The effect of mineral nuclei modes in the particle distributions of Sahara mineral dust aerosols

Abstract

In this work, the aerosols optical depth (AOD) of Sahara aerosols were extracted from Optical Properties of Aerosols and Clouds (OPAC) software package (4.0)at eight relative humidities RHs (00,50, 70, 80, 90, 95, 98,and 99)% and ultraviolet to visible wavelengths (0.25 to 0.9) μm. The mixture or composition of Sahara aerosols are water, mineral nuclei mode nonspherical, mineral accumulation mode nonspherical and mineral coarse mode nonspherical. The number concentrations of mineral nuclei modes nonspherical were varied. The angstrom exponent (α) which is normally used to analyze the type of particle and size distributions using inverse law was applied at all RHs. The inverse law was linearized by using natural logarithm and also into the second order polynomial with the natural log. From the linear part α were determined and analyzed while in quadratic part α2 (known as curvature)were also analyzed. It was observed that α is less than 1.0 signified coarse mode particles and increases with the increase in the concentration of nuclei mode particles and RHs. The α2 which gives additional information of particle mode distributions is positive at the RHs between 0 to 95%, this shows bimodal type of size distributions and negative at 98 and 99% which shows monomode type of size distributions with the dominance of coarse mode particles. With the increased in nuclei mode and RHs, it was clearly shown that from the figures 1 to 5 values of AOD is high at shorter wavelengths and decreases towards longer wavelengths indicating the loading of fine and coarse mode particles into the atmosphere.

Keywords: Angstrom exponent, aerosols optical depth, curvature, wavelengths, relative humidity