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Acid rain

2007 Schools Wikipedia Selection. Related subjects: Environment; General
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   The term acid rain also known as acid precipitation is commonly used to
   mean the deposition of acidic components in rain, snow, dew, or dry
   particles. The more accurate term is "acid precipitation." Acid rain
   occurs when sulfur dioxide and nitrogen oxides are emitted into the
   atmosphere, undergo chemical transformations and are absorbed by water
   droplets in clouds. The droplets then fall to earth as rain, snow,
   mist, dry dust, hail, or sleet. This increases the acidity of the soil,
   and affects the chemical balance of lakes and streams.

   The term "acid rain" is sometimes used more generally to include all
   forms of acid deposition — both wet deposition, where acidic gases and
   particles are removed by rain or other precipitation, and dry
   deposition removal of gases and particles to the Earth's surface in the
   absence of precipitation.

   Acid rain is defined as any type of precipitation with a pH that is
   unusually low. Dissolved carbon dioxide dissociates to form weak
   carbonic acid giving a pH of approximately 5.6 at typical atmospheric
   concentrations of CO[2]. Therefore a pH of less than 5.6 has sometimes
   been used as a definition of acid rain. However, natural sources of
   acidity mean that in remote areas, rain has a pH which is between 4.5
   and 5.6 with an average value of 5.0 and so rain with a pH of less than
   5 is a more appropriate definition. The US EPA says, "Acid rain is a
   serious environmental problem that affects large parts of the US and
   Canada" Acid rain accelerates weathering in carbonate rocks and
   accelerates building weathering. It also contributes to acidification
   of rivers, streams, and forest damage at high elevations. When the acid
   builds up in rivers and streams it can kill fish.

History and trends

   Evidence for an increase in the levels of acid rain comes from
   analyzing layers of glacial ice. These show a sudden decrease in pH
   from the start of the Industrial Revolution of 6 to 4.5 or 4. Other
   information has been gathered from studying organisms known as diatoms
   which inhabit ponds. Over the years these die and are deposited in
   layers of sediment on the bottoms of the ponds. Diatoms thrive in
   certain pH levels, so the numbers of diatoms found in sediment layers
   of increasing depth give an indication of the change in pH over the
   years.

   Since the industrial revolution, emissions of sulfur and nitrogen
   oxides to the atmosphere have increased. Occasional pH readings of well
   below 2.4 (the acidity of vinegar) have been reported in industrialized
   areas. Industrial acid rain is a substantial problem in the People's
   Republic of China, Eastern Europe, Russia and areas down-wind from
   them. These areas all burn sulfur-containing coal to generate heat and
   electricity.The problem of acid rain not only has increased with
   population and industrial growth, but has become more widespread. The
   use of tall smokestacks to reduce local pollution has contributed to
   the spread of acid rain by releasing gases into regional atmospheric
   circulation. Often deposition occurs a considerable distance downwind
   of the emissions, with mountainous regions tending to receive the most
   (simply because of their higher rainfall). An example of this effect is
   the low pH of rain (compared to the local emissions) which falls in
   Scandinavia.

   Acid rain was first found in Manchester, England. In 1852, Robert Angus
   Smith found the relationship between acid rain and atmospheric
   pollution. Though acid rain was discovered in 1852, it wasn't until the
   late 1960s that scientists began widely observing and studying the
   phenomenon. Canadian Harold Harvey was among the first to research a
   "dead" lake. Public awareness of acid rain in the U.S increased in the
   1990s after the New York Times promulgated reports from the Hubbard
   Brook Experimental Forest in New Hampshire of the myriad deleterious
   environmental effects demonstrated to result from it.

Emissions of chemicals leading to acidification

   The most important gas which leads to acidification is sulfur dioxide.
   Emissions of nitrogen oxides which are oxidized to form nitric acid are
   of increasing importance due to stricter controls on emissions of
   sulfur containing compounds. 70 Tg(S) per year in the form of SO[2]
   comes from fossil fuel combustion and industry, 2.8 Tg(S) from
   wildfires and 7-8 Tg(S) per year from volcanoes.

Natural Phenomena

   The principal natural phenomena that contribute acid-producing gases to
   the atmosphere are emissions from volcanoes and those from biological
   processes that occur on the land, in wetlands, and in the oceans. The
   major biological source of sulfur containing compounds is dimethyl
   sulfide.

   The effects of acidic deposits have been detected in glacial ice
   thousands of years old in remote parts of the globe.

Human gas

   The coal-fired Gavin power plant in Cheshire, Ohio
   The coal-fired Gavin power plant in Cheshire, Ohio

   The principal cause of acid rain is sulfuric and nitrogen compounds
   from human sources, such as electricity generation, factories and motor
   vehicles. The gases can be carried hundreds of miles in the atmosphere
   before they are converted to acids and deposited.

Gas phase chemistry

   In the gas phase sulfur dioxide is oxidized by reaction with the
   hydroxyl radical via a intermolecular reaction:

          SO[2] + OH· → HOSO[2]·

   which is followed by:

          HOSO[2]· + O[2] → HO[2]· + SO[3]

   In the presence of water sulfur trioxide (SO[3]) is converted rapidly
   to sulfuric acid:

          SO[3](g) + H[2]O(l) → H[2]SO[4](l)

   Nitric acid is formed by the reaction of OH with Nitrogen dioxide:

          NO[2] + OH· → HNO[3]

   For more information see Seinfeld and Pandis (1998).

Chemistry in cloud droplets

   When clouds are present the loss rate of SO[2] is faster than can be
   explained by gas phase chemistry alone. This is due to reactions in the
   liquid water droplets

   Hydrolysis

   Sulfur dioxide dissolves in water and then, like carbon dioxide,
   hydrolyses in a series of equilibrium reactions:

          SO[2] (g)+ H[2]O ⇌ SO[2]·H[2]O
          SO[2]·H[2]O ⇌ H^++HSO[3]^-
          HSO[3]^- ⇌ H^++SO[3]^2-

   Oxidation

   There are a large number of aqueous reactions that oxidise sulfur from
   S( IV) to S(VI), leading to the formation of sulfuric acid. The most
   important oxidation reactions are with ozone, hydrogen peroxide and
   oxygen (reactions with oxygen are catalysed by iron and manganese in
   the cloud droplets).

   For more information see Seinfeld and Pandis (1998).

Acid deposition

   Processes involved in acid deposition (note that only SO2 and NOx play
   a significant role in acid rain).
   Processes involved in acid deposition (note that only SO[2] and NO[x]
   play a significant role in acid rain).

Wet deposition

   Wet deposition of acids occurs when any form of precipitation (rain,
   snow, etc) removes acids from the atmosphere and delivers it to the
   Earth's surface. This can result from the deposition of acids produced
   in the raindrops (see aqueous phase chemistry above) or by the
   precipitation removing the acids either in clouds or below clouds. Wet
   removal of both gases and aerosol are both of importance for wet
   deposition.

Dry deposition

   Acid deposition also occurs via dry deposition in the absence of
   precipitation. This can be responsible for as much as 20 to 60% of
   total acid deposition. This occurs when particles and gases stick to
   the ground, plants or other surfaces.

Adverse effects

   This chart shows that not all fish, shellfish, or the insects that they
   eat can tolerate the same amount of acid; for example, frogs can
   tolerate water that is more acidic (i.e., has a lower pH) than trout.
   This chart shows that not all fish, shellfish, or the insects that they
   eat can tolerate the same amount of acid; for example, frogs can
   tolerate water that is more acidic (i.e., has a lower pH) than trout.

   Acid rain has been shown to have adverse impacts on forests,
   freshwaters and soils, killing off insect and aquatic lifeforms as well
   as causing damage to buildings and having possible impacts on human
   health.

Surface waters and aquatic animals

   Both the lower pH and higher aluminium concentrations in surface water
   that occur as a result of acid rain can cause damage to fish and other
   aquatic animals. At pHs lower than 5 most fish eggs will not hatch and
   lower pHs can kill adult fish. As lakes become more acidic biodiversity
   is reduced. Acid rain has eliminated insect life and some fish species,
   including the brook trout in some Appalachian streams and creeks.

Soils

   Soil biology can be seriously damaged by acid rain. Some tropical
   microbes can quickly consume acids but other microbes are unable to
   tolerate low pHs and are killed. The enzymes of these microbes are
   denatured (changed in shape so they no longer function) by the acid.
   The hydronium ions of acid rain also mobilize toxins and leach away
   essential nutrients and minerals

Forests and other vegetation

   Acid rain can slow the growth of forests, cause leaves and needles to
   turn brown and fall off and die. In extreme cases trees or whole areas
   of forest can die. The death of trees is not usually a direct result of
   acid rain, often it weakens trees and makes them more susceptible to
   other threats. Damage to soils (see above) can also cause problems.
   High altitude forests are especially vulnerable as they are often
   surrounded by clouds and fog which are more acidic than rain.

   Other plants can also be damaged by acid rain but the effect on food
   crops is minimized by the application of fertilizers to replace lost
   nutrients. In cultivated areas, limestone may also be added to increase
   the ability of the soil to keep the pH stable, but this tactic is
   largely unusable in the case of wilderness lands. Acid Rain depletes
   minerals from the soil and then it stunts the growth of the plant.

Human health

   Some scientists have suggested direct links to human health, but none
   have been proven.. However, fine particles, a large fraction of which
   are formed from the same gases as acid rain (sulfur dioxide and
   nitrogen dioxide), have been shown to cause illness and premature
   deaths such as cancer and other deadly diseases For more information on
   the health effects of aerosol see: Particulate#Health effects.

Other adverse effects

   Acid rain can also cause damage to certain building materials and
   historical monuments. Acid rain can cause weathering on ancient and
   valuable statues and has caused considerable damage. This is because
   the sulfuric acid in the rain chemically reacts with the calcium
   compounds in the stones (limestone, sandstone, marble and granite) to
   create gypsum, which then flakes off. This is also commonly seen on old
   gravestones where the acid rain can cause the inscription to become
   completely illegible. Acid rain also causes an increased rate of
   oxidation for iron. Visibility is also reduced by sulfate and nitrate
   in the atmosphere.

Prevention methods

   American efforts to reduce acid rain have produced positive results.
   Sulfur dioxide emissions are down by one-third from 1980s levels.

Technical solutions

   In the United States, many coal-burning power plants use Flue gas
   desulfurization (FGD) to remove sulfur-containing gases from their
   stack gases. An example of FGD is the wet scrubber which is commonly
   used in the U.S. and many other countries. A wet scrubber is basically
   a reaction tower equipped with a fan that extracts hot smoky stack
   gases from a power plant into the tower. Lime or limestone in slurry
   form is also injected into the tower to mix with the stack gases and
   combine with the sulfur dioxide present. The calcium carbonate of the
   limestone produces pH-neutral calcium sulfate that is physically
   removed from the scrubber. That is, the scrubber turns sulfur pollution
   into industrial sulfates.

   In some areas the sulfates are sold to chemical companies as gypsum
   when the purity of calcium sulfate is high. In others, they are placed
   in landfill. However, the effects of acid rain can last for
   generations, as the effects of pH level change can stimulate the
   continued leaching of undesirable chemicals into otherwise pristine
   water sources, killing off vulnerable insect and fish species and
   blocking efforts to restore native life.

International treaties

   A number of international treaties on the long range transport of
   atmospheric pollutants have been agreed e.g. Sulphur Emissions
   Reduction Protocol and Convention on Long-Range Transboundary Air
   Pollution.

Emissions trading

   An even more benign regulatory scheme involves emissions trading. In
   this scheme, every current polluting facility is given an emissions
   license that becomes part of capital equipment. Operators can then
   install pollution control equipment, and sell parts of their emissions
   licenses. The main effect of this is to give operators real economic
   incentives to install pollution controls. Since public interest groups
   can retire the licenses by purchasing them, the net result is a
   continuously decreasing and more diffused set of pollution sources. At
   the same time, no particular operator is ever forced to spend money
   without a return of value from commercial sale of assets.

   Retrieved from " http://en.wikipedia.org/wiki/Acid_rain"
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   with only minor checks and changes (see www.wikipedia.org for details
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