Solar radiative effects of non-spherical mineral dust particles during SAMUM 2006

Kurzfassung

During the SAMUM campaign comprehensive measurements were performed in May-June 2006 concerning the physico-chemical and radiative properties of Saharan mineral dust. Based on these observations optical properties for a dust plume were calculated using measured number size distributions as well as information on the particles’ chemical composition and predominant shape. For a wavelength of 550 nm size-resolved complex refractive indices of 1.5-1.58 and 0.004-0.005 were derived for the real and imaginary part, respectively. For 550 nm the single scattering albedo and the asymmetry parameter of the phase function attained values of about 0.8, so that the SAMUM Saharan mineral dust appears to be absorbing and forward scattering. In addition, a considerable impact on the optical properties can be traced back to the presence of coarse dust particles. Mie simulated backscatter coefficients and lidar ratios were not in agreement with independent data measured by various lidars confirming that the observed Saharan mineral dust particles had non-spherical shapes. The lidar measurements provided key-information to quantify the predominant shape of the particles. Assuming spheroidal model particles comprehensive simulations demonstrated that a mean oblate shape (axis ratio, AR, of about 1:1.6) led to the best agreement with the lidar data. This was confirmed by single particle analyses using a scanning electron microscope, too. For the purpose of extensive radiative transfer simulations for ensembles with non-spherical particles, a comprehensive database of optical properties for single oblate and prolate spheroidal particles was built up. Our computations show significant non-sphericity effects of the observed dust: For oblate particles (AR 1:1.6) the single scattering albedo is weakly affected by only 1, but the asymmetry parameter is reduced by important 4%. Assumptions on how to interpret particle ‘size’ and ‘shape’ may cause positive and negative changes for all optical properties. The spectral optical depths for extinction and scattering may vary by +/-7% as a function of the particular size-shape equivalence relation adopted. The downward radiative transport is simulated to be less influenced by the particles’ non-sphericity. On the other hand, reflectance is predominantly forced, i.e. non-spherical particles enhance the population’s backscattering. For example, for SAMUM conditions and for the wavelength range 0.3-2.2 nm, the top-of-atmosphere local radiative forcing over a non-reflecting surface increases from 6 W/m² (spheres) to 16 W/m² (oblate spheroids, AR 1:1.6). These and other issues of non-spherical dust particles during SAMUM will be discussed.