Physical Evaluation of Atmospheric Optical Properties and Solar Radiation Analysis over Tarhuna – Libya by using NASA POWER Data

This study presents a physical evaluation of atmospheric optical properties and solar radiation parameters over Tarhuna, Libya (398 m, above sea level) by using  resolutions daily for the year 2025. Given the lack of local ground based meteorological stations in Tarhuna, daily global horizontal irradiance   was sourced from the NASA POWER database. To assess the atmospheric conditions, the Erbs decomposition model was applied to partition the global radiation into its direct and diffuse components. This mathematical approach was chosen because it delivers reliable data for solar energy applications, overcoming the high margins of error that are typically associated with raw satellite observations. The direct horizontal componentwas converted into direct normal irradiation  using the solar zenith angle  while the atmospheric optical depth () was derived by the Beer-Lambert law as a function of solar noon air mass. The statistical and optical results showed that the atmosphere reached its highest levels of clarity and stability in    summer. July recorded  the lowest daily variance   due to clear skies,  resulting in positive findings in contrast, seasonal shifts caused disturbances in the optical properties. In April, desert  dust storms shift the atmospheric transmission into a  Mie scattering regime, pushing the atmospheric optical depth to its annual peak () and reducing  direct solar irradiance   to its minimum (2.5 kWh/m²/ day). However, global horizontal irradiance  remains high (6.7617 kWh/m²/ day) because large dust particles scatter light downward as diffuse horizontal irradiation , a phenomenon further amplified by multiple scattering with the Earth's surface. In November, autumn rains effectively washed aerosols from the atmosphere, reducing the optical depth to its annual minimum  and raising the clearness index to its peak  where molecular Rayleigh scattering of clear air domineer. dominates Daily resolution Pearson correlation matrix analysis revealed a near-perfect inverse relationship between () and () () and a strong negative correlation between  and ()  The correlation between  and () was found to be , the non-perfect linearity of this specific relationship is physically explained by dust storms converting direct radiation into diffuse radiation. In conclusion ,this study provides a basic framework for solar energy projects and optical properties of the atmosphere in the study area ,where the annual average global horizontal irradiance of (5.7239 kWh/m²/day) accompanied by  annual clearness index of ( = 0.6006).