Calculation of surface spectral irradiance using the Geant4 Monte Carlo toolkit

The spectral direct normal irradiance (DNI) of the surface spectral irradiance, with a spectral resolution of 1 nm in the range of 280-1600 nm, and 5 nm in the range of 1600-3000 nm, has been calculated for the first time with the Geant4 Monte Carlo simulation toolkit using the US Standard Atmosphere 1976 model and the reference extraterrestrial solar spectrum SOLAR-ISS. The atmospheric attenuation of DNI has been studied with and without aerosols. In the presence of aerosols corresponding to 10 km visibility, the total solar DNI attenuation can reach about 84% in the UV domain, 56% in the visible domain and 35.08% in the rest of the infrared domain. The Geant4 results have been compared to those calculated by the MYSTIC code of LibRadtran package and the Beer-Lambert law under the same atmospheric conditions. The average relative transmittance difference between Geant4 and MYSTIC is of the order of few percent in the entire solar spectrum. In the case of atmosphere with aerosols corresponding to 23 km visibility, the most important average of the relative difference between Geant4 and MYSTIC transmittances is about 7.5% in the UV domain and stays between 1.19% and, 3.31% in the rest of the spectrum. More importantly, in this last case of the atmosphere, the values of the spectral transmittance of Geant4 remain intermediate between those of MYSTIC and Beer-Lambert law in the entire visible range up to 800 nm.