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What is Acid Deposition?

已有 3766 次阅读 2008-7-5 10:56 |个人分类:英语交流

         Acid deposition, commonly known as acid rain, occurs when emissions from the combustion of fossil fuels and other industrial processes undergo complex chemical reactions in the atmosphere and fall to the earth as wet deposition (rain, snow, cloud, fog) or dry deposition (dry particles, gas). Rain and snow are already naturally acidic, but are only considered problematic when less than a pH of 5.
The main chemical precursors leading to acidic conditions are atmospheric concentrations of sulfur dioxide (SO2) and nitrogen oxides (NOx). When these two compounds react with water, oxygen, carbon dioxide, and sunlight in the atmosphere, the result is sulfuric (H2SO4) and nitric acids (HNO3), the primary agents of acid deposition.
        Airborne chemicals can travel long distances from their sources and can therefore affect ecosystems over broad regional scales and in locations far from the sources of emissions.
Factors that Place Ecosystems at Risk of Acidification
A variety of ecological, chemical, physical, and human factors determine the vulnerability of an ecosystem to acid deposition:
  • Watershed bedrock composition: Certain rocks, such as granite, weather slowly and do not produce neutralizing chemicals. Watersheds that contain these rocks are more vulnerable to acidification.
  • Land use history: Various management techniques, such as clear cutting, can change a forest ecosystem’s capacity to neutralize acid inputs.
  • Disturbances: Insect defoliation, fires, and other types of disturbances can stress ecosystems making them less resilient to acidification.
  • Vegetation type: Different plant species respond in different ways to acid deposition and also contribute differently to ecological processes that regulate acidification.
  • Landscape features: Elevation, edges (e.g., the edge between a forest and field), the presence or absence of vegetation, and the steepness and directional face of a slope all affect acidification.
  • Soil depth: Shallow soils are often more sensitive to acidification than deep soils.
  • Base nutrient reserves: Elements such as calcium, potassium, and magnesium are base nutrients that help buffer acid inputs. Higher amounts of these nutrients increase an ecosystem’s capacity to neutralize acids.
 
Types of Acid in Acid Deposition
Fossil fuel burning creates sulfur and nitrogen emissions. (Courtesy of the US Geological Survey) Source: ESA
Enlarge
Fossil fuel burning creates sulfur and nitrogen emissions. (Courtesy of the US Geological Survey) Source: ESA
Sulfuric Acid Deposition
     Sulfur is stored in various organic substances such as coal, oil, and peat. These long-term pools of stored sulfur are released into the air as a result of fossil fuel combustion and natural phenomena such as volcanic eruptions. Sulfur dioxide in the atmosphere, which is converted to sulfuric acid, is identified as one of the major causes of acid deposition.
  Human sources of SO2 emissions: The primary sources of human created sulfur dioxide include electric utility and power plants that burn coal and oil products. Automobile exhaust also contributes to SO2 emissions. In 1992, the U.S emitted 22.7 million tons of SO2 into the atmosphere. Government policies have helped to reduce these emissions. Between 1995-1998, total national SO2 emissions averaged 19.6 millions tons/year, nearly a 14% reduction from 1992 levels.
Nitric Acid Deposition
      Nitrogen exists in several different forms and is an essential element to plant and animal growth. Nitrogen is only considered a pollutant when it exists in high enough concentrations to cause acid deposition or ecosystem eutrophication (excessive nutrient enrichment). Nitrogen in the form of nitrogen dioxide (NO2) is very reactive in the atmosphere, becoming nitric acid which is a key contributor to acid conditions.
      Human sources of NOx emissions: The primary human source of nitrogen that is emitted to the atmosphere comes from the combustion of fossil fuels for transportation, electric utilities, and industrial processes. In 1997, the U.S released 23.8 tons of NOx into the atmosphere, and these emissions are expected to increase with growth in electricity demand and vehicle miles traveled. However, current government regulations and changes in industrial practices are expected to help reduce the rate of these emissions from the electricity sector over time.

From : The encyclopedia of Earth



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