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

Abstract

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 averaging of a large amount 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. 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 reveal 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.

Effect Of The Aerosol Vertical Distribution On The Modelling Of Solar Radiation

“…Thus, using measured profiles for climatological studies of the radiative effects of aerosols is not critical. This is in agreement with the findings of Petrzala et al for urban aerosols [23]. According to Mishra et al [20] differences can be more significant for the thermal infrared (4 -20 μm) spectral region (longwave radiation).…”

“…In climate modelling studies, as well as in many case studies relative to the radiative effects of aerosols, simplified extinction coefficient profiles are commonly used [21,22]. Very few information is available on how this simplification affects the simulated radiation (e.g., [20,23]). In the present study, we exploited the recently developed LIVAS climatology [54] and investigated how changes in the vertical distribution of aerosols affect the distribution of shortwave radiation in the atmosphere under cloudless skies.…”

“…In most climatic studies, default aerosol profiles are used which commonly assume exponential decrease of aerosol extinction (with aerosols concentrated in the first 1-2 km from the surface). In many cases this assumption does not affect the modelled irradiance at the BOA and the TOA significantly (e.g., [23]). However, as shown in the following section for high aerosol loads and under certain conditions (e.g., high SZA or high surface albedo, aerosol layers at very high altitudes) this assumption can result to non-negligible errors in the simulations of the GI at the BOA or the UDI at the TOA when large fraction of the aerosols is located at higher altitudes (e.g., at 3 or 4 km).…”

“…Due to inadequate information for the vertical distribution of aerosols it is common to use simplified extinction coefficient profiles (assuming that aerosols are mostly concentrated near the surface and/or that they decrease exponentially with increasing altitude) [21,22] for the modelling of their radiative effects. Although the assumption of exponential decrease is relatively accurate for the estimation of the SSR over certain environments (e.g., for urban aerosols) [23], few information is available for the effect of using a more realistic instead of a theoretical profile to perform radiative transfer (RT) simulations when the real profiles of aerosols decline significantly from the ideal theoretical profile. For example, volcanic, dust, or biomass burning aerosols are frequently transported to long distances away from their sources, travelling at high altitudes in the atmosphere [24][25][26][27][28][29], resulting to aerosol profiles which differ significantly from the theoretical.…”

Effect of Aerosol Vertical Distribution on the Modeling of Solar Radiation

“…Due to inadequate information on the vertical distribution of aerosols, it is common to use simplified extinction coefficient profiles (assuming that aerosols are mostly concentrated near the surface or decrease exponentially with increasing altitude) [21,22] for the modeling of their radiative effects. Although the assumption of exponential decrease is relatively accurate for the estimation of SSR over certain environments (e.g., for urban aerosols) [23], limited information is available for the effect of using a more realistic instead of theoretical profile to perform radiative transfer (RT) simulations when the real profiles of aerosols deviate significantly from the ideal theoretical profile. For example, volcanic, dust, or biomass burning aerosols are frequently transported over long distances away from their sources, traveling at high altitudes in the atmosphere [24][25][26][27][28][29], resulting in aerosol profiles that differ significantly from the theoretical.…”