sulphur dioxide(SO₂) The gaseous product of the oxidation of sulphur, which is released into the atmosphere when fuels containing sulphur compounds are burned. In the atmosphere, a series of reactions oxidize SO₂ to particles of sulphate (SO₄). Sulphate is also produced in the atmosphere from dimethyl sulphide (DMS, (CH₃)₂S), released by some species of phytoplankton (especially coccolithophorids). Sulphate crystals act as condensation nuclei, but lower the pH of the water into which they dissolve and SO₂ is implicated in acid precipitation.

sulphur dioxide (SO₂) Preservative used in gaseous form or as salts (sulphites) for fruit drinks, wine, comminuted meat, and as a processing aid to control physical properties of flour; also prevents enzymic and non‐enzymic browning. Protects vitamin C but destroys vitamin B₁. Prepared by ancient Egyptians and Romans by burning sulphur and used to disinfect wine.

sulphur dioxide (sulphur(IV) oxide) A colourless liquid or pungent gas, SO₂, formed by sulphur burning in air. Sulphur dioxide is produced by burning fossil fuels in power stations; it dissolves in water to give a mixture of sulphuric and sulphurous acids. See also acid rain; air pollution.