The hydration of SO₃ plays an important role in atmospheric sulfuric acid formation. Some atmospheric species can be involved in and facilitate the reaction. In this work, using quantum chemical calculations, we show that oxalic acid, the most common dicarboxylic acid in the atmosphere, can effectively catalyze the hydration of SO₃. The energy barrier of the SO₃ hydration reaction catalyzed by oxalic acid (cTt, tTt, tCt and cCt conformers) is a little higher or less than 1 kcal mol⁻¹, which is lower than the energy barrier of 5.17 kcal mol⁻¹ for water-catalyzed SO₃ hydration. Compared with the rates of the SO₃ hydration reaction catalyzed by oxalic acid and water, it can be found that in the upper troposphere the OA-catalyzed SO₃ hydration can play an important role in promoting SO₃ hydration. It leads us to conclude that the involvement of oxalic acid in SO₃ hydration to form H₂SO₄ is significant in the atmosphere.

Methanesulfinic acid (MSIA) is an important intermediate in the oxidation of dimethyl sulfide (DMS) in the marine boundary layer. The oxidation of MSIA by ozone in the gas phase to form methanesulfonic acid (MSA) was investigated using theoretical calculations in this paper. Three pathways can be found for the reaction of MSIA with ozone. The highest energy barrier is 13.02 kcal mol⁻¹ in the most favorable pathway. By comparing the reaction rate of MSIA + O₃ with that of MSIA + OH, it can be concluded that the oxidation of MSIA by O₃ to form MSA is of minor significance relative to its oxidation by OH radical in the gas phase. This study can provide some information for the theoretical and experimental studies in the significantly heterogeneous and aqueous-phase oxidation of MSIA by O₃.