Acid Deposition is the scientific term used to describe "Acid Rain". When atmospheric pollutants such as sulphur dioxide and nitrogen oxides mix with water vapour in the air, they are converted to sulphuric and nitric acids. These acids make the rain acidic, hence the term "acid rain". Rain returns the sulphur and nitrogen acids to Earth, and in high concentrations, can cause damage to natural environments including forests and freshwater lakes. This form of acid deposition is known as wet deposition. 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. Sulphur 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.