Retrieval of complex refractive index and size distribution of spherical particles from Dual-Polarization Polar Nephelometer data

An algorithm to retrieve the complex refractive index and size distribution of spherical particles from Dual-Polarization Polar Nephelometer data are presented. The assessment of the refractive index is based on the lookup-table approach, which is safe against converging to a local minimum, i.e. false solutions. The results of tests of the algorithm against real experimental data are outlined. The retrieved characteristics are in the expected ranges and in good agreement with the nature of particles and the specifications of the aerosol generators. It is demonstrated that, when a population of small particles, e.g., atmospheric aerosols, is dealt with and a gradient algorithm is employed to estimate the complex refractive index, either the algorithm must be able to overcome the problem of local minima or one should have some a priori information about credible ranges of the real and imaginary parts.