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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.
Chehab et al. (Mon,) studied this question.