A New Mixing Rule for Imaginary Refractive Indices of Aerosols or Colloids in the Rayleigh Regime

JUSTIN MAUGHAN, Hans Moosmüller, Prakash Gautam, Christopher M. Sorensen, Pratt Community College

     Abstract Number: 255
     Working Group: Aerosol Physics

Abstract
A new Rayleigh mixing rule that relates the effective imaginary part of the refractive index of a composite medium to the complex refractive index of the Rayleigh particles within a non-absorbing medium is derived using Rayleigh scattering theory. The new Rayleigh mixing rule is compared with widely used mixing rules ( i.e., volume, Maxwell Garnett, and Bruggeman mixing rules) and we show that in the small volume fraction regime both the Maxwell Garnet and Bruggeman mixing rules agree with the Rayleigh mixing rule. The Multiple Sphere T-Matrix (MSTM) code by Mackowski is used to calculate the total extinction, absorption, and scattering cross sections for a spherical non-absorbing medium with absorbing Rayleigh particles inclusions randomly imbedded within the medium. The MSTM results are compared to that of Mie theory calculations using the effective refractive indices given by the Rayleigh, volume, Maxwell Garnett, and Bruggeman mixing rules. Comparison of the MSTM and Mie results for volume fractions of 0.01, 0.03, 0.1, and 0.3 show that for all volume fractions except 0.3, and all mixing rules except the volume mixing rule, there was good agreement. Fractional differences were within approximately 10% or less when comparing mixing rule results to the actual MSTM results for relative refractive indices of the Rayleigh inclusions smaller than 2.0+i.