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 1kcalmol?1, which is lower than the energy barrier of 5.17kcalmol?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.
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