Explore Air
What goes up must come down. Air pollutants eventually deposit into ecosystems, causing damage. The amount of pollution that can be tolerated before harm occurs is known as the CRITICAL LOAD.

What is a critical load?

Air pollution emitted from a variety of sources may eventually deposit out of the air into ecosystems.
Atmospheric deposition explained
In ecosystems, these pollutants may cause undesirable effects, such as acidification, soil nutrient imbalances, and loss of biodiversity.
A CRITICAL LOAD is the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment are not expected to occur according to present knowledge.
Critical loads can be developed for any type of air pollutant, but most commonly they are developed for nitrogen or sulfur compounds emitted by vehicles, powerplants, industry, agriculture, and other sources. More »

How are critical loads calculated?

Scientists calculate critical loads using ecosystem modeling, observations, or experiments. A critical load is specific to a certain resource, such as alpine plant diversity, and a defined effect, such as a 10 percent loss in diversity.
 Example of critical load at Rocky Mountain National Park
Critical loads are expressed as loading rates of the pollutant in a given area over time, usually kilograms of sulfur or nitrogen per hectare per year. Critical loads are used to evaluate resource condition, establish benchmarks for resource protection and ecosystem recovery, guide park planning, and inform air quality management. Each region is unique in how it receives and assimilates airborne pollutants.

How is the National Park Service developing and using critical loads?

The National Park Service is dedicated to resource protection and recognizes critical loads as valuable tools for evaluating and communicating ecosystem condition. We are encouraging and supporting critical loads research in parks and are participating in multiagency efforts to advance the science of critical loads. One of these efforts, the Critical Loads Ad-Hoc Committee (CLAD), includes the National Park Service, the Environmental Protection Agency, the Forest Service, and the U.S. Geological Survey. CLAD is sponsoring several critical loads projects in parks, including Shenandoah, Great Smoky Mountains, Rocky Mountain, and Glacier National Parks.
updated on 10/23/2009  I   http://nature.nps.gov/air/Studies/criticalLoads/index.cfm   I  Email: Webmaster

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