Infrared Spectroscopy of Stepwise Hydration Motifs of Sulfur Dioxide.

Experimental characterization of microscopic events and behaviors of SO 2 -H 2 O interactions is crucial to understanding SO 2 atmospheric chemistry but has been proven to be very challenging due to the difficulty in size selection. Here, size-dependent development of SO 2 hydrate structure and cluster growth in the SO 2 (H 2 O) n ( n = 1-16) complexes was probed by infrared spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. Spectral changes with cluster size demonstrate that the sandwich structure initially formed at n = 1 develops into cycle structures with the sulfur and oxygen atoms in a two-dimensional plane ( n = 2 and 3) and then into three-dimensional cage structures ( n ≥ 4). SO 2 is favorably bound to the surface of larger water clusters. These stepwise features of SO 2 hydration on various sized water clusters contribute to understanding the reactive sites and electrophilicity of SO 2 on cloud droplets, which may have important atmospheric implications for studying the SO 2 -containing aerosol systems.