Effects of acid rain on a forest, in the Czech Republic
**Acid rain** is rain that is unusually acidic and highly corrosive in nature. It is rain with high levels of hydrogen ions (low pH). It may be defined as "rain water having pH less than 5.6".

Acid rain can have harmful effects on plants, animals and humans. It is caused when gaseous compounds of ammonium, carbon, nitrogen, and sulphur are released into the atmosphere. The wind carries the gases high into the sky. There the compounds react with the water in the atmosphere and acids are made. In 1852, Robert Angus Smith showed the relationship between acid rain and atmospheric pollution in Manchester. He coined the term "acid rain" in 1872.

Causes
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Acid rain is caused by acids mixing with air. The largest source of acid is sulphur dioxide. Carbon dioxide and various oxides of nitrogen also make acid in the atmosphere. These chemicals are both natural and artificial.

There are various natural causes, such as gases from volcanoes. However, it is thought that air pollution by people now causes most acid rain. People started producing more acidic gases when they started building factories and power stations. These buildings as well as houses and vehicles burn coal or oil that have sulphur in them. This releases gases into the air that produce acid rain. Governments have tried since the 1970s to reduce the amount of sulphur being released into the Earth's atmosphere, and have had good results so far. However, it is expensive to clean the smoke from factories and power stations. In 2001 Great Britain still produced about five million tons of these gases every year; and China produced 18 million tons. The United States produced more than 20 million tons then, which declined to 8.1 million in 2010.

Trees are destroyed by acid rain. Fish are also killed by acid rain.

Acid rain can also be caused naturally. For example, acids can be made by nitrogen compounds made by lightning, and volcanic eruptions can release sulphur dioxide into the atmosphere.

Acid rain has a devastating impact on forests, freshwater and soil. It kills insect and aquatic life-forms as well as causes damage to buildings and having impacts on human health. Due to the corrosive nature of the acids it damages both non-living things as well as living organisms.

Impact
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Acid rain poisons rivers and lakes. Fish and other animals cannot live in acid water. It is also bad for buildings as the acid damages calcium carbonate stone. The acid dissolves it. Many buildings and monuments have been damaged by acid rain.

Consequences
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An acid rain involves deposition of aqueous acids, acidic gases and acidic salts.

***Acid deposition has 2 parts: wet and dry.***

✓ Wet deposition refers to acidic rain, fog & snow.

✓ Dry deposition refers to acidic gases and particles.

✓ Half of the acidity in the atmosphere falls back to earth through dry deposition.

Acid rain is a regional air pollution problem. Canada and North Western USA are worst affected. The average pH of rainfall recorded in Toronto in Feb. 1979 was 3.5. In 1989 fog in Los Angeles had a pH as low as 2.2. However till now the record of having the most acidic rain is with US in Wheeling West Virginia where the pH was as low as 1.5.