Acid rain occurs when fossil fuels such as coal, gasoline, and fuel oils are burned and emit oxides of sulfur, carbon, and nitrogen into the air. These oxides combine with moisture in the air to form sulfuric acid, carbonic acid, and nitric acid. When it rains or snows, these acids are brought to Earth in what is called acid rain.

– www.britannica.com

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Acid rain is defined as any type of precipitation with a pH of below 5.0. Normal rain has a pH which can range between 4.5 and 5.6 with an average value of 5.0. This natural acidity is caused by dissolved carbon dioxide dissociating to form weak carbonic acid.

Acid rain is formed by the oxidation of sulfur and nitrogen containing compounds which have both natural and human caused emissions. This oxidation occurs in both the gas phase and in raindrops and produces sulfuric acid and nitric acid.

The resulting increased acidity in soil and waterways has proven to be harmful to fish and vegetation. Acid rain also 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.

Origin

Volcanic "injection"

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

The principal cause of acid rain is from human sources, such as industrial and power-generating plants and vehicles. The gases can be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.

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.

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 by him in 1872. He observed that acid rain could lead to natural destruction.

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.

Adverse effects

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 trout 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 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 leache 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 less 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 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.

Prevention Methods

In the U.S., 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.

Some people oppose regulation of power generation, believing that pollution from power generation is inevitable. However, nuclear reactors generate less than one-millionth the toxic waste (measured by net biological effect) per watt, when wastes of both power generation facilities are properly handled. On the other hand, nuclear power has a well-known potential for catastrophic accidents or nuclear proliferation.

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

• Rodhe, H., et. Al. “The Global Distribution of Acidifying Wet Deposition.” Environmental Science & Technology. v. 36 no. 20 (October 15 2005) p. 4382-8.

See also

• List of environment topics