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Deposition and Critical Loads

A diagram explaining the acid deposition cycle. Diagram adapted from the US EPA.

Deposition occurs when compounds of various types of air pollution are deposited on the earth's surface through rain, clouds, snow, fog, or as dry particles. The amount of deposition received in a given area is affected both by the concentration of pollution in the atmosphere and the way in which it is deposited. General factors such as, meteorology and topography influence how much pollution reaches the area from both local and distant sources, as well as how much of that pollution actually lands on the earth's surface via the various wet and dry forms of deposition. There are several types of ecosystem effects associated with deposition that tie to the pollutant being deposited. These include acidic deposition, excess amounts of nitrogen and heavy metals (including mercury).

Sulfate Effects

Sulfate is the primary component of acidic deposition in the eastern U.S. with the highest levels of emissions coming from the heavily industrialized Ohio River Valley. Despite recent emission reductions across the eastern U.S., sulfate deposition still exceeds the tolerance level for healthy ecosystems in the Appalachian states.

Nitrogen Effects

Nitrogen deposition is the more dominant component of acid rain in the mid and western United States. In addition to contributing to acid rain, nitrogen can impact other ecosystem by excessively fertilizing land and water. These enhanced inputs of nitrogen can disrupt the natural flora and fauna by allowing certain species that would not naturally occur in abundance to out-compete those that thrive in pristine nitrogen-limited systems. The end result is an unnatural shift in species composition that may, in turn, have a subsequent impact on other components of the ecosystem.

Critical Loads

The term critical load is used to describe the threshold of nitrogen deposition or sulfur deposition that causes harm to sensitive resources in an ecosystem. A critical load is technically 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 are not expected to occur according to present knowledge." In the USDA-Forest Service, critical loads are used in air quality analyses for forest and project planning, wilderness stewardship, Clean Air Act related permit reviews, and national scale assessments. The USDA-Forest Service conducts deposition monitoring as a participating agency in the National Atmospheric Deposition Program and is also a member of the Critical Loads of Atmospheric Deposition Science Committee (CLAD), which facilitates coordination of the efforts of multiple federal and state agencies, scientists, and other partners related to the science of critical loads.