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One way to decrease ground-level air pollution from sulfur dioxide, SPM, and nitrogenoxides when coal and oil are burned in electric power plants, metal smelters, and other industrial plants is to discharge these emissions from smokestacks tall enough to pierce the thermal inversion layer. Use of tall smokestacks in the United States, Canada, and western Europe has led to considerable reduction of ground-level pollution in many urban areas.
This approach, however, leads to increased levels of these pollutants and various secondary pollutants in downwind rural and urban areas. As emissions of sulfur dioxide and nitric oxide are transported over long distances by wind currents, they are chemically transformed into a variety of secondary pollutants such as nitrogen dioxide, droplets of sulfuric and nitric acids, and solid particles of sulfate and nitrate salts.
These chemicals fall or are washed out of the atmosphere onto downwind land and bodies of water. Wet deposition occurs when some of the suspended droplets of sulfuric acid and nitric acid return to the earth as acid rain or its variants, consisting of these acids and snow, sleet, hail, fog, or dew. Dry deposition occurs when solid particles of sulfate and nitrate salts and gases such as sulfur dioxide fall or are washed out of the atmosphere, usually near the original pollution sources. These deposited solids can then react with water in soil and bodies of water to form sulfuric and nitric acids. The combined wet and dry deposition of acids or acid-forming substances onto the surface of the earth is known as acid deposition. This phenomenon is commonly called acid rain, but this is a misleading term because these acids and acid-forming substances are deposited not only in rain but also in snow, sleet, fog, and dew and as dry particles and gas.
The relative levels of acidity and basicity of water solutions of substances are commonly expressed in terms of pH. The lower the pH value, the higher the acidity, with each whole-number decrease in pH representing a tenfold increase in acidity. Natural precipitation has an average pH value of 5.1 (with a range of 5.0 to 5.6 depending on location), caused when carbon dioxide and traces of natural sulfur and nitrogen compounds and organic acids in the atmosphere dissolve in atmospheric water. This slight acidity of natural precipitation helps water deposited on soil to dissolve minerals for use by plants and animals. It also deposits some sulfur and nitrogen used as plant nutrients.
However, deposition of acids and acid-forming substances with higher levels of acidity (pH values of 5.0 and less) than those in natural precipitation can damage materials; leach certain nutrients from soil; and kill fish, aquatic plants, and microorganisms in lakes and streams. Acid deposition, in combination with other air pollutants such as ozone, sulfur dioxide, and nitrogen oxides, can damage trees, crops, and other plants. It can also affect human health.
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