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<p>The hydration of <span class="inline-formula">SO₃</span> 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 <span class="inline-formula">SO₃</span>. The energy barrier of the <span class="inline-formula">SO₃</span> hydration reaction catalyzed by oxalic acid (cTt, tTt, tCt and cCt conformers) is a little higher or less than 1&thinsp;kcal&thinsp;mol<span class="inline-formula">⁻¹</span>, which is lower than the energy barrier of 5.17&thinsp;kcal&thinsp;mol<span class="inline-formula">⁻¹</span> for water-catalyzed <span class="inline-formula">SO₃</span> hydration. Compared with the rates of the <span class="inline-formula">SO₃</span> hydration reaction catalyzed by oxalic acid and water, it can be found that in the upper troposphere the OA-catalyzed <span class="inline-formula">SO₃</span> hydration can play an important role in promoting <span class="inline-formula">SO₃</span> hydration. It leads us to conclude that the involvement of oxalic acid in <span class="inline-formula">SO₃</span> hydration to form <span class="inline-formula">H₂SO₄</span> is significant in the atmosphere.</p>