Assessment of Influence of Urban Aerosol Vertical Profile on Clear-Sky Diffuse Radiance Pattern

Aerosol particles spread in the atmosphere play an important role in solar light scattering and thus co-determine the sky radiance/luminance pattern as well as diffuse irradiances/illuminances at the ground. The particular influence is given by their optical properties and by their distribution in the atmosphere. The dependence of the aerosol extinction coefficient on altitude is usually described by the exponential law, which results from the averaging of a large number of aerosol realizations. This is also frequently the case of simulating of the solar diffuse radiance/luminance distribution over the sky, when it is based on solving the radiative transfer problem. However, the aerosol vertical profile can sometimes be significantly different from the exponential one. Mainly in the urban environment, the aerosol is often well-mixed within the atmospheric boundary layer, so its volume extinction coefficient is almost constant there. Under specific conditions also the so-called elevated aerosol layer may occur. This work investigates how such different profiles affect the clear sky radiance pattern and consequently also the ground-based horizontal diffuse irradiance. The numerical simulations revealed that the discrepancies are negligible in practice. So, it appears that the aerosol vertical distribution does not play any important role in sky radiance calculations, and the standard exponential law is general enough to cover also various specific aerosol conditions.