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.