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

Abstract

Ångström Turbidity Coefficients (ÅTC) 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 arid 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 ÅTC. 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 ÅTC, over the all period, are α=0.477, 0.817 and β=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.