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Acid Rain - Factors Affecting Acid Deposition

pollutants united air water

The interplay of soil, water, climate, and winds can have a profound impact on the effects of acid deposition. The effects of acid rain can be greatly reduced by the presence of alkali (basic) substances. Sodium, potassium, magnesium, calcium, and bicarbonate are examples of chemicals with buffering (neutralizing) capacity. In areas where soils contain limestone (calcium carbonate) or other minerals with high buffering capacities, acidity is reduced as runoff travels over the soil, mixes with dust, FIGURE 9.2
Transported air pollutants: emissions to effects
SOURCE: Acid Rain and Transported Air Pollutants: Implications for Public Policy, U.S. Congress, Office of Technology Assessment, 1984
and percolates through the soil. Brackish and salt water are more resistant to pH change from acid deposition than freshwater because they contain many substances with good buffering capacity.

Areas most sensitive to acid deposition have hard, crystalline bedrock and very thin surface soils. When no buffering particles are in the soil, acid rainfall and runoff directly affect surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity neutralizes acid rain better. Generally, lakes tend to be most susceptible to acid rain because of low alkaline content in lakebeds, the water, and the watershed soils, and the longer residence time of water in lakes.

Like lakes, freshwater streams flowing over stream-beds and draining watersheds with low buffering capacity can also be susceptible to acid deposition. For example, according to the Environmental Protection Agency (EPA) in "Effects of Acid Rain: Lakes & Streams" (, November 12, 2003), 580 freshwater streams in the Mid-Atlantic Coastal Plain have been identified as acidic because of acid deposition.

In drier climates, such as the western United States, windblown alkaline dust blows freely and tends to help neutralize atmospheric acidity. On the other hand, the more acidic dust on the eastern seaboard contributes to atmospheric acidity.

Some acid deposition events are more severe than others. Episodic acidification refers to brief periods during which pH levels drop because of runoff from the influx of large amounts of water, such as heavy downpours and snowmelt. An example would be heavy rainfall following a long dry period. The runoff would be very acidic because of the combination of the acidic rain and the dry acid deposition washed from all surfaces.

Freshwater lakes and streams in many areas of the United States are susceptible to episodic acidification, that is, they become temporarily acidic during and immediately after storms and snowmelt. Effects may last for several hours or days, depending on the water flow, as opposed to waters that are acidic year-round. For example, during rainstorms and snowmelts, mountain streams in New York, North Carolina, Pennsylvania, Tennessee, and Arkansas have shown acidity from three to twenty times the level occurring the rest of the year. If episodic acidification occurs during periods when fish are spawning or seed is germinating, the results can be devastating. In severe cases, it has caused fish kills.

The prevailing winds in an area are determinants in the transport systems that distribute acid pollutants in FIGURE 9.3
The potential hydrogen (pH) scale
SOURCE: Acid Rain, U.S. Environmental Protection Agency, 1980
definite patterns across the planet. The movement of air masses transports air pollutants many miles, during which time these pollutants are transformed into sulfuric and nitric acid by mixing with other pollutants, clouds, and water. This process is known as "transport and transformation." For example, a typical European transport pattern carries pollutants from the smokestacks of the United Kingdom over Sweden. In southwestern Germany, many trees of the famed Black Forest (mostly coniferous) are dying from the effects of acid rain transported from industrial sites to the region by wind.

Northeastern United States Hit Hardest

The areas of greatest acidity (lowest pH values) in the nation occur in the northeastern United States. This high acidity is caused by the large number of cities, dense populations, and the concentration of power and industrial plants in the Northeast. Because the area is characterized by generally acidic soils and copious freshwater lakes and streams with low buffering capacity, it is very vulnerable to the effects of acid deposition.

The prevailing wind direction in the Northeast also brings storms and air pollutants from the Ohio River Valley, an area rich in coal-fired utilities. A typical transport pattern brings pollutants from the Ohio River Valley to the northeastern United States on prevailing winds that tend to move from west to east and from south to north. As the Attorney General for the State of Maine, Steven Rowe, said in a November 2002 press release, the Clean Air Act benefits people in Maine most because the state is at the end of the country's "air pollution tailpipe"("AG Rowe to Sue Bush Administration for Gutting Clean Air Act,", November 22, 2002). About one-third of the total sulfur compounds deposited over the eastern United States originate from sources in the Midwest, more than 300 miles away. In addition to the problems in the northeastern United States, eastern areas of Canada have also been affected by pollutants from the Ohio River Valley.

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almost 9 years ago

I learned alot from this essay. If you were to add anything at all, maybe more factors?