COS, a gas that is very stable and unreactive in the troposphere, but, it is thought, photolyzes to form carbon monoxide, CO, and sulfur, S, in the stratosphere. Through stratospheric chemical reactions, the sulphur atoms are converted to SO₂ and H₂SO₄ which form sulphate aerosol and cloud condensation nuclei, but eventually settle into the troposphere and react to form sulfuric acid, a component of acid rain. Volcanic eruptions contribute some of this COS to the atmosphere. The major biospheric sources of COS are thought to be biological.