Acid rain

Acid rain is rain or any other form of precipitation which is unusually acidic. It has harmful effects on plants, aquatic animals and buildings. Acid rain is mostly caused by human emissions of sulfur and nitrogen compounds which react in the atmosphere to produce acids. In recent years, many governments have introduced laws to reduce these emissions.

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Definition

The term "acid rain" is commonly used to mean the deposition of acidic components in rain, snow, fog, dew, or dry particles. The more accurate term is "acid precipitation." Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. "Clean" or unpolluted rain is slightly acidic, its pH being about 5.0, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid.

H₂O (l) + CO₂ (g) → H₂CO₃ (aq)

Carbonic acid then can ionize in water forming low concentrations of hydronium ions:

2H₂O (l) + H₂CO₃ (aq) ⇌ CO₃^2- (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 vehicles and industrial and power-generating plants.

History

Since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased.^[1] Acid rain was first found in Manchester, England. In 1852, Robert Angus Smith found the relationship between acid rain and atmospheric pollution.^[2] 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.^[3]

Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas.^[1] Industrial acid rain is a substantial problem in China^[4], Eastern Europe, Russia and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity.^[5] 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.^[6]

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 SO₂ comes from fossil fuel combustion and industry, 2.8 Tg(S) from wildfires and 7-8 Tg(S) per year from volcanoes.^[7]

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 activity

  The principal cause of acid rain is sulphuric and nitrogen compounds 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; thus, factories now have longer smoke funnels. However, this causes pollutants to be carried farther, causing greater ecological damage.

Chemical processes

Gas phase chemistry

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

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

NO₂ + OH· → HNO₃

For more information see Seinfeld and Pandis (1998).^[2]

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 ⇌ SO₂·H₂O

SO₂·H₂O ⇌ H⁺+HSO₃^-

HSO₃^- ⇌ H⁺+SO₃^2-

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 peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).

For more information see Seinfeld and Pandis (1998).^[2]

Acid deposition

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.^[8] This occurs when particles and gases stick to the ground, plants or other surfaces.

Adverse effects

  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 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 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.^[9] However, there has been some debate on the extent to which acid rain contributes to lake acidity (i.e., that many acid lakes may result primarily from characteristics of the surrounding watershed, and not the rain itself).^[10] The 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".^[9]

Soils

Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids^[11] 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^[12]

nH⁺ (aq)+ Mn^{2+ (clay)⇌ nH+ (clay)+ Mn2+(aq)}

Forests and other vegetation

  Acid rain can slow the growth of vulnerable forests and cause leaves and needles to turn brown and fall off.^[13]. 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 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.^[14]. However, fine particles, a large fraction of which are formed from the same gases as acid rain (sulphur dioxide and nitrogen dioxide), have been shown to cause illness and premature deaths such as cancer and other deadly diseases^[15] 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. 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.

CaCO₃ (s) + H₂SO₄ (aq) ⇌ CaSO₄ (aq) + CO₂ (g) + H₂O (l)

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.^[16] Visibility is also reduced by sulfate and nitrate in the atmosphere.^[17]

Prevention methods

Technical solutions

In the United States, many coal-burning power plants use Flue gas desulfurization (FGD) to remove sulphur-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 sulphur 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.

Automobile emissions control reduces emissions of nitrogen oxides from motor vehicles.

International treaties

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

Emissions trading

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.

See also

• List of environment topics

References

Further reading

• John McCormick, Acid Earth: The Global Threat of Acid Pollution (London: Earthscan, 1989) ISBN 185383033X