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

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.