Air pollution travels to parks with the wind, depositing nitrogen and sulfur compounds that can acidify or artificially fertilize (enrich) soils and surface waters. Deposited nitrogen comes from both natural (wildfires and lightning) and human sources (power plants, industrial facilities, and agriculture). The largest sources of deposited sulfur are sulfur dioxide emissions from fossil fuel combustion at power plants and other industrial facilities. National parks protect high elevation lakes and streams, alpine meadows, lichen, and desert shrublands that are sensitive to this deposition.
Atmospheric nitrogen and sulfur deposition can lead to changes in water chemistry (acidification). This effects water plants and animals ranging from diatoms and insects, to amphibians and fish. Deposition can also cause chemical changes in soils that alter microorganisms, plants, and trees. Nitrogen is an essential plant nutrient but, excess nitrogen from atmospheric deposition can stress ecosystems. Excess nitrogen acts as a fertilizer, favoring some plants and leaving others at a competitive disadvantage. This creates an imbalance in natural ecosystems, and over time may lead to:
Excess nitrogen can also cause toxic algal blooms, fish deaths, and loss of plant and animal diversity in coastal and estuarine ecosystems. NPS acidification risk assessments and nitrogen risk assessments rank park risk according to pollutant exposure, ecosystem sensitivity, and park protection mandates. Learn more about the effects of air pollution in parks by searching for acid sensitive tree species by park or visiting the Park Air Profiles and AQRV Inventory Products pages.
Research in parks around the country has shown that nitrogen and sulfur deposition cause ecosystem changes most often in nutrient-poor environments. Plant species and soil nutrient cycling can rapidly change when there is too much nitrogen in these environments. High elevation ecosystems in the Rocky Mountains, Cascades, and Sierra Nevada are very sensitive to increased nitrogen. Alpine lakes in these areas are naturally nutrient poor, leading to sudden changes in algae with small additions. Also, the health of land and water ecosystems on the east side of Rocky Mountain National Park have been affected by nitrogen deposition. At Joshua Tree National Park non-native grasses, fertilized by nitrogen deposition, have spread so much that fire frequency has increased.
The biggest stress to water bodies from nitrogen and sulfur deposition in the Eastern U.S. is acidification. Shenandoah and Great Smoky Mountains National Parks have both experienced chronic acidification. This resulted from years of heavy pollution, much of which occurred before emission reductions required by the Clean Air Act. Many affected streams are still uninhabitable for native brook trout and their food sources.
Learn more about the effects of nitrogen and sulfur deposition for selected parks in our park profiles section.
Changes in species and habitat quality from deposition can also impact ecosystem services such as water purification, local businesses, climate regulation, aesthetic enjoyment, fishing, and biodiversity maintenance.
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). If deposition stays below these levels, ecosystems are usually protected from damage. Some parts of an ecosystem are more sensitive than others, so the effects of pollutant loading will be different within ecosystems. For example, the critical loads for surface waters that can support healthy fish are different than critical loads for healthy forest. Learn more about critical loads and discover which ecosystems are at risk from nitrogen deposition in each national park.
The National Park Service monitors air pollution in and near many national parks.
Nitrogen and sulfur compounds are monitored by the National Park Service and interagency partners in and near many national parks. We also track park-specific information about nitrogen and sulfur conditions and trends. Learn more about:
Last updated: August 26, 2022