Properties of Sharav (Khamsin) Dust–Comparison of Optical and Direct Sampling Data

Abstract

Simultaneous measurements of optical depth and Size distribution in a dust storm are presented. The measured and derived properties of the aerosol are compared with each other and with other results published in the scientific literature. We observe some global commonality in the measured size spectra of desert aerosols especially for post-frontal conditions. On the other hand, during the passage of the front itself, high aerosol concentrations with a sharp peak in radius at −1 μm were observed. Generally, these were not similar to other size distributions reported in the literature. The imaginary part of the refractive index in the spectral region 0.3-1.7 μm was found to be similar to that found in other deserts. Comparison of the optical measurements with the direct sampling data suggests that the general time trends of the size distributions, as measured in situ, are followed by the optical depth and its variations with wavelength. On the other hand, detailed short-term fluctuations detected by our direct measurements are not followed by the optical method. We have observed that a simple power law for the size distribution, in the range r>0.15μm is a reasonable approximation only during clear and calm conditions with small optical depth. During the dust storm itself, the deviations from a power law are lame as shown by both direct in situ and optical observations.