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Acid deposition is the scientific term used to describe acid rain (which includes including acid fog, acid sleet, and acid snow).

Acid deposition (acid rain) occurs when sulfur dioxide (SO₂) and, to a lesser extent, NO_x emissions are transformed in the atmosphere and return to the earth as dry deposition or in rain, fog, or snow. Acid rain is another environmental problem that affects many industrialized area of the world resulting in damage crops, forests, wildlife populations, and causing respiratory and other illnesses in humans.

When atmospheric pollutants such as sulfur dioxide and nitrogen oxides mix with water vapors in the air, they are converted to sulfuric and nitric acids.

Sulfur dioxide with water in produce sulfurous acid:

In the gas phase, sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:

In the presence of water sulfur trioxide (SO₃) is converted rapidly to sulfuric acid:

Nitric acid is formed by the reaction of water with nitrogen dioxide and by the reaction of carbon dioxide with water:

Although carbonic acid is a weaker acid the nitric acid, sulfurous acid, and sulfuric acid, it does, however, contribute to acid rain which, in high concentrations, can cause damage to natural environments including forests and freshwater lakes.

Acid deposition can be classified as wet deposition such as acid rain, snow, sleet and fog or dry deposition such as deposition as particulate matter even less than PM_2.5. Effects of acid rain can be either chronic or episodic.

Chronic acidification is a long-term effect due to years of acid rain. Episodic acidification is due to heavy rain storms; it also occurs in spring as concentrated nitrate and sulphate in lower layer of snow pack get released when snow melts. A second method of acid deposition is known as dry deposition. Whilst wet deposition involves the precipitation of acids, dry deposition occurs when the acids are first transformed chemically into gases and salts, before falling under the influence of gravity back to Earth. Sulfur dioxide, for example, is deposited as a gas and as a salt.

The gases present in acid deposition are found to occur naturally in the environment. They are given off from a number of sources including volcanic eruptions and the rotting of vegetation. They become a problem when humans produce the gases in large amounts and at high concentrations by the burning of fossil fuels. It is arguable which gives off the most acid gases – fossil fuels or natural events such as eruption of volcanoes. The distances that pollutant gases travel means that acid deposition is an international or trans-boundary problem. This means that acid pollutants are not necessarily deposited in the same country where they were produced.

Acid rain falling over regions with alkaline soils or rocks is quickly neutralized but in areas with little acid-neutralizing capacity is the biosphere sensitive to acid rain. Over North America these areas include New England, eastern Canada, and mountainous regions, which have granitic bedrock and thin soils.

In areas where the biosphere is sensitive to acid rain, there has been ample evidence of the negative effects of acid rain on freshwater ecosystems. Elevated acidity in a lake or river is directly harmful to fish because it corrodes the organic gill material and attacks the calcium carbonate skeleton. In addition, the acidity dissolves toxic metals such as aluminum from the sediments. There is also ample evidence that acid rain is harmful to terrestrial vegetation, mostly because it leaches nutrients such as potassium and allows them to exit the ecosystem by runoff.

See also: Acid Rain.