Abstract
Solar irradiance is attenuated spectrally when passing through the earth׳s atmosphere and it is strongly dependent on sky under cloudless conditions. The prevailing winds, which may transport moisture or aerosol particles from distant sources, play a major role in the seasonal variation of turbidity. A direct measurement of the aerosol transmittance is not possible due to the strong influence of the other atmospheric components. During the last century, a number of atmospheric turbidity indices were introduced and several methods were developed to determine their values. This paper provides a review of the effect of aerosols on solar radiation budget by considering two common turbidity parameters including the Linke turbidity factor (TL) and Angstrom turbidity coefficients (β) and (α). Furthermore, an investigation of atmospheric turbidity has been undertaken in Egypt (Cairo, Aswan and Marsa-Matruh), from 1990 to 2013: turbidity indexes, namely, Linke factor (TL) and Ångström coefficient (β). The Angstrom turbidity coefficients (β) have been determined at fixed air masses during the first and second halves of the day at different spectroscopic solar energy bands, the maximum values of turbidity in the summer due to dust storms and vertical convection. And low values of turbidity index in the winter, due to precipitation removal as well as relative humidity-impacted deposition, but the middle values of turbidity were observed in the spring and autumn seasons. The maximum values of the Angstrom turbidity coefficient occur around the noon time. Annual variations show lowest values in winter months and highest values in both spring months, due to the khamsin continental wind lead to more polluting effects on the prevailing air masses and summer months due to hot air mass and large water vapor contents. The Angstrom turbidity coefficient decreases with increase in both wavelengths and optical air masses.