Linear solvation energy parameters for model tropospheric aerosol surfaces

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Aerosol particles play a crucial role in atmospheric processes.
• Understanding the surface properties of aerosols is vital for atmospheric modeling.
• Coumarin derivatives are useful probes for studying solvation environments.

Purpose of the Study:

• To determine the linear solvation energy (LSE) relationships for aerosol surfaces.
• To investigate the polarity of various aerosol particles, including NaCl and (NH4)2SO4.
• To understand the factors controlling surface polarity, such as ion-dipole forces and hydrogen bonding.

Main Methods:

• Measurement of excited-state absorption spectra of coumarin derivatives adsorbed on aerosol particles.
• Application of the Kamlet-Taft scale to quantify aerosol surface polarity (pi*) and hydrogen bond acidity (alpha).
• Systematic investigation of various inorganic and mineral aerosol compositions.

Main Results:

• Aerosol surfaces exhibit high Kamlet-Taft dipolarity/polarizability (pi*) values (0.73-1.69), exceeding those of many molecular solvents.
• High polarity is attributed to strong ion-dipole interactions, particularly at surface defect sites.
• Lower pi* values correlate with significant surface hydrogen bond acidity (alpha), indicating surface interactions can reduce polarity.

Conclusions:

• Aerosol surfaces possess unique solvation properties distinct from homogeneous solvents.
• Surface polarity is modulated by both inherent surface chemistry and interactions with adsorbed species like water.
• These findings are critical for accurately modeling chemical reactions and physical processes occurring on atmospheric aerosols.