New approach for the determination of aerosol refractive indices – Part I: Theoretical bases and numerical methodology

Abstract The knowledge of aerosol complex refractive indices on wide spectral range with high spectral resolution is important for many research fields and applications. Various combinations of experimental/theoretical/numerical approaches have been employed to determine the optical indices of aerosol particles. However, each approach has its own advantages and limitations that restrict its generalization. This article is first part of a work aimed at proposing a new technique for determining the optical constants of aerosols. Experimentally, the method is based on recording transmittance spectra of an aerosol flow from thermal infrared to UV–visible combined with the size distribution measurements. Herein, we present the theoretical and numerical bases of the algorithm developed to retrieve the imaginary and real parts of refractive indices. This model associates the Mie theory, the single subtractive Kramers-Kronig relations, and the optimal estimation method with an iterative process. In order to quantify the capabilities of the algorithm to retrieve complex refractive indices, inverse calculations are performed from simulated extinction spectra of Quartz particles whose some of optical properties are available in the literature. We have detailed each step of the procedure and performed some comparisons with the most currently employed methods. The impact of experimental accuracy and numerical simulation are investigated in terms of errors, and uncertainties on the retrieved real and imaginary parts of the complex optical index.