Abbreviation for various oxides of sulfur, including SO₂ (sulfur dioxide) and SO₃ (sulfur trioxide). These gaseous and solid pollutants can be transformed in the presence of water to sulfuric acid H₂SO₄, one of the acids in acid rain. They are formed anthropogenically by combustion of coal or coal that contains sulfur impurities. They are also produced naturally by volcanoes and by biological activity in swamps, tidal areas, oceans, and by some soil bacteria.