Scientists reveal stepwise hydration motifs of sulfur dioxide

A research team led by Prof. Jiang Ling and Zhang Zhaojun from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) identified the infrared signature of stepwise hydration motifs of sulfur dioxide (SO₂).

Their findings were published in the Journal of Physical Chemistry Letters on June 16.

The researchers revealed the size-dependent development of SO₂ hydrate structure and cluster growth in the SO₂(H₂O)_n (n = 1-16) complexes and highlighted a general model to elucidate the formation mechanism of the SO₂-containing aerosol systems.

SO₂ is an important atmospheric pollutant and is involved in many atmospheric processes such as the formation of cloud condensation nuclei and acid rain. Characterizing the chemical composition, structure and growth of nucleating precursors is essential for understanding the underlying mechanisms of atmospheric new particle formation.

Experimental characterization of microscopic events and behaviors of SO₂-H₂O interactions is challenging due to the difficulty in size selection.

Based on the recently-developed infrared spectroscopy using a tunable vacuum ultraviolet free electron laser (VUV-FEL), the researchers measured infrared spectra for the neutral SO₂(H₂O)_n (n = 1-16) clusters in the 2700-3900 cm^-1 spectral regions.

They conducted quantum mechanical calculations to identify the low-lying isomers and to assign the experimental spectral features. They found that the sandwich structure initially formed at n = 1 developed 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) with the binding of SO₂ on the outer side of water clusters.

"Since the structures of hydrated SO₂ could affect the reactive sites and electrophilicity of SO₂, the present cluster perspectives would deepen our understanding of the solvation behaviors of SO₂ on the water nanodroplets and surfaces and have atmospheric implications for studying the SO₂-containing aerosol systems," said Prof. Jiang.