Does optically effective complex refractive index of internal-mixed aerosols have a physically-based meaning?

Area of Science:

• Atmospheric Chemistry and Physics
• Aerosol Science
• Optical Properties of Particles

Background:

• Retrieving low real parts of aerosol complex refractive index (ACRI) from particle optical properties is problematic.
• The physical basis of optically effective ACRI remains an open question.

Purpose of the Study:

• To numerically determine the optically effective ACRI for internal-mixed aged BC particles.
• To compare ACRI derived from Mie theory with the volume-weighted average (VWA) method.
• To assess the physical meaningfulness of optically effective ACRI.

Main Methods:

• Core-shell Mie theory was used to calculate scattering and absorption properties of coated BC particles.
• A Mie theory-based data analysis scheme was employed.
• Look-up tables were generated based on particle composition and optical properties.

Main Results:

• Optically effective ACRI for coarse coated BC can exhibit extreme real parts when using unconstrained look-up tables.
• Retrieved imaginary ACRIs for coarse coated BC are significantly lower (nearly 3x) than VWA approximations.
• VWA overestimates absorption of coarse coated BC by ~2x for large shell/core ratios.

Conclusions:

• Optically effective ACRI for internal-mixed BC particles possesses physical meaning, contrary to some retrieval outcomes.
• Limited ACRI look-up tables that do not consider aerosol composition can lead to unrealistic retrieved values.
• Optically effective ACRI, not VWA-derived ACRI, should be used for coarse internal-mixed particles in climate models.