Abstract
Excited-state absorption spectra for several coumarin derivatives adsorbed to aerosol particles provide linear solvation energy (LSE) relationships for the aerosol surfaces. This study focuses on NaCl and (NH4)2SO(4) particles as models for tropospheric aerosol. We investigate several others, including NH(4)Cl, NaBr, KI, Na(2)SO(4), NaNO(3), Al(2)O3, and CaCO(3), to establish trends and understand the factors that control polarity for surfaces. The Kamlet-Taft dipolarity/polarizability parameter, pi*, for these particles ranges from 0.73 to 1.69. The values are high compared to most homogeneous molecular solvents and are attributable to ion-dipole forces, especially at defect sites. We also find that the smaller values of pi* (1.01 for (NH4)2SO(4) and 0.73 for NH(4)Cl) correlate with appreciable hydrogen bond donor acidity in the surface (alpha = 0.23 and 1.06, respectively). Strong hydrogen bonds with the surface lead to a drop in overall polarity either by making interaction with very polar defect sites less likely or orienting the probe molecule away from the surface. Adsorbed water layers mainly alter the alpha value of the surface, but can have indirect effects on pi* by changing the interaction of the adsorbed molecule with the surface.
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