Aerosols complex refractive indices determination from far infrared to UV: application to dust and residual ashes of biomass burning

Due to their ability to absorb and scatter solar radiation, major injections of aerosols can have a significant effect on the atmosphere including impacts on the radiation balance of the Earth and changes in temperature. Given the variability and spatial heterogeneity of their concentration, size and chemical composition, it is important to quantify these aerosols, from remote sensing techniques, in order to better identify their sources and understand their environmental impact from regional to global scale. Satellite instruments, such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder AIRS for the thermal IR region and FORUM for the far infrared, can give us information about chemical composition (Alalam et al. 2022) and microphysical parameters of the aerosols such as the effective radius, concentration and mass (Deguine et al. 2023). Nonetheless, these techniques require accurate information about the optical properties, specifically the complex refractive index (CRI)   .CRI databases available in the literature however, span over limited wavelength ranges and provide mainly reflectance measurements on bulk materials or pressed pellets. In particular, the latter can have several limitations such as the modification of the microphysical properties of the particles (size distribution and morphology). Furthermore, in pellet samples, the particles are present in a compressed matrix causing modifications of the vibrational modes. For bulk measurements, there is strong underestimation of the scattering signal.  Therefore, the optical constants coming from such techniques are not fitted for aerosols and atmospheric applications (McPheat et al. 2002).

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) from far infrared (FIR) (50 µm /200 cm^-1) up to UV (0.25 µm /40,000 cm^-1) and the recording of the size distribution of both fine and coarse particles (Hubert et al. 2017). 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 to scattering theories and the single subtractive Kramers-Kronig (SSKK) relation (Herbin et al. 2017).

Kaolinite, one of the main clays found in dust, has been used as a first application of this methodology. For the first time, homogenous values of CRI have been retrieved continuously from FIR to UV for suspended particles. This methodology is also being used to retrieved CRI of biomass burning aerosols (BBA). Preliminary result obtained from residual ashes will be present, showing IR extinction spectra as well as chemical analysis.