For more information about National Park Service air resources, please visit http://www.nature.nps.gov/air/.


Critical Loads for Resource Protection

Air pollution emitted from a variety of sources eventually deposits out of the air into natural environments. Airborne pollutants cause undesirable effects, such as acidification, soil nutrient imbalances, and loss of biodiversity. Certain ecosystems in national parks are particularly vulnerable to pollutant deposition, including high elevation lakes and streams, alpine meadows, sugar maple forests, and desert shrublands. The use of critical loads has become a valuable tool for assessing ecosystem health and guiding resource management decisions.

» Explore critical loads in parks

  • Basics
  • Research
  • Application
  • Case Study

What are critical loads?

“Critical loads” is a term used to describe the amount of pollution initiating harmful changes in sensitive ecosystems. A critical load is defined as “the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.”

Critical loads can be used to assess a variety of ecosystem responses to air pollution deposition, including changes in aquatic and terrestrial 0plant diversity, soil nutrient levels, or fish health. Some parts of an ecosystem are more sensitive than others; therefore the effects of pollutant loading will differ within ecosystems. For example, critical loads to maintain healthy fish in lakes and streams are different than those for soil conditions needed to sustain healthy forests. When critical loads are exceeded, the environmental effects can cascade. For instance, excess nitrogen can act as a fertilizer, encouraging exotic grass species to establish in desert ecosystems in sufficient densities to fuel wildfires in areas not adapted to fire.

Critical loads are being developed in the U.S. for nitrogen or sulfur compounds, which are emitted by vehicles, power plants, industry, agriculture, and other sources. Critical loads are usually expressed as kilograms per hectare per year (kg /ha/yr) of wet or total (wet and dry) deposition.

Nitrogen Example

As nitrogen deposition increases, impacts to sensitive ecosystem components also increase. Because pollutants accumulate in soils and lakes over time, it can take centuries to reverse ecosystem degradation once it occurs. Prevention, therefore, is preferable (click on image to enlarge).

How are critical loads developed?

Subalpine meadow ecosystem at North Cascades NP
A U.S. Forest Service 2011 report identifies nitrogen critical loads for ecosystems across the U.S.

Scientists calculate critical loads by using ecosystem modeling, field observations, or experiments. For example, water chemistry measurement from hundreds of western lakes have been used to determine nitrogen deposition critical load causing lake nutrient enrichment. In the Rocky Mountains, this critical load is 3.0 kg/ha/yr of total (wet and dry) nitrogen deposition, while it is 2.0 kg/ha/yr in the Sierra Nevada (Baron et al. 2011).

Scientists are currently developing critical loads, and air quality regulators and natural resource managers are beginning to select target loads for sensitive ecosystems. A recent research report published by the U.S. Forest Service compiled atmospheric nitrogen deposition critical loads for ecosystems across the U.S. (Pardo et al. 2011). This report significantly augments the critical loads approach as a management and policy tool because it provides hundreds of ecosystem-specific critical loads relevant to all ecoregions in the country.


How are critical loads used?

Critical loads can be used to assess ecosystem condition, evaluate effectiveness of emission reduction strategies, establish goals for resource protection and ecosystem recovery, and guide park planning.

Critical loads are valuable tools for evaluating and communicating ecosystem condition to policy makers and the public. Identifying areas where current deposition exceeds critical loads can help air quality regulators and land managers determine where emission reductions are most needed. While critical loads are science-based, managers may identify a “target” load to guide policy or management decisions such as emission reduction goals. For areas where the critical loads have not been exceeded, a protective target load may be set at a level lower than the critical load to prevent future resource harm.

Setting a target load plays an important role in guiding regulatory or voluntary measures to reduce air pollutants. Target loads can be set and communicated to the public via land management planning processes and other collaborative forums, and provided as input into air pollution permit reviews and NEPA processes.

Target loads are usually based on a critical load and represent a policy or management decision specifying the amount of deposition that would result in an acceptable level of resource protection. If current deposition is below the critical load, a protective target load is set below the critical load to prevent degradation. An interim target load can also be set to represent a benchmark for progress toward reducing deposition(click on image to enlarge).

Critical Loads: A Rocky Mountain National Park Case Study

Critical loads have been successfully applied in Canada and Europe as air management tools. Although critical loads have not been widely used in the U.S., several federal agencies, including the National Park Service, are employing critical loads to protect and manage sensitive ecosystems.

In an effort to address harmful impacts of nitrogen deposition and related air quality issues at Rocky Mountain National Park (RMNP), the Colorado Department of Public Health and Environment, the National Park Service, and the U.S. Environmental Protection Agency entered into a Memorandum of Understanding (MOU). After much collaboration, the MOU agencies issued the Nitrogen Deposition Reduction Plan (NDRP) in 2007.

As part of the NDRP, the Rocky Mountain NP Superintendent set a resource management goal (target load) of 1.5 kilograms of nitrogen per hectare per year (kg N/ha/yr) wet deposition as the threshold for adverse ecosystem effects in RMNP. This threshold is based on decades of research and is equal to the “critical load” of nitrogen that can be absorbed by ecosystems within RMNP before detrimental changes occur.

To achieve this goal, the stakeholders developed a “glidepath” for deposition reductions. This type of approach anticipates gradual reductions over time and is similar to the approach used for regional haze plans. The glidepath approach allows for the target load to be met over the course of 25 years. The process includes monitoring and assessing nitrogen deposition over time, and exploring and recommending emissions reductions strategies if interim goals are not met.

To implement the Rocky Mountain National Park resource management goal of 1.5 kg/ha/yr wet nitrogen deposition, Agency partners developed a “glidepath” process to monitor and assess whether emissions reductions strategies were resulting in sufficient deposition decreases to meet this goal by the 2032 target year (click on image to enlarge).

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Last Updated: December 18, 2014