Surface Confinement of Finite-Size Water Droplets for SO 3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field.

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO 3 ) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H 2 SO 4 )-(H 2 O) clusters and ionic (HSO 4 ) - -(H 3 O) + clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO 3 with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO 3 and water clusters of different sizes. Generally, neutral (H 2 SO 4 )-(H 2 O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO 3 (H 2 O) n ≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H 2 O) 8 , a (HSO 4 ) - -(H 3 O) + ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H 2 O) n ≥32 ), the (SO 4 ) 2- [(H 3 O) + ] 2 ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO 3 hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.