mountains.

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.

Mycorrhizal fungi community response

Many plants have a symbiotic relationship with mycorrhizal fungi (MF). Through the roots, the plants supply the fungi with carbon from photosynthesis and in exchange the MF enhance nutrient availability within soils, increase drought tolerance, and provide physical resistance to soil erosion (George et al., 1995; Cheng et al., 2021; Burri et al., 2013). Anthropogenic Nitrogen (N) deposition can disrupt this symbiotic relationship resulting in a shift from N sensitive to N tolerant mycorrhizal fungi and plant communities.

With increased N deposition to the soil, MF become less important for nutrient uptake and many plants will cease the exchange of nutrients altogether making them more vulnerable to stressors such as drought (Lilleskov et al., 2019). The CL-N for the shift in mycorrhizal community is 5-6 kg-N ha-1 yr-1 in coniferous forests and 10-20 kg-N ha-1 yr-1 broadleaf forests.
Rocky Mountain National Park has 718.3 km2 of coniferous forests, 1.3 km2 of broadleaf forests. Using the range in critical loads above, the minimum CL is exceeded in 21.1% of forested area and the maximum CL is exceeded in 1.7% of forested area based on 2019-2021 TDep Total N deposition.

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 ROMO 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 4.5 to 3.5 kg-N ha-1 yr-1 and maximum N deposition decreased from 7.6 to 7.4 kg-N ha-1 yr-1.
  • Minimum S deposition decreased from 1.1 to 0.3 kg-S ha-1 yr-1 and maximum S deposition decreased from 2.1 to 1.0 kg-S ha-1 yr-1.
Two maps showing ROMO boundaries. The left map shows the spatial distribution of estimated total nitrogen deposition levels from 2000-2002. The right map shows the spatial distribution of estimated total sulfur deposition levels from 2000-2002. Two maps showing ROMO boundaries. The left map shows the spatial distribution of estimated total nitrogen deposition levels from 2000-2002. The right map shows the spatial distribution of estimated total sulfur deposition levels from 2000-2002.

Estimated total nitrogen and sulfur deposition levels from 2000-2002 (top) compared to the 2019-2021 (bottom) average at ROMO. 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.

Persistent Pollutants

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 consumption. 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 US EPA’s human consumption guidance in 2%, 15%, and 3% of fish sampled, respectively (Eagles-Smith et al. 2014). Fish consumption advisories may be in effect for mercury and other contaminants (NPS 2022).
  • 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). However, 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 (Schreck and Kent 2013; Schwindt et al. 2009).
  • 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 (Battaglin et al. 2018; Keteles 2011). 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. 2010; Landers et al. 2008; 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).

The NPS Air Resources Division reports on park conditions and trends for mercury. Visit the webpage to learn more.


Scenic view in Rocky Mountain NP
Clean, clear air is essential to appreciating the scenic vistas at Rocky Mountain NP.

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.

Visibility effects:

  • 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!

Visit the NPS air quality conditions and trends website for park-specific visibility information. Rocky Mountain NP has been monitoring visibility since 1979. Explore air monitoring »

Ground-Level Ozone

Ozone garden in Rocky Mountain NP
Rocky Mountain NP uses gardens of ozone-sensitive plants to educate park visitors on the harmful effects of ground-level ozone.

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 grow and store carbon. The US Environmental Protection Agency led ozone exposure experiments on trees that are abundant in United States forests. Experimental results showed ozone slowed tree seedling growth. NPS uses W126 values of 7 and 13 ppm-h from averaged tree response in those experiments to describe Vegetation Health condition in parks. A recent re-analysis of those experiments established critical levels of ozone protective of each tree species tested, rather than to tree seedlings as a class of vegetation (Lee et al. 2022).

The ozone critical levels are W126 values that will prevent 5% or greater loss of tree seedling biomass. Some tree species are ozone sensitive and have critical levels lower than the current ARD breakpoints of 7 and 13 ppm-h. Some tree species are ozone tolerant, and their critical levels are higher than the current breakpoints.

ARD Conditions and Trends reports a 5-year average of W126 for each park. In 2018-2022, the average W126 value for Rocky Mountain National Park was 2.6 ppm-h. Based on this [FQE1] ozone level, trees present in the park (NPSpecies) are at risk of the following ozone effects:

  • The tree species Ponderosa Pine (Pinus Ponderosa, ssp.scopulorum), with an ozone critical level of 6 ppm-h, is at risk of 13% biomass loss in seedlings. The tree species Quaking Aspen (Populus Tremuloides), with an ozone critical level of 9 ppm-h, is at risk of 14% biomass loss in seedlings. Recent ozone levels in the park exceed critical levels that protect these species
  • Tree species Douglas-Fir (Pseudotsuga menziesii, critical level of 51.4) is at low risk of ozone effects. Recent ozone levels in the park are below critical levels that protect these from 5% or greater biomass loss.

Ozone critical levels are for tree seedlings, which represent the regenerative capacity and long-term stability of sensitive species within a forest. These tree species are also known to be sensitive to ozone as adults (Bell et al 2020), but critical values for seedling growth do not predict ozone effects on adult trees. ARD is currently working with collaborators to establish critical levels for adult trees using data from forest monitoring plots.

Visit the NPS air quality conditions and trends website for park-specific ozone information. Rocky Mountain NP has been monitoring ozone since 1987. Explore air monitoring data »

Explore Other Park Air Profiles

There are 47 other Park Air Profiles covering parks across the United States and its territories.


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Part of a series of articles titled Park Air Profiles.

Rocky Mountain National Park

Last updated: May 24, 2024