In the gas phase this process happens when compounds like nitrogen oxides and sulphur oxides are converted in a chemical reaction in the gas phase or in clouds into acidic substances. These acids are rained-out or dry deposited. Significant amounts of the compounds containing nitrogen and sulphur are a direct result of anthropogenic activity. An example reaction that takes place in soil occurs from the oxidation of reduced sulphur (for instance, pyrite) exposed during, for instance, strip mining of lignite. This can be represented as:

2FeS₂ + 6H₂O + 7O₂ = 4SO₄^2- + 8H⁺ + 2Fe(OH)₂