Strengthened H-bond networks compensate for entropy loss to facilitate NH3-assisted SO2 hydration in aqueous microdroplets.

Hydration of SO2 in aqueous microdroplets assisted by NH3 is a key precursor step in the formation of atmospheric sulfate aerosols. In this study, ab initio molecular dynamics (AIMD) and metadynamics simulations were performed to investigate the microscopic mechanism of the SO2-NH3-H2O system, with particular emphasis on the effects of water-cluster size on NH3-assisted SO2 hydration and the kinetic and thermodynamic stability of the hydration product NH4HSO3. Our results indicate that the alkaline microenvironment created by NH3 significantly promotes SO2 hydration by lowering the reaction barrier. The hydration product NH4HSO3 is metastable and readily hydrolyzes in small water clusters (4H2O), whereas larger water clusters (10H2O) effectively stabilize NH4HSO3 by increasing the hydrolysis barrier. Larger water clusters exhibit greater entropy loss (-182.36 J mol-1 K-1) during the SO2 hydration process, reflecting enhanced structural ordering and stronger hydrogen-bond interactions. A cooperative enthalpy-entropy effect is therefore proposed, in which a stronger hydrogen bond network for larger water cluster compensates for the entropic penalty and facilitates proton transfer. Moreover, lower temperatures further favor the hydration reaction. This work provides molecular-level insight into aerosol-phase SO2 conversion and offers theoretical guidance for atmospheric sulfate control.