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

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