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Great Lakes Air Deposition

National Information

Pollutants emitted into the atmosphere can be transported and then deposited to aquatic ecosystems nearby or far from their original sources. Chemicals from anthropogenic sources are now present in Arctic and Antarctic ecosystems, and studies from fish on a remote island in northern Lake Superior have shown contamination from chemicals in use only outside, and in some cases, far away from this isolated island.

U.S EPA Region 5 is home to the Great Lakes - an ecosystem containing 18 percent of the world's fresh water supply, and 95 percent of the surface freshwater within the United States. Scientific evidence shows that air certain persistant and toxic air may contribute significantly to water pollution and bioaccumulate in the food web. Therefore Congress created the Great Water Program in section 112(m) in the 1990 Clean Air Act Amendments. The purpose of the Great Waters program is to evaluate the atmospheric deposition of air pollutants to the Great Lakes and determine whether the current Clean Air Act provisions are sufficient to prevent serious adverse effects to public health and the environment.

What Are We Doing About Air Deposition of Pollutants to the Great Lakes?

Our strategy is a two-track approach: A fast track of actions that can be taken right now including pollution prevention and the "virtual elimination" project; and a science track for study and assessment of the problems and solutions through modeling, monitoring, and emission inventories.

Virtual Elimination of Pollutants of Concern:

The Canadian - U.S. strategy to virtually eliminate persistent toxic substances in the Great Lakes Basin (the Bi-national Strategy) seeks to achieve quantifiable reduction goals between now and 2005 for specific toxic substances. more information. Exit EPA Disclaimer

Monitoring the Pollutants of Concern:

Monitoring stations (one per lake) are collecting wet and dry toxic air deposition samples. This is the Integrated Atmospheric Deposition Network (IADN), in operation since 1992 and is designed to monitor a background, or 'continental' signal, away from local or urban influences. Other complementary stations around the Lakes and ship-based intensive collection efforts are measuring urban influence or collecting data for other special studies. Monitoring itself can't provide all the information as monitoring is very expensive and limited in spatial scope. But monitoring can give loading estimates and can be used to validate or provide feedback into models.

Modeling the Pollutants of Concern:

We need to understand the transport of toxic pollutants from their points of emission to their eventual deposition, while also understanding the physical and chemical transformations which also affect the probability for removal from the air into the water.
Airborne chemicals can take a variety of forms: gases, condensed vapors, large or small particles, or adsorption onto other particles. Some removal processes such as dry deposition of gases to land or water are continual, while others, like rain out are episodic. The meteorology is complex and so are micro processes which occur at the air-water interface. The information from these dispersion and deposition models can be further used as inputs to a mass balance model. The modeler must have good and comprehensive information describing emissions from a large number of sources as input.

The Mass Balance

Tracking Emissions:

An air emission inventory is typically a mathematical estimate of pollutants from sources through the use of emission factors. These emission factors are derived from actual measurements of the emissions from representative sources and are derived specifically for one type of process or processequipment. Emission factors can be used to estimate both the amount and type of pollutants being emitted based on the material processed. Emission inventories are the most practical way for agencies to estimate emissions given the large number of sources. Air pollution can be emitted from point, area, or mobile sources. Point sources exist as a single point at a fixed location, such as a smokestack; area sources are small individual sources categorized by geographic area (e.g., dry cleaners or degreasers); and mobile sources are cars, planes, and other vehicles which release pollutants while moving.

U.S.EPA, the Great Lakes States, Ontario, and the Great Lakes Commission are working together to create the Great Lakes Regional Air Toxics Emissions Inventory. The inventory data is publically available at The Centralized Air emission Repository On-Line (CAROL) Exit EPA Disclaimer website.

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