A physical model of the atmospheric aerosol turbidity for estimating the illuminance of direct sunlight

Abstract

The aerosol turbidity index is the key variable in simple parameterisation models of direct normal solar radiation on a plane perpendicular to the direction of the sun. Techniques that use fixed design standard values of the turbidity index do not adequately account for variations in aerosol attenuation. This study presents a simple physical model to estimate the Unsworth—Monteith illuminance turbidity index T_Ui, using aerosol mass concentration (PM₁₀) data that are routinely available from air pollution monitoring stations. T_Ui is parameterised in terms of the aerosol scale height and the ground level aerosol extinction coefficient. A measured set of hourly average `clear sun' direct normal illuminances, and PM₁₀ data for the urban location of Sheffield UK was used to validate the proposed model.