Retrieval of complex refractive indices of mineral aerosols from the UV to the thermal infrared

High spectral resolution extinction measurements of calibrated particlesfrom the UV to the thermal infrared region of the electromagnetic spectrum have been obtained to derive accurate values of the corresponding complex refractive indices. The latter are generally performed through absorbance or transmittance measurements from bulk material or diluted particles in solid pellets leading to possible experimental limitation, such as lack of knowledge of the particle size distribution.
Calibrated amorphous silicasphere particles (99.9%, AngströmSphere) of diameters D = 0.5 and 1 µm were dispersed by a mechanical way in a flow of nitrogen (5 L/min) within a glass container. The continuous flow of aerosol particles was introduced into a 10 m multi-pass cell within an FTIR (Fourier Transform InfraRed) spectrometer (Antaris IGS Analyser, Thermo Scientific) and a 1 m single-pass cell within a UV-visible-NIR spectrometer (MAYA 2000 PRO, Ocean Optics). Aerosol size distributions have been measured at the exit of the spectrometers either with an Aerodynamic Particle Sizer spectrometer (TSI APS 3321), or with a Scanning Mobility Particle Sizer (TSI SMPS 3936L75).
The complex refractive indices are determined by combining Kramers-Krönig relations, the Mie theory and an iterative process. This allows minimizing errors in the retrieval procedure.