Interaction of SO2 with the Surface of a Water Nanodroplet.

We present a comprehensive computational study of interaction of a SO2 with water molecules in the gas phase and with the surface of various sized water nanodroplets to investigate the solvation behavior of SO2 in different atmospheric environments. Born-Oppenheimer molecular dynamics (BOMD) simulation shows that, in the gas phase and at a temperature of 300 K, the dominant interaction between SO2 and H2O is (SO2)S···O(H2O), consistent with previous density-functional theory (DFT) computation at 0 K. However, at the surface of a water nanodroplet, BOMD simulation shows that the hydrogen-bonding interaction of (SO2)O···H(H2O) becomes increasingly important with the increase of droplet size, reflecting a marked effect of the water surface on the SO2 solvation. This conclusion is in good accordance with spectroscopy evidence obtained previously (J. Am. Chem. Soc. 2005, 127, 16806; J. Am. Chem. Soc. 2006, 128, 3256). The prevailing interaction (SO2)O···H(H2O) on a large droplet is mainly due to favorable exposure of H atoms of H2O at the air-water interface. Indeed, the conversion of the dominant interaction in the gas phase (SO2)S···O(H2O) to the dominant interaction on the water nanodroplet (SO2)O···H(H2O) may incur effects on the SO2 chemistry in atmospheric aerosols because the solvation of SO2 at the water surface can affect the reactive sites and electrophilicity of SO2. Hence, the solvation of SO2 on the aerosol surface may have new implications when studying SO2 chemistry in the aerosol-containing troposphere.