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Deposition


A diagram explaining the acid deposition cycle

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 impacts 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 more of a component of acid rain in the mid and western United States. In addition to contributing to acid rain, nitrogen can impact other ecosystem by unnaturally fertilizing land and water. These excess 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.

There are several types of ecosystem effects associated with deposition that tie to the pollutant being deposited. These include acidic deposition (acid rain), excess amounts of nitrogen and heavy metals (including mercury).

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