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{{Pollution}}
The term '''acid rain''' is commonly used to mean the deposition of acidic components in [[rain]], [[snow]], [[dew]], or dry particles. The more accurate term is "acid precipitation." <ref>[http://pubs.usgs.gov/gip/acidrain/2.html (USGS)]</ref> Acid rain occurs when [[sulfur dioxide]] and [[nitrogen oxide]]s are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in [[cloud]]s. 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.<ref name="acidrain intro">http://www.epa.gov/region1/eco/acidrain/intro.html</ref>
The term "acid rain" is sometimes used more generally to include all forms of [[acid]] deposition - both [[Deposition (Aerosol physics)|wet deposition]], where acidic gases and particles are removed by rain or other [[Precipitation (meteorology)|precipitation]], and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.<ref>http://www.epa.gov/acidrain/</ref>
Acid rain is defined as any type of [[Precipitation (meteorology)|precipitation]] with a [[pH]] that is unusually low.<ref name="Brimblecombe 1996">Brimblecombe, P (1996). Air Composition and Chemistry. Cambridge University Press. ISBN</ref> Dissolved carbon dioxide dissociates to form weak [[carbonic acid]] giving a pH of approximately 5.6 at typical atmospheric concentrations of CO<sub>2</sub>.<ref name="Seinfeld 1998">Seinfeld, John H.; Pandis, Spyros N (1998). Atmospheric Chemistry and Physics - From Air Pollution to Climate Change. John Wiley and Sons, Inc. ISBN</ref> Therefore a pH of less than 5.6 has sometimes been used as a definition of acid rain.<ref name="archive glossary">http://www.airquality.co.uk/archive/glossary.php</ref> 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.<ref>http://www.enviroliteracy.org/article.php/2.html</ref>
The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada" <ref>US EPA: [http://www.epa.gov/airmarkets/acidrain/ Clean Air Markets - Environmental Issues]</ref>
Acid rain accelerates [[weathering]] in [[carbonate rocks]] and accelerates [[building weathering]]. It also contributes to acidification of [[river]]s, [[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{{Fact|date=February 2007}}.
Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased.<ref name="NASA Glossary"> http://asd-www.larc.nasa.gov/biomass_burn/glossary.html </ref> Occasional pH readings of well below 2.4 (the acidity of [[vinegar]]) have been reported in industrialized areas.<ref name="NASA Glossary"/> Industrial acid rain is a substantial problem in the [[People's Republic of China]]<ref>http://www.saag.org/%5Cpapers20%5Cpaper1944.html</ref>, [[Eastern Europe]], [[Russia]] and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity{{Fact|date=February 2007}}. 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]].{{Fact|date=February 2007}}
Acid rain was first reported in [[Manchester]], [[England]], which was an important city during the [[Industrial Revolution]]. In 1852, [[Robert Angus Smith]] found the relationship between acid rain and atmospheric pollution. The term "acid rain" was used for the first time by him in 1872.<ref name="Seinfeld 1998"/> 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{{Fact|date=February 2007}}.
==Emissions of chemicals leading to acidification==
[[Image:Volcanic injection.jpg|thumb|400px|right|Volcanic injection.]]
The most important gas which leads to acidification is sulfur dioxide. Emissions of [[nitrogen oxide]]s 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<sub>2</sub> comes from [[fossil fuel]] combustion and industry, 2.8 Tg(S) from [[wildfires]] and 7-8 Tg(S) per year from [[volcanoes]].<ref name="Berresheim 1995">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 </ref>
===Natural emissions===
The principal natural [[phenomena]] that contribute acid-producing gases to the [[Earth's atmosphere|atmosphere]] are emissions from [[volcano]]es and those from [[biology|biological]] processes that occur on the land, in [[wetland]]s, and in the [[ocean]]s. The major biological source of sulfur containing compounds is [[dimethyl sulfide]].
The effects of acidic deposits have been detected in [[glacier|glacial ice]] thousands of years old in remote parts of the globe.
===Human emissions===
[[Image:Gavin Plant.JPG|thumb|right|280px|The 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 vehicle]]s. 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 [[termolecular]] reaction:
:SO<sub>2</sub> + OH· → HOSO<sub>2</sub>·
which is followed by:
:HOSO<sub>2</sub>· + O<sub>2</sub> → HO<sub>2</sub>· + SO<sub>3</sub>
In the presence of water [[sulfur trioxide]] (SO<sub>3</sub>) is converted rapidly to [[sulfuric acid]]:
:SO<sub>3</sub>(g) + H<sub>2</sub>O(l) → H<sub>2</sub>SO<sub>4</sub>(l)
[[Nitric acid]] is formed by the reaction of OH with [[Nitrogen dioxide]]:
:NO<sub>2</sub> + OH· → HNO<sub>3</sub>
For more information see Seinfeld and Pandis (1998).<ref name="Seinfeld 1998"/>
===Chemistry in cloud droplets===
When clouds are present the loss rate of SO<sub>2</sub> 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, [[hydrolysis|hydrolyses]] in a series of [[Chemical equilibrium|equilibrium]] reactions:
:SO<sub>2</sub> (g)+ H<sub>2</sub>O {{unicode|⇌}} SO<sub>2</sub>·H<sub>2</sub>O
:SO<sub>2</sub>·H<sub>2</sub>O {{unicode|⇌}} H<sup>+</sup>+HSO<sub>3</sub><sup>-</sup>
: HSO<sub>3</sub><sup>-</sup> {{unicode|⇌}} H<sup>+</sup>+SO<sub>3</sub><sup>2-</sup>
; Oxidation
There are a large number of aqueous reactions that [[redox|oxidise]] sulfur from S([[oxidation state|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).<ref name="Seinfeld 1998"/>
==Acid deposition==
[[Image:Origins.gif|thumb|right|350px|Processes involved in acid deposition (note that only SO<sub>2</sub> and NO<sub>x</sub> 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.<ref name="archive glossary"/> This occurs when particles and gases stick to the ground, plants or other surfaces.
==Adverse effects==
[[Image:Waterspecies.gif|thumb|right|350px|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.<ref>US EPA: [http://www.epa.gov/acidrain/effects/surface_water.html Effects of Acid Rain - Surface Waters and Aquatic Animals]</ref>''
=== Soils ===
[[Soil#Biological processes in soil|Soil biology]] can be seriously damaged by acid rain. Some tropical [[microbes]] can quickly consume acids<ref name="Rodhe 2005">Rodhe, H., et. Al. “The Global Distribution of Acidifying Wet Deposition.” Environmental Science & Technology. v. 36 no. 20 (October) p. 4382-8.</ref> but other microbes are unable to tolerate low pHs and are killed. The [[enzymes]] of these microbes are [[Denaturation (biochemistry)|denatured]] (changed in shape so they no longer function) by the acid. The [[hydronium]] ions of acid rain also mobilize [[toxin]]s and [[Leaching|leach]] away essential nutrients and minerals<ref name="EPA: Forests">US EPA: [http://www.epa.gov/acidrain/effects/forests.html Effects of Acid Rain - Forests]</ref>
===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.<ref name="EPA: Forests"/>
Other plants can also be damaged by acid rain but the effect on food crops is minimised 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.<ref name="EPA: Forests"/> 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.<ref name="acidrain intro"/>. However, [[Particulate|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.<ref>US EPA: [http://www.epa.gov/acidrain/effects/health.html Effects of acid rain - human health. ]</ref> 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.<ref>US EPA: [http://www.epa.gov/acidrain/effects/materials.html Effects of Acid Rain - Materials ]</ref> Visibility is also reduced by sulfate and nitrate in the atmosphere.<ref>US EPA: [http://www.epa.gov/acidrain/effects/visibility.html Effects of Acid Rain - Visibility]</ref>
==Prevention methods==
{{main|emissions control}}
American efforts to reduce acid rain have produced positive results. Sulfur dioxide emissions are down by one-third from 1980s levels.<ref>http://canberra.usembassy.gov/hyper/2002/1125/epf111.htm</ref>
===Technical solutions===
In the United States, many coal-burning [[power plant]]s 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 [[Restoration ecology|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.
== See also ==
*[[List of environment topics]]
== References ==
<references/>
==Further reading==
* [[John McCormick]], ''Acid Earth: The Global Threat of Acid Pollution'' (London: Earthscan, 1989) ISBN 185383033X
==External links==
* [http://www.al.noaa.gov/AQRS/reports/napapreport05.pdf National Acid Precipitation Assessment Program] - a 98-page report to Congress
* [http://www.geography-site.co.uk/pages/environ/acid.html Acid rain for schools]
* 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://www.hubbardbrook.org/education/Glossary/AcidRain.pdf Acid Rain Revisited] - Hubbard Brook Research Foundation
* [http://pages.cthome.net/rwinkler/acidrain.htm Acid rain linked to decline of the wood thrush, a songbird of the Eastern Forest]
[[Category:Environmental chemistry]]
[[Category:Precipitation]]
[[Category:Sulfur]]
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