A region in the lower stratosphere (approximately 15–25 km in altitude and worldwide) where submicrometer-sized particles composed of aqueous sulfuric acid are present. Oxidation of sulfur compounds (primarily carbonyl sulfide, OCS, and sulfur dioxide, SO₂) is believed to be the source of sulfuric acid (H₂SO₄). Volcanic eruptions directly inject large quantities of H₂S or SO₂ into the stratosphere, causing large increases in the amount of particulate sulfuric acid. The particles undergo a slow falling-out process (timescale of years) into the lower atmosphere. Because these sulfuric acid particles are present in the ozone layer, heterogeneous chemical reactions that occur on them can have a significant effect on ozone by influencing the abundance of trace species that affect ozone concentrations.