Acid rain

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, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams. [1]. 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.[2]

Acid rain is defined as any type of precipitation with a pH that is unusually low (Brimblecombe, 1996). Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO₂ (Seinfeld and Pandis, 1998). Therefore a pH of <5.6 has sometimes been used as a definition of acid rain [3]. 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 <5 is a more appropriate definition [4].

Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidic rivers, streams, and damage to trees at high elevation. Efforts to combat this phenomenon are ongoing.

History and trends

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 (Seinfeld and Pandis, 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.

Evidence for an increase in the levels of acid rain comes from analysing 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 pHs, so the numbers of diatoms found in 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. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulfur oxides. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas. These sources, plus the transportation sector, are the major originators of increased nitrogen oxides.

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 from its formation, with mountainous regions tending to receive the most (simply because of their higher rainfall). An example of this effect is the frequent low pH of rain which falls in Scandinavia compared to the local emissions.

Industrial acid rain is a substantial problem in China, Eastern Europe, Russia and areas down-wind from them. Acid rain from power plants in the midwest United States has also harmed the forests of upstate New York and New England. These areas all burn sulfur-containing coal to generate heat and electricity.

Emissions of chemicals leading to acidification

The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidised 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. (Berresheim et al, 1995).

Natural emissions

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 emissions

The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation 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 oxidised by reaction with the hydroxyl radical via a thermolecular reaction:

SO₂ + OH· + M→ HOSO₂· + M

which is followed by:

HOSO₂· + O₂ → HO₂· + SO₃

In the presence of water sulfur trioxide is converted rapidly to sulfuric acid:

SO₃ + H₂O + M → H₂SO₄ + M

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

NO₂ + OH· + M → HNO₃ + M

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 ⇌ SO₂·H₂O

SO₂·H₂O ⇌ H⁺+HSO₃^-

HSO₃^- ⇌ H⁺+SO₃^2-

Oxidation

There are a large number of aqueous reactions of sulfur which oxidise it 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).

Aerosol formation

In the gas phase sulfuric and nitric can condense on existing aerosols or nucleate to form new aerosols. The nucleation process is an important source of new particles in the atmosphere and so emissions of sulfur containing compounds, as well as causing acidification also have a climate effect.

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

Adverse effects

Decades of enhanced acid input has increased the environmental stress on high elevation forests and aquatic organisms in sensitive ecosystems. In extreme cases, it has altered entire biological communities and eliminated some fish species from certain lakes and streams. In many other cases, the changes have been more subtle, leading to a reduction in the diversity of organisms in an ecosystem. This is particularly true in the northeastern United States and Canada, where the rain tends to be most acidic, and often the soil has less capacity to neutralize the acidity. The adverse effect acid rain has on forests has decimated Canada's and Germany's national forests.

Acid rain also can damage certain building materials and historical monuments.

Some scientists have suggested links to human health, but none have been proven. [6]

Effects on lake ecology

There is a strong relationship between lower pH values and the loss of populations of fish in lakes. Below 4.5 virtually no fish survive, whereas levels of 6 or higher promote healthy populations. Acid in water inhibits the production of enzymes which enable fish's larvae to escape their eggs. It also mobilizes toxic metals such as aluminium in lakes. Aluminium causes some fish to produce an excess of mucus around their gills, preventing proper ventilation. Phytoplankton growth is inhibited by high acid levels, and animals which feed on it suffer.

Many lakes are subject to natural acid runoff from acid soils, and this can be triggered by particular rainfall patterns that concentrate the acid. An acid lake with newly-dead fish is not necessarily evidence of severe air-pollution.

Effects of acid rain on soil chemistry

The effect of acid rain on soil chemistry is consistent with the soil forming process, podzolisation. Podzolisation is a complex process (or number of sub-processes) in which organic material and soluble minerals (commonly iron and aluminium) are leached from the A to the B horizon. In nature, podsols form under moist, cool, and acidic conditions.

Effects of acid rain on soil biology

Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids (Rodhe, 2005) but other types of microbe 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.

Forest soils tend to be inhabited by fungi, but acid rain shifts forest soils to be more bacterially dominated. In order to fix nitrogen many trees rely on fungi in a symbiotic relationship with their roots. If acidity inhibits the growth of these mycorrhizae associations this could lead to trees struggling to fix nitrogen without their symbiotic partners.

Other adverse effects

Trees are harmed by acid rain in a variety of ways. The waxy surface of leaves is broken down and nutrients are lost, making trees more susceptible to frost, fungi, and insects. Root growth slows and as a result fewer nutrients are taken up. Toxic ions are mobilized in the soil, and valuable minerals are leached away or (as in the case of phosphate) become bound to aluminium or iron compounds, or to clay.

The toxic ions released due to acid rain form the greatest threat to humans. Mobilized copper has been implicated in outbreaks of diarrhea/diarrhoea in young children and it is thought that water supplies contaminated with aluminium cause Alzheimer's disease.

Acid rain can cause erosion on ancient and valuable statues and has caused considerable damage. This is because the sulfuric acid in the rain chemically reacts with the calcium in the stones (lime stone, 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.

Prevention methods

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 a land-fill.

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 emission 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.

References

Berresheim, H.; Wine, P.H. and Davies D.D., (1995). Sulfur in the Atmopshere. In Composition, Chemistry and Climate of the Atmophere, ed. H.B. Singh. Van Nostran Rheingold.
Brimblecombe, P (1996). Air Composition and Chemistry. Cambridge University Press. ISBN 0-521-45366-6
Rodhe, H., et. Al. “The Global Distribution of Acidifying Wet Deposition.” Environmental Science & Technology. v. 36 no. 20 (October 15 2005) p. 4382-8.
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

External links

U.S. Environmental Protection Agency - New England Acid Rain Program (superficial)
Acid Rain (more depth than ref. above)
U.S. Geological Survey - What is acid rain?
Acid rain analysis - freeware for simulation and evaluation of titration curves and pH calculations