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H 2 SO 4 is an important pollutant in the atmosphere, and it mainly be formed through the oxidation of SO 2. SO 3 produced from the oxidation of SO 2 is a precursor of H 2 SO 4. Considering that understanding of SO 3 hydration reaction at the air-water interface is limited, we combined the classical molecular dynamics simulations (classical MD) with Born–Oppenheimer MD simulations and ab initio calculations to study the SO 3 reaction with H 2 O at air-water interface. The free energy and impinging results indicate that the SO 3 molecules can easily be trapped by water droplets. After uptake by the interface, the SO 3 can react rapidly with water molecules to form the ion pair of HSO 4 − and H 3 O+ or H 2 SO 4 within a few picoseconds. In the interfacial hydration mechanism, the loop-structure formation is not prerequisite of the hydration reaction, which is obviously different from the corresponding gas-phase hydration reaction of SO 3. The work is meaningful for understanding the heterogeneous formation of H 2 SO 4 and the contribution of interfacial reaction to the new particle formation. • SO 3 can easily be trapped by water droplets. • The SO 3 hydration mechanism at the air-water interface was considered. • The loop-structure formation is not prerequisite of interfacial hydration of SO 3. [ABSTRACT FROM AUTHOR]