First complex refractive indices retrieval from FIR to UV: Application to kaolinite particles

Abstract

Knowledge of the optical properties, in particular the complex refractive indices (CRI), of aerosols is crucial to better quantify their impact on the environment from remote sensing techniques. However, the CRI data available in the literature provide mainly reflectance measurements on bulk materials or pressed pellets and span over limited wavelength ranges. Here, we present an improved retrieval methodology combining an experimental setup that allows simultaneously the measurement of high spectral-resolution extinction spectra (up to 0.5 cm⁻¹) and the recording of the size distribution (SD) of both fine (down to 10 nm) and coarse (up to 20 µm) particles. Introducing these experimental measurements in a numerical iterative process, the real and imaginary parts of the CRI are retrieved using an optimal estimation method (OEM) associated with scattering theories and the single subtractive Kramers–Kronig (SSKK) relation. Using this methodology, we are able to accurately determine for the first time CRI of an aerosol flow for kaolinite over a wide spectral range from far infrared (FIR) (50 µm/200 cm⁻¹) up to UV (0.25 µm/40,000 cm⁻¹). The mean values of the total uncertainty of the retrieved real and imaginary parts are 1.6% and 0.6%, respectively.

Copyright © 2024 American Association for Aerosol Research

Graphical Abstract

EDITOR:

• Hans Moosmüller

Acknowledgments

This work is a contribution to the LabEx CaPPA (Chemical and Physical Properties of the Atmosphere) project funded by the French National Research Agency (ANR) under contract “ANR-11-LABX-0005-01” and to the CPER research project CLIMIBIO/ECRIN funded by the French Ministère de l’Enseignement Supérieur et de la Recherche. The authors thank the Institut on Environmental Sciences (IRePSE, FED 4129, Univ. Lille) for its financial support.

The authors thank Professor Etienne Balan from Institut de Mineralogie, Physique des Materiaux et Cosmochimie (IMPMC) at Sorbonne University for providing us with his experimental data.

Disclosure statement

The authors declare there is no Conflict of Interest.