Analysis of the atmospheric turbidity levels at two Tunisian sites

Abstract

Atmospheric turbidity coefficients have been estimated in many locations, either on the basis of spectral irradiance or on broadband direct irradiance data. The Ångström turbidity coefficient, β, linked to the absorption and diffusion of aerosols, is very important since many solar radiation models for clear days use this parameter. The Linke turbidity factor, T L, is of specific importance for the calculation of beam and diffuse solar radiation in a determined locality. Furthermore, an atmospheric turbidity index in a determined locality can be associated with the pollution level at that place. Direct solar radiation data, measured with a pyrheliometer in a coastal tourist location in Tunisia (Sidi Bou Saïd), are used to investigate the diurnal and monthly variations of the atmospheric turbidity using Linke's factor T L during 1999. Computed T L values vary between 2.5 and 8 with a frequency of about 53% greater than 5.0. Highest and lowest turbidity levels have been observed in August and December, respectively. Values of the turbidity coefficient β have been determined in Thala (a continental and nonindustrialized region in Tunisia), for 8 months in 2001, based on a direct fitting method of the Angström power low expression using sun-photometer data. Monthly averaged values of β coefficient vary between 0.15 and 0.33 with highest turbidity recorded in July and October and lowest in April. Measured meteorological parameters represent a valuable supplement to the data sets with the sun-photometer and the pyrheliometer measurements. The analysis of the results shows that there is a high correlation between atmospheric turbidity and local weather conditions, especially wind speed and wind direction.