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By theoretical calculations, the gas-phase SO 2 hydration reaction assisted by methylamine (MA) and dimethylamine (DMA) was investigated, and the potential contribution of the hydrated product to new particle formation (NPF) also was evaluated. The results show that the energy barrier for aliphatic amines (MA and DMA) assisted SO 2 hydration reaction is lower than the corresponding that of water and ammonia assisted SO 2 hydration. In these hydration reactions, nearly barrierless reaction (only a barrier of 0.1 kcal mol −1 ) can be found in the case of SO 2 + 2H 2 O + DMA. These lead us to conclude that the SO 2 hydration reaction assisted by MA and DMA is energetically facile. The temporal evolution for hydrated products (CH 3 NH 3 + -HSO 3 - -H 2 O or (CH 3 ) 2 NH 2 + -HSO 3 - -H 2 O) in molecular dynamics simulations indicates that these complexes can self-aggregate into bigger clusters and can absorb water and amine molecules, which means that these hydrated products formed by the hydration reaction may serve as a condensation nucleus to initiate the NPF. [ABSTRACT FROM AUTHOR]

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]

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]