Determination of Angstrom coefficients from spectral aerosol optical depth at two sites in Egypt

Angstrom Turbidity Coefficients (ATC) are estimated from Aerosol Optical Depth by using spectral broadband data. These data are carried out from Pyrheliometric measurements in the period 1991-96 for two sites of different climatological and environmental view; the first one is a highly polluted urbanized site (Cairo), while the other is an unpolluted and site (Aswan). SMARTS2 model proposed by Gueymard and SPCTRAL2 model proposed by Bird and Riordan, with two pairs of spectral broadbands, are used to select the suitable spectral broadband for estimating ATC. The turbidity levels increase during the two transition seasons, spring (due to Khamasin depressions coming from Great Sahara) and autumn (due to extend of Sudan monsoon trough), in addition to summer season. The subsidence inversion is stronger and leads to trap the pollution in the boundary layer in summer. The mean average values of ATC, over the all period, are alpha=0.477, 0.817 and beta=0.283, 0.144 for Cairo and Aswan respectively. Turbidity level in Cairo is higher than that in Aswan because the two big industrial areas Helwan and Shoubra El-Kheima surround Cairo, in addition to traffic. (C) 2002 Elsevier Science Ltd. All rights reserved.