Vibrational exciton coupling in pure and composite sulfur dioxide aerosols.

Icy aerosol particles determine weather processes in planetary atmospheres and are discussed as sites of chemical reactions in interstellar dust. One of the interesting ice systems in this context are pure and composite SO2 particles. Vibrational exciton coupling is exploited in the present study to understand the properties of SO2 aerosol particles from their infrared spectroscopic signatures. The condensation to pure SO2 ice particles leads to a partially amorphous structure of the particles, whose infrared spectra do not show pronounced shape effects. The simultaneous condensation of SO2 and CO2 gas results in aerosol particles with a core-shell structure. Icy SO2 forms the core, whereas CO2 remains in the shell. This structure is mainly determined by the different thermodynamic properties of the two substances and the less favorable intermolecular interactions between SO2 and CO2 compared with the pure substances. The example of mixed sulfur dioxide/ammonia particles demonstrates that nanosized aerosol particles are ideal nanoreactors. They facilitate an easy homogeneous mixture of the reactants providing an interface-free reaction environment.