Gas-Phase Formation of Sulfurous Acid (H 2 SO 3 ) in the Atmosphere.

Sulfurous acid (H 2 SO 3 ) is known to be thermodynamically instable decomposing into SO 2 and H 2 O. All attempts to detect this elusive acid in solution failed up to now. Reported H 2 SO 3 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 H 2 SO 3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH 3 S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53 - 17 + 7 ${{53}_{-17}^{+\ 7}}$  %. Further main products are SO 2 , SO 3 and methanesulfonic acid. CH 3 S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H 2 SO 3 production of ∼8 million metric tons from the OH+CH 3 S(O)OH reaction. The investigated H 2 SO 3 depletion in the presence of water vapor results in k(H 2 O+H 2 SO 3 ) <3×10 -18  cm 3  molecule -1  s -1 , which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H 2 SO 3 , once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.