Abstract
The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51–1.55 and imaginary parts of 0.0008–0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ωo and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ωo is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.
Highlights
Saharan mineral dust is one of the most important aerosol components in the Earth’s atmosphere
The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition
This aspect ratio was confirmed by independent single particle analyses using a scanning electron microscope
Summary
Saharan mineral dust is one of the most important aerosol components in the Earth’s atmosphere. The size and concentration measurement of large particles is complicated and influenced by measurement uncertainties (Reid et al, 2003) and technical limitations (Haywood et al, 2003) Their importance becomes obvious by the fact that remote sensing techniques (Wendisch and von Hoyningen-Huene, 1994; Kaufman et al, 2001) and retrieval algorithms as used within the scope of AERONET (Dubovik et al, 2002a, 2006) yield optical properties and size distribution information for mineral dust ensembles, different to those measured in-situ (Formenti et al, 2000; Schmid et al, 2000) and modelling studies (Cattrall et al, 2003; Balkanski et al, 2007).
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