Abstract

Abstract In the Middle East various problems such as air pollution, reducing the visibility and human health hazards are caused by aerosols, specifically in the last two decades. In this paper, to better understand the effects of aerosols in the Middle East, aerosol optical properties and radiative forcing over 7 sites in the Middle East have been studied during 2013. Using the ground-based AErosol RObotic NETwork (AERONET) data, aerosol optical properties like Aerosol Optical Depth (AOD), Angstrom Exponent (AE), ASYmmetry parameter (ASY), Single Scattering Albedo (SSA), and phase function have been analyzed. An inverse relation between AOD and AE were observed for all of sites in most months. A high value for AOD and a low value for AE were observed generally in the spring and summer in all sites except one site which showed the presence of coarse mode particles and dust storms in these seasons. In contrast, a high value of AE represented the dominance of fine mode particles like anthropogenic aerosol. Except 2 sites, which high values of AE were observed in the summer and fall, in other sites the peak value of AE occurred in the fall and winter. As wavelength increased initially due to the dominance of anthropogenic absorbing aerosol, ASY decreased in the visible spectrum. Then with increasing wavelength in the infrared region, the ASY rose due to the dominance of the coarse mode particle. Except 2 sites, the maximum values of ASY were observed in the spring and summer. In most sites SSA increased with increasing the wavelength especially in the spring and summer because of the dominance of desert dust. In other sites with increasing wavelength, SSA decreased, which signifies the coexistence of scattering water-soluble and absorbing aerosols. In one site SSA showed a negligible dependence on wavelength. In the spring and summer in most sites the phase function was high. High phase functions in small scattering angle were due to coarse mode particles. In angles between 0-10 degrees, phase function reduced uniformly. At angles bigger than 10 degrees, because of occurrence of fine mode particles, the values of phase function were low. Phase function was almost stable at larger angles. A ten-day period with high AOD has been selected to calculate ARF using The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model for 4 sites. For surface ARF, a negative value obtained which means cooling of the earth's surface because of the loss of radiation due to aerosols. While the positive values of atmosphere ARF showed heating of the atmosphere due to absorbing particles. A value of 0.99 for correlation coefficient between the calculated ARF using SBDART and the AERONET inversion products of 4 sites represents the accuracy of the SBDART model in this study. Keywords: SBDART; AERONET; Aerosol Optical Depth; Aerosol Radiatve Forcing; Middle East.