Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations, and are absorbed by water droplets in the clouds. The droplets then fall to the earth as either rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.

Acid rain is a serious environmental problem that affects several parts of the world. It accelerates weathering in carbonate rocks and accelerates building weathering.

How did acid rain originate?

This weather phenomenon is a natural occurrence on Earth, although it is limited to volcanic emissions and other biological processes that occur on land, wetlands, and in the oceans. However, the increased incidences of acid rain coincided with the beginnings of man's industrialization. Acid rain was first reported in Manchester, England, which was an important city during the Industrial Revolution. It was in 1852 when the connection between acid rain and atmospheric pollution was connected. The term “acid rain” was first used in 1872.

Since the Industrial Revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased dramatically. The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. For instance, the use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing harmful gases into regional atmospheric circulation.

Although acid rain was discovered in the 19th century, it wasn't until the late 1960s that scientists began widely observing and studying the phenomenon. Increased public awareness of acid rain in the US began 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.

What causes acid rain?

There are two reasons that causes acid rain: natural emissions and human emissions. The former involves natural phenomena that contribute acid-producing gases to the atmosphere such as volcanoes. The major biological source of sulfur-containing compounds is Dimethy sulfide. The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.

Meanwhile, the principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, factories, and motor vehicles. The gases can be carried hundreds of miles in the atmosphere before they are converted to acids and deposited as rain.

What are the effects of acid rain?

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.

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.

Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids but other types of microbe are unable to tolerate low pHs and are killed. The enzymes of these microbes are denatured (changed in shaped so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins and leach away essential nutrients and minerals.

Acid rain can also slow the growth of forests, cause leaves and needles to turn brown and fall off. In extreme cases, trees or whole areas of forest can die. The death of trees is not usually a diret result of acid rain, often it weakens trees and makes them more susceptible to other threats. Damage to soils can also cause problems to trees. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog that 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 minerals. In cultivated areas, limestone may also be added to 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.

Some scientists have suggested direct links to the effects of acid rain to human health, but none have been proven yet. However, fine particles—a large fraction of which are formed from the same gases as acid rain—have been shown to cause illness and premature deaths.

Acid rain can also cause damage to certain building materials and historical monuments. 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 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 in increased rate of oxidation for iron. Visibility in the sky is also reduced by sulfate and nitrate in the atmosphere.

What should we do to lower the emissions that causes acid rain?

In the United States, many coal-burning power plants use Flue gas desulfurization (or FGD) to remove sulfur-containing gases from their stack gases. 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. 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.

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.

A number of international treaties on the long-range transport of atmospheric pollutants have been agreed.