Atmospheric turbidity over Egypt

Atmospheric turbidity conditions were monitored during the period 1989-1995 in rural and urban sites in Egypt. The rural site (Aswan) is in the south region of Egypt (desert climate), while the urban site is greater Cairo (Cairo and its surroundings). The measurements were used to recover the aerosol optical depth, Linke turbidity factor, and Angstrom turbidity indices, from the total direct solar radiation measurements and knowledge of the atmospheric ozone and water vapor content. Solar direct radiation measurements were carried out with an Eppley pyrheliometer, following World Meteorological Organization recommendations, improving on previous work that made use of Voltz sun photometers. An examination of the methodologies for calculating aerosol optical depth and comparisons to observations suggests that the Davies-Hay method is superior to the method of Unsworth-Monteith. A comparison of the seasonal cycle of aerosol optical characteristics at both sites showed: aerosol of photochemical origin in the summer; a significant impact of temperature, relative humidity and dust storms in the autumn; low values of aerosol optical characteristics in the winter (due to precipitative removal as well as relative humidity-impacted deposition); and high values in the spring resulting from seasonal dust storms. An examination of the inter-annual variability of the global direct solar radiation and the anomalous aerosol optical depth at both rural and urban sites showed the signature of both the El-Chichon and Mt. Pinatubo eruptions. The Pinatubo eruption was studied in detail: anomalously high aerosol optical depth (AOD) values and low global direct radiation was observed in mid-1991 over Cairo (AOD average: 0.04) and early 1992 over Aswan (AOD average: 0.039). The annual behavior of the Angstrom turbidity coefficient and Linke turbidity factor recorded maximum values (0.27 and 7.0, respectively) over Cairo in late 1991/early 1992, with maximum values over Aswan being (0.18 and 5.57, respectively) in late 1991. (C) 2004 Elsevier Ltd. All rights reserved.