In the stratosphere, the atmospheres second layer that is generally between 10 and 50 km above Earths surface, sulfuric acid is formed by the oxidation of volcanic sulfur dioxide by the hydroxyl radical:
Because sulfuric acid reaches supersaturation in the stratosphere, it can nucleate aerosol particles and provide a surface for aerosol growth via condensation and coagulation with other water-sulfuric acid aerosols. This results in the stratospheric aerosol layer.