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Acid rain facts for kids

Kids Encyclopedia Facts
Acid rain woods1
Acid rain can make the leaves fall off trees
- Acid rain damaged gargoyle -
A gargoyle that has been damaged by acid rain

Acid rain is rain that is unusually acidic. It is rain with high levels of hydrogen ions (low pH).

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.


Origins of acid rain
Origins of acid rain

Acid rain is caused by acids mixing with air. The largest source of acid is sulfur 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 mankind 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 sulfur in them. This releases gases into the air that produce acid rain.

Governments have tried since the 1970s to reduce the amount of sulfur 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 tonnes of these gases every year; and China produced 18 million tonnes. The United States produced more than 20 million tonnes then, which declined to 8.1 million in 2010.

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

Adverse effects

Effects on lake ecology

Cloud formation from refinery in Curacao
Acid clouds can grow on SO2 emissions from refineries, as seen here in Curaçao

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

Weathered limestone (20125209969)
Chemical weathering occurs when rocks are broken down by a chemical change - Acid rain is also a contributor to chemical weathering

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 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

Mount Mitchell - These spruce-fir forests in the Appalachian Mountains have been damaged by introduced pests and 'acid rain' precipitation

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

Children play in the yard of a Ruston, Wash., home, while a Tacoma smelter stack showers area with arsenic and lead residue in August 1972
Children play in the yard of a Ruston, Washington home, while a Tacoma smelter stack showers area with arsenic and lead residue

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.

Phillips Power Station wet scrubber facility

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.

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