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In recent years, Criegee chemistry has become an important research focus due to its relevance in regulating concentrations of tropospheric OH radicals, hydroperoxides, sulfates, nitrates, and aerosols. However, to date, its interface behavior remains poorly understood. Thus, in this study, we used the Born-Oppenheimer molecular dynamics (BOMD) simulation method to explore the reaction mechanisms between Criegee intermediates (CIs) and methylsulfonic acid (MSA) at the air-water interface, then compared the observed behaviors with those in the gas phase. The addition of Criegee intermediates to MSA is nearly a barrierless reaction and follows a loop-structure mechanism in the gas phase. The high rate constants indicate that the Criegee intermediates and MSA reactions are the main acid removal channels. At the water's surface, the interaction of Criegee intermediates with MSA includes three main channels: 1) direct addition reaction, 2) H 2 O-mediated hydroperoxide formation, and 3) MSA-mediated Criegee hydration. These reaction channels follow a loop-structure or a stepwise mechanism and proceed at the picosecond time-scale. The results of this work broaden our understanding of Criegee atmospheric behaviors in polluted urban and marine areas, which in turn will aid in developing more effective pollution control measures. Unlabelled Image • The ab initio dynamics simulation was adopted to simulate the reactions at the air-water interface. • The reaction of MSA with CH 3 CHOO plays an active role in the MSA removal channel. • At the water's surface, the reaction channels follow a loop-structure or a stepwise mechanism. • MSA also acts as a trigger to promote Criegee hydration at the air-water interface. [ABSTRACT FROM AUTHOR]