| Abstract
| - Excited-state absorption spectra for several coumarin derivatives adsorbed to aerosol particles provide linearsolvation energy (LSE) relationships for the aerosol surfaces. This study focuses on NaCl and (NH4)2SO4particles as models for tropospheric aerosol. We investigate several others, including NH4Cl, NaBr, KI, Na2SO4, NaNO3, Al2O3, and CaCO3, to establish trends and understand the factors that control polarity for surfaces.The Kamlet-Taft dipolarity/polarizability parameter, π*, for these particles ranges from 0.73 to 1.69. Thevalues 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 π* (1.01 for (NH4)2SO4 and 0.73 for NH4Cl)correlate with appreciable hydrogen bond donor acidity in the surface (α = 0.23 and 1.06, respectively).Strong hydrogen bonds with the surface lead to a drop in overall polarity either by making interaction withvery polar defect sites less likely or orienting the probe molecule away from the surface. Adsorbed waterlayers mainly alter the α value of the surface, but can have indirect effects on π* by changing the interactionof the adsorbed molecule with the surface.
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