Acid rain is
rainor any other form of precipitation that is unusually acidic. It has harmful effects on plants, aquatic animals, and infastructure. Acid rain is mostly caused by human emissions of sulfurand nitrogencompounds which react in the atmosphere to produce acids. In recent years, many governments have introduced laws to reduce these emissions.
"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog and cloudwater, dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”.
Distilled water, which contains no carbon dioxide, has a neutral pHof 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are basic. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals. [Likens, G. E., W. C. Keene, J. M. Miller and J. N. Galloway. 1987. Chemistry of precipitation from a remote, terrestrial site in Australia. J. Geophys. Res. 92(D11):13,299-13,314.]
Carbonic acid then can ionize in water forming low concentrations of
:2H2O (l) + H2CO3 (aq) unicode| CO32- (aq) + 2H3O+(aq)
The extra acidity in rain comes from the reaction of primary air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and
nitric acid). The main sources of these pollutants are industrial power-generating plants and vehicles.
Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased. [http://asd-www.larc.nasa.gov/biomass_burn/glossary.html New Science Directorate Bio Mass Burning Redirect ] ] [Weathers, K. C. and G. E. Likens. 2006. Acid rain. pp. 1549-1561. In: W. N. Rom (ed.). Environmental and Occupational Medicine. Lippincott-Raven Publ., Philadelphia. Fourth Edition.] In 1852, Robert Angus Smithwas the first to show the relationship between acid rain and atmospheric pollution in Manchester, England.Seinfeld, John H.; Pandis, Spyros N (1998). Atmospheric Chemistry and Physics - From Air Pollution to Climate Change. John Wiley and Sons, Inc. ISBN 0-471-17816-0] Though acidic rain was discovered in 1852, it wasn't until the late 1960s that scientists began widely observing and studying the phenomenon. The term "acid rain" was generated in 1972. [Likens, G. E., F. H. Bormann and N. M. Johnson. 1972. Acid rain. Environment 14(2):33-40.] Canadian Harold Harvey was among the first to research a "dead" lake. Public awareness of acid rain in the U.S increased in the 1970s after the New York Timespromulgated reports from the Hubbard Brook Experimental Forestin New Hampshireof the myriad deleterious environmental effects demonstrated to result from it. [Likens, G. E. and F. H. Bormann. 1974. Acid rain: a serious regional environmental problem. Science 184(4142):1176-1179.] [ [http://www.hubbardbrook.org/pubs/pub_search.php Search the HBES Publications ] ]
Occasional pH readings in rain and fog water of well below 2.4 (the acidity of
vinegar) have been reported in industrialized areas. Industrial acid rain is a substantial problem in Europe, China, [Galloway, J. N., Zhao Dianwu, Xiong Jiling and G. E. Likens. 1987. Acid rain: a comparison of China, United States and a remote area. Science 236:1559-1562.] [ [http://www.saag.org/%5Cpapers20%5Cpaper1944.html CHINA: Industrialization pollutes its country side with Acid Rain ] ] Russiaand areas down-wind from them. These areas all burn sulfur-containing coalto generate heat and electricity. [ [http://www.asl-associates.com/sulphur1.htm] ] 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 pollutionhas contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. [Likens, G. E., R. F. Wright, J. N. Galloway and T. J. Butler. 1979. Acid rain. Sci. Amer. 241(4):43-51.] [Likens, G. E. 1984. Acid rain: the smokestack is the “smoking gun.” Garden 8(4):12-18.] Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the greatest deposition (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. [http://www.emep.int/publ/common_publications.html]
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 acidare of increasing importance due to stricter controls on emissions of sulfur containing compounds. 70 Tg(S) per year in the form of SO2 comes from fossil fuelcombustion and industry, 2.8 Tg(S) from wildfiresand 7-8 Tg(S) per year from volcanoes.Berresheim, H.; Wine, P.H. and Davies D.D., (1995). Sulfur in the Atmosphere. In Composition, Chemistry and Climate of the Atmophere, ed. H.B. Singh. Van Nostran Rheingold ISBN ]
=Natural Phenomena= The principal natural
phenomenathat 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. Acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe. [Likens, G. E., R. F. Wright, J. N. Galloway and T. J. Butler. 1979. Acid rain. Sci. Amer. 241(4):43-51.]
The principal cause of acid rain is sulfur and
nitrogencompounds from human sources, such as electricity generation, factories, and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometres in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke, but this caused many problems locally; thus, factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage.
=Gas phase chemistry= In the
gas phasesulfur dioxide is oxidized by reaction with the hydroxylradical via an intermolecularreaction: :SO2 + OH· → HOSO2· which is followed by: :HOSO2· + O2 → HO2· + SO3 In the presence of water, sulfur trioxide(SO3) is converted rapidly to sulfuric acid: :SO3(g) + H2O(l) → H2SO4(l) Nitric acidis formed by the reaction of OH with nitrogen dioxide: :NO2 + OH· → HNO3 For more information see Seinfeld and Pandis (1998).
=Chemistry in cloud droplets= When clouds are present the loss rate of SO2 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: :SO2 (g)+ H2O unicode| SO2·H2O :SO2·H2O unicode| H++HSO3- : HSO3- unicode| H++SO32- ; Oxidation There are a large number of aqueous reactions that oxidize sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with
ozone, hydrogen peroxideand oxygen(reactions with oxygen are catalyzed by ironand manganesein the cloud droplets).
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 aerosols are both of importance for wet 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. [http://www.airquality.co.uk/archive/glossary.php UK National Air Quality Archive: Air Pollution Glossary ] ] This occurs when particles and gases stick to the ground, plants or other surfaces.
Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic lifeforms as well as causing damage to buildings and having impacts on human health.
urface waters and aquatic animals
Both the lower pH and higher aluminum 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 and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the
brook troutin some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States.US EPA: [http://www.epa.gov/acidrain/effects/surface_water.html Effects of Acid Rain - Surface Waters and own Aquatic Animals] ] However, the extent to which acid rain contributes directly or indirectly via runoff from the catchment to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable. The US EPA's website states: "Of the lakes and streams surveyed, acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams".
Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pHs and are killed. [Rodhe, H., et al. The global distribution of acidifying wet deposition. Environmental Science and TEchnology. vlo. 36, no. 20 (October) p. 4382-8] 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, e.g. aluminium, and leach away essential nutrients and minerals.US EPA: [http://www.epa.gov/acidrain/effects/forests.html Effects of Acid Rain - Forests] ]
: 2H+ (aq)+ Mg2+ (clay)unicode| 2H+ (clay)+ Mg2+(aq)
Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain thereby affecting sensitive species, such as
sugar maple( Acer saccharum). [Likens, G. E., C. T. Driscoll, D. C. Buso, M. J. Mitchell, G. M. Lovett, S. W. Bailey, T. G. Siccama, W. A. Reiners and C. Alewell. 2002. The biogeochemistry of sulfur at Hubbard Brook. Biogeochemistry 60(3):235-316.] [Likens, G. E., C. T. Driscoll and D. C. Buso. 1996. Long-term effects of acid rain: response and recovery of a forest ecosystem. Science 272:244-246.]
Forests and other vegetation
Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. 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 lime and 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. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death [DeHayes, D.H., Schaberg, P.G., and G.R. Strimbeck. 2001. Red Spruce Hardiness and Freezing Injury Susceptibility. In: F. Bigras, ed. Conifer Cold Hardiness. Kluwer Academic Publishers, the Netherlands.] [Lazarus, B. E., P. G. Schaberg, G. Hawley and D. H. DeHayes. 2006. Landscape-scale spatial patterns of winter injury to red spruce foliage in a year of heavy region-wide injury. Can. J. For. Res. 36:142-152.] .
Scientists have suggested direct links to human health. [http://www.epa.gov/region1/eco/acidrain/intro.html Introduction | Acid Rain | New England | US EPA ] ] . 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 diseases [US EPA: [http://www.epa.gov/acidrain/effects/health.html Effects of acid rain - human health. ] ] For more information on the health effects of aerosols see particulate health effects.
Other adverse effects
Acid rain can also cause damage to certain building materials and historical monuments. This results when 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.
:CaCO3 (s) + H2SO4 (aq) unicode| CaSO4 (aq) + CO2 (g) + H2O (l)
This result 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. [US EPA: [http://www.epa.gov/acidrain/effects/materials.html Effects of Acid Rain - Materials ] ] Visibility is also reduced by sulfate and nitrate aerosols and particles in the atmosphere. [US EPA: [http://www.epa.gov/acidrain/effects/visibility.html Effects of Acid Rain - Visibility] ]
Particularly badly affected places around the globe include most of Europe (particularly
Scandinaviawith many lakes with acidic water containing no life and many trees dead) many parts of the United States(states like New Yorkare very badly affected) and South Western Canada. Other affected areas include the South Eastern coast of Chinaand Taiwan.
Potential problem areas in the future
Places like much of South Asia (
Indonesia, Malaysiaand Thailand), Western South Africa(the country), Southern Indiaand Sri Lankaand even West Africa (countries like Ghana, Togoand Nigeria) could all be prone to acidic rainfall in the future.
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 smoke 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
gypsumwhen 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. Automobile emissions controlreduces emissions of nitrogen oxides from motor vehicles.
A number of international treaties on the long range transport of atmospheric pollutants have been agreed e.g.
Sulphur Emissions Reduction Protocolunder the Convention on Long-Range Transboundary Air Pollution.
A more recent 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 intention of this is to give operators economic incentives to install pollution controls.
List of environmental issues
Lists of environmental topics
= References =
John McCormick, "Acid Earth: The Global Threat of Acid Pollution" (London: Earthscan, 1989) ISBN 185383033X
* Likens, G. E., R. F. Wright, J. N. Galloway and T. J. Butler. 1979. Acid rain. Sci. Amer. 241(4):43-51.
* Weathers, K. C. and G. E. Likens. 2006. Acid rain. pp. 1549-1561. In: W. N. Rom (ed.). Environmental and Occupational Medicine. Lippincott-Raven Publ., Philadelphia. Fourth Edition.
* [http://www.esrl.noaa.gov/csd/AQRS/reports/napapreport05.pdf National Acid Precipitation Assessment Program Report] - a 98-page report to Congress
* [http://www.geography-site.co.uk/pages/environ/acid.html Acid rain for schools]
* [http://www.hubbardbrookfoundation.org/education_outreach/ Acid rain for schools - Hubbard Brook]
* U.S. Environmental Protection Agency - [http://www.epa.gov/region1/eco/acidrain/index.html New England Acid Rain Program] (superficial)
* [http://www.enviroliteracy.org/article.php/2.html Acid Rain] (more depth than ref. above)
*U.S. Geological Survey - [http://pubs.usgs.gov/gip/acidrain/2.html What is acid rain?]
* [http://www2.iq.usp.br/docente/gutz/Curtipot_.html Acid rain analysis - freeware for simulation and evaluation of titration curves and pH calculations]
* [http://archives.cbc.ca/IDD-1-75-584/science_technology/acid_rain/ CBC Digital Archives – Acid Rain: Pollution and Politics]
* [http://pubs.acs.org/subscribe/journals/esthag/40/i02/html/011506feature_larssen.html Larssen, Thorjørn et al. “Acid Rain in China”. Environmental Science and Technology, 40:2, 2006, pp 418-425.]
* [http://www.adirondackcouncil.org/acrapub.pdf Acid Rain: A Continuing National Tragedy] - a report from The Adirondack Council on acid rain in the Adirondack region
* [http://www.bookrags.com/Acid_rain Assortment of Summaries on Acid Rain]
* [http://pages.cthome.net/rwinkler/acidrain.htm Acid rain linked to decline of the wood thrush, a songbird of the Eastern Forest]
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