A detailed characterization of the Saharan dust collected during the Fennec campaign in 2011: in situ ground-based and laboratory measurements

Abstract

Abstract. Millions of tons of mineral dust are lifted by the wind from arid surfaces and transported around the globe every year. The physical and chemical properties of the mineral dust are needed to better constrain remote sensing observations and are of fundamental importance for the understanding of dust atmospheric processes. Ground-based in situ measurements and in situ filter collection of Saharan dust were obtained during the Fennec campaign in the central Sahara in 2011. This paper presents results of the absorption and scattering coefficients, and hence single scattering albedo (SSA), of the Saharan dust measured in real time during the last period of the campaign and subsequent laboratory analysis of the dust samples collected in two supersites, SS1 and SS2, in Algeria and in Mauritania, respectively. The samples were taken to the laboratory, where their size and aspect ratio distributions, mean chemical composition, spectral mass absorption efficiency, and spectral imaginary refractive index were obtained from the ultraviolet (UV) to the nearinfrared (NIR) wavelengths. At SS1 in Algeria, the time series of the scattering coefficients during the period of the campaign show dust events exceeding 3500 Mm −1 , and a relatively high mean SSA of 0.995 at 670 nm was observed at this site. The laboratory results show for the fine particle size distributions (particles diameter < 5µm and mode diameter at 2-3 µm) in both sites a spectral dependence of the imaginary part of the refractive index I m(m) with a bow-like shape, with increased absorption in UV as well as in the shortwave infrared. The same signature was not observed, however, in the mixed particle size distribution (particle diameter < 10 µm and mode diameter at 4 µm) in Algeria. I m(m) was found to range from 0.011 to 0.001i for dust collected in Algeria and 0.008 to 0.002i for dust collected in Mauritania over the wavelength range of 350-2500 nm. Differences in the mean elemental composition of the dust collected in the supersites in Algeria and in Mauritania and between fine and mixed particle size distributions were observed from EDXRF measurements, although those differences cannot be used to explain the optical properties variability between the samples. Finally, particles with low-density typically larger than 10 µm in diameter were found in some of the samples collected at the supersite in Mauritania, but these low-density particles were not observed in Algeria.

Analysis of Atmospheric Aerosol Optical Properties in the Northeast Brazilian Atmosphere with Remote Sensing Data from MODIS and CALIOP/CALIPSO Satellites, AERONET Photometers and a Ground-Based Lidar

“…The detection limit of MODIS is reported as 0.03-0.05 [56]. The low values are in accordance with [57], where the authors performed a characterization of AOD in all regions in the world, and indicated that South America, in general, has mostly low AOD values, except during the South American main biomass burning season (from July to October). The NEB is not affected by these emissions, due to the typical atmospheric circulation patterns.…”

Coarse-mode mineral dust size distributions, composition and optical properties from AER-D aircraft measurements over the tropical eastern Atlantic

“…The PCASP was calibrated before and after the campaign, and the CDP was calibrated and cleaned before most flights during the campaign as described in Rosenberg et al (2012).…”

“…The PCASP and CDP employ light scattering in order to determine particle size; thus, the nominal size bins have been adjusted for a refractive index (RI) of dust of 1.53-0.001i, informed by the refractive index closure results from Sect. 2.5, using the CStoDConverter software (Rosenberg et al, 2012). This results in an increase in diameter of the largest size bins due to the more absorbing imaginary part of the refractive index as compared to PSL (polystyrene latex) used in the manufacturer's calibrations.…”

Complex refractive indices and single scattering albedo of global dust aerosols in the shortwave spectrum and relationship to iron content and size

“…2.2). In the calculations, the real part of the complex refractive index of dust was assumed to be wavelength-independent and fixed at a value of 1.53, while the imaginary part was set to 0.003 at 450 and 550 nm and to 0.001 at 700 nm, according to pre-existing information (Sinyuk et al, 2003;Schladitz et al, 2009;Formenti et al, 2011;Rocha-Lima et al, 2018). For the different dust samples, Ctrunc ranged between 1.2 and 1.7 and decreased with wavelength and the dust residence time in the chamber, following the relative importance of the coarse component in the dust population (Anderson and Ogreen, 1998).…”

“…CC BY 4.0 License. Lima et al, 2018). Albeit some variability being instrumental or analytical (differences in the sampled size fraction or in the method used to retrieve optical parameters), differences persist when the same measurement approach and retrieval method are applied, e.g., in AERONET inversions, supporting the dependence of dust k and SSA with location (Dubovik et al, 2002;Koven and Fung, 2006;Su and Toon, 2011).…”