Electronic Properties of SO2 in Aqueous Nanodroplets from Machine Learning Molecular Dynamics and Electron Propagator Theory.

Sulfur dioxide (SO2) plays a central role in atmospheric chemistry and is efficiently processed in aqueous aerosols, where solvation profoundly alters its reactivity. We investigate the electronic properties of SO2 in a series of aqueous nanodroplets using machine learning molecular dynamics simulations combined with electron propagator theory. Our simulations show a pronounced surface propensity of SO2 and a preferred orientation at the interface with the sulfur atom directed toward the liquid phase. Both the electron affinity and ionization energy exhibit nonmonotonic variations across the droplet, reaching extrema at the interface, where oxidation is facilitated and reduction is hindered. These variations correlate strongly with the local electrostatic potential and arise from the interplay between chalcogen-chalcogen and hydrogen-bonding interactions. The results demonstrate that the SO2 electronic properties at aqueous interfaces cannot be described as intermediate between bulk and gas phases and highlight the key role of interfacial solvation in controlling redox processes in atmospheric aerosols.