5.8 Acid Deposition

pH Scale

Acid deposition – is the general term for acid coming down to Earth’s surface from the air (can be wet or dry) -> rain, snow, dew, fog, gas, dry particles, etc. growing problem in China (40% of its land), former Soviet Union, India, Nigeria, Brazil, Venezuela, Columbia

PRIMARY POLLUTANTS (sulfur dioxide & nitrogen oxides) are emitted directly by combustion. Primary pollutants can travel as far as 1,000 km.

Primary pollutants can be changed to secondary pollutants by reactions with other substances in the atmosphere. The primary pollutants are sulfur dioxide (SO2) and Nitrogen oxides (NO and NO2) These are produced through combustion of fossil fuels in cars, industry and electricity plants. (Sulfur common from coal and oil burning).

As the primary pollutants react with H 2 O vapor and O 2 and become SECONDARY POLLUTANTS (nitric acid vapor, droplets of sulfuric acid, and particles of acid-forming sulfate and nitrate salts).

Sulfur dioxide will react with oxygen from the atmosphere to form sulfur trioxide (SO3). This can react with water to then form sulfurous acid (H2SO3) and sulfuric acid (H2SO4). Nitric acid can also form (HNO3).

Acidic substances descend to the earth’s surface in two forms: 1.Wet deposition (as acid rain, snow, fog, and cloud vapor). 2.Dry deposition as acidic particles. Most dry deposition falls near the emission sources. Most wet deposition falls in more distant downwind areas.

Soils and lakes vary in their ability to buffer or remove excess acidity. Wind Transformation to sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ) Nitric oxide (NO) Acid fog Ocean Sulfur dioxide (SO 2 ) and NO Windborne ammonia gas and particles of cultivated soil partially neutralize acids and form dry sulfate and nitrate salts Dry acid deposition (sulfur dioxide gas and particles of sulfate and nitrate salts) Farm Lakes in deep soil high in limestone are buffered Lakes in shallow soil low in limestone become acidic Wet acid deposition (droplets of H 2 SO 4 and HNO 3 dissolved in rain and snow) movie

Buffering Soils that contain basic compounds such as calcium carbonate (CaCO 3 ) or limestone can neutralize or buffer some inputs of acids. Soils that don’t have basic compounds (granite & depleted soils) are more sensitive to acid deposition. Coral reefs contain CaCO 3, pH in saltwater environments doesn’t change much. Freshwater ecosystems are more sensitive to acid deposition.

Effects on Humans Contributes to human respiratory diseases. Can leach toxic metals into drinking water. Damage statues, buildings, metals and car finishes. Decreases atmospheric visibility. Can lower profits and causes job losses because of lower productivity.

Effects on Aquatic Ecosystems Fish kills below pH 4.5. Al 3 from soil can stimulate production of excess mucus which clogs gills. Toxic shock is the damage caused sudden runoff of large amounts of highly acidic water and Al 3.

Water boatman Whirligig Yellow perch Lake trout Brown trout Salamander (embryonic) Mayfly Smallmouth bass Mussel pH Fish and other aquatic organisms vary in their sensitivity to acidity.

Effects of Plants & Soil Chemistry Can harm forests and crops (pH below 5.1). Damage leaves directly. Leaching reduces plant productivity and buffering abilities of soil. Al 3 can hinder uptake and use of soil nutrients and water by plants. Dissolving insoluble soil compounds and ions of metals are toxic to both plants & animals. Promoting growth of acid resistant competing and parasitic mosses.

Emission Acid deposition SO 2 H 2 O 2 PANs NO X O 3 Others Direct damage to leaves and bark Reduced photosynthesis and growth Increased Susceptibility to drought, extreme cold, insects, mosses, and disease organisms Soil acidification Leaching of soil nutrients Acid Release of toxic metal icons Root damage Reduced nutrient and water uptake Tree death Air pollutants are one of several interacting stresses that can damage, weaken or kill trees. movie

Acid Rain Effects on Buildings and Sculptures Acids have a corrosive effect on limestone or marble buildings or sculptures. It is well established that either wet or dry deposition of sulfur dioxide significantly increases the rate of corrosion on limestone, sandstone, and marble There are many examples in both the U. S. and Europe of the corrosive effects of acid rain on sculptures. Many sculptures have been destroyed, a few have been preserved by bringing them inside.

The same reactions can be simulated in a demonstration by putting an egg in vinegar. The eggshell is made from the same material as limestone - calcium carbonate. The acetic acid produces an immediate bubbling as the carbonate converts first to carbonic acid and then to carbon dioxide bubbling. Within an hour the egg shell will be dissolved.

PreventionCleanup Reduce air pollution by improving energy efficiency Reduce coal use Increase natural gas use Increase use of renewable resources Burn low-sulfur coal Remove SO 2 particulates, and NO x from smokestack gases Remove No x from motor vehicular exhaust Tax emissions of SO 2 Add lime to neutralize acidified lakes Add phosphate fertilizer to neutralize acidified lakes Methods for reducing acid deposition.

Reducing Primary Pollutants Sulfur dioxide emissions have been reduced by installing smokestack scrubbers in coal-fired power plants. Using low-sulfur coal reduced sulfur dioxide, but increased CO 2 emissions. Nitrogen oxide emissions have not declined due to more motor vehicles traveling farther.

Using limestone to neutralize acids Temporary and expensive. Limestone can kill some types of plankton. Can harm wetland plants that need acidic water. It is difficult to know how much lime to put where.

The regional nature of acid deposition Effects of acid depo. are regional, as compared with global warming or ozone depletion It is mostly the localized area affected, or areas downwind of the pollution