Gas-Phase Formation of Sulfurous Acid (H₂SO₃) in the Atmosphere

Sulfurous acid (H₂SO₃) is known to be thermodynamically instable decomposing into SO₂ and H₂O. All attempts to detect this elusive acid in solution failed up to now. Reported H₂SO₃ formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H₂SO₃ is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH₃S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of $$  %. Further main products are SO₂, SO₃ and methanesulfonic acid. CH₃S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H₂SO₃ production of ∼8 million metric tons from the OH+CH₃S(O)OH reaction. The investigated H₂SO₃ depletion in the presence of water vapor results in k(H₂O+H₂SO₃) <3×10^-18 cm³ molecule^-1 s^-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H₂SO₃, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.