Optical properties of Gobi dust and its pure compounds: experimental extinction spectra and complex refractive indices determination.

Abstract

<p>Spectrometers are powerful instruments to detect atmospheric aerosols, especially on satellites since they allow measurements at a global scale and over different spectral ranges with high spectral resolution. However, to fully exploit their capabilities and to link optical properties, chemical composition and mass concentration, it is essential to have reference optical properties of various particles and mainly the complex refractive indices (CRI). The CRI of a natural aerosol source can be determined from a real sample of it or applying the effective medium approximation using the CRI of the pure compounds present in the natural sample. But in that case, it is necessary to know the mass fraction of each individual compound and above all their CRI. Nevertheless, the literature and CRI databases provide only reflectance measurements on bulk materials or pressed pellets and over a limited wavelength range (Querry <em>et al.</em>, 1987).</p><p>In the present work, dust from the Gobi desert is studied as it is the second most active dust source, after the Sahara desert, in terms of mass emissions (Querol <em>et al.</em>, 2019). For that extinction spectra have been recorded for natural Gobi dust sample and for its major compounds (Illite, Calcite and Quartz). Particles as a powder in a vessel are generated thanks to a magnetic stirring and a flow of nitrogen (Hubert <em>et al.</em>, 2017). The continuous flow of aerosols is directed into a 10-meters multipass cell fitted to a Fourier transform infrared spectrometer and a 1-meter singlepass cell within a UV-Visible spectrometer which cover a continuous spectral range from 650 cm<sup>-1</sup> to 40000 cm<sup>-1</sup>. Moreover, at the exit of the spectrometers the size distribution is recorded by an aerodynamic particle sizer and a scanning mobility particle sizer which allow to measure size particles from 14 nm to 20 µm. An inversion algorithm is carried out using experimental extinction spectra and the size distribution as input data (Herbin <em>et al.</em>, 2017). Applying the Mie theory and the single subtractive Kramers-Kröning integral, the real and the imaginary part of the CRI are retrieved at each wavelength with an optimal estimation method.</p><p>For the first time, CRI of Illite has been retrieved with a high spectral resolution (1 cm<sup>-1</sup>) and over a wide spectral range for suspended particles. For calcite and quartz particles, the crystalline phase has to be considered by introducing the ordinary and extraordinary indices. These pure compound sets of CRI will be used for testing effective medium approximation on Gobi dust for which effective CRI have been also retrieved.</p>

Aerosol Mineralogical Study Using Laboratory and IASI Measurements: Application to East Asian Deserts

“…The extinction coefficient of the Gobi dust sample in Figure 10a shows absorption peaks identical to the peaks of the pure mineral extinction coefficients of quartz, illite, and calcite illustrated in Figure 10b [52]. These extinction coefficients present a measurement of each individual mineral and were normalized to remove the dependency on concentration.…”

“…The experimental study was performed by two laboratories: PhysicoChimie des Processus de Combustion et de l'Atmosphère (PC2A) and Laboratoire d'Optique Atmosphérique (LOA). The analyzed samples included pure quartz crystal, pure amorphous illite, and pure calcite crystal [52], in addition to the analysis of Gobi dust samples that were collected in Yanchi, Ningxia province in China (107.475211 • E, 36.487333 • N) [53]. Sample particles (5 g of mass) were placed in suspension by maintaining mechanical agitation using a magnetic stirrer.…”

Spatial variability of Saharan dust deposition revealed through a citizen science campaign

“…where i is the extinction spectrum of the mineral; i and C i are coefficients that are fitted by a least-squares fitting method, minimising the differences between the calculated extinction and the experimental extinction (i = q, il, and c for quartz, illite, and calcite). The calculation of the extinction spectrum for each mineral was performed using Mie optical theory (Herbin et al, 2017) by introducing the complex refractive indices (CRIs) recently determined by Deschutter (2022) as input data and a size distribution calculated using the fitted moments of the distribution listed in part 6.3.1. of Deschutter (2022).…”

Spatial variability of Saharan dust deposition revealed through a citizen science campaign

“…where i is the extinction spectrum of the mineral i and C i are coefficients that are fitted by a least-squares fitting method minimising the differences between the calculated extinction and the experimental extinction (i = q, il and c for quartz, illite and calcite). The calculation of the extinction spectrum for each mineral was performed using Mie optical theory (Herbin et al, 2017) by introducing their complex refractive indices (CRI) recently determined by Deschutter (2022) as input data and a size distribution calculated using the fitted moments of the distribution listed in part 6.3.1. of Deschutter (2022).…”