Study of Atmospheric Turbidity in a Northern Tropical Region Using Models and Measurements of Global Solar Radiation

Abstract

Radiative transfer in the Earth’s atmosphere under clear-sky conditions strongly depends on turbidity due to aerosols and hydrometeors. It is therefore important to know its temporal radiative properties for a given site when the objective is to optimize the solar energy that is collected there. Turbidity can be studied via measurements and models of the global solar radiation reaching the ground in cloudless conditions. These models generally depend on two parameters, namely the Angström turbidity coefficient and the Linke factor. This article aims to do a comparative study of five models of global solar radiation, all dependent on the Linke factor, based on real data. The measurements are provided by the Tamanrasset Meteorological Center (Algeria), which has a long series of global solar radiation data recorded between 2005 and 2011. Additional data from AERONET and MODIS onboard the TERRA satellite were also used to perform the comparison between the two estimated parameters and those obtained from AERONET. The study shows that the ESRA models are the most reliable among the five models for estimating the Linke factor with a correlation coefficient R of the data fits of 0.9995, a RMSE of 13.44 W/m2, a MBE of −0.64 W/m2 and a MAPE of 6.44%. The maximum and minimum statistical values were reached, respectively, in June and during the autumn months. The best correlation is also observed in the case of ESRA models between the Linke parameter and the joint optical thickness of aerosols and the total column-integrated water vapor. The Angström turbidity coefficient β, calculated from the Linke factor and MODIS data, has values less than 0.02 at 9% of the cases, and 76% present values ranging between 0.02 and 0.15 and 13% higher than 0.15. These β values are validated by AERONET measurements since a very good correlation (R≈0.87) is observed between the two datasets. The temporal variations of β also show a maximum in June. Satellite observations confirm more aerosols during the summer season, which are mostly related to the African monsoon.

The Variation in Atmospheric Turbidity over a Tropical Site in Nigeria and Its Relation to Climate Drivers

“…In the atmosphere, solar radiation undergoes significant attenuation due to two primary mechanisms: scattering and absorption by air molecules, hydrometeors, and aerosol particles. These processes result in a remarkable reduction in the direct solar component and a moderate increase in the diffuse component [4,[6][7][8][9][10][11][12][13][14][15]. The attenuation processes of solar radiation exhibit significant variability due to factors such as the apparent motion of the sun, changes in meteorological conditions, and fluctuations in aerosol properties with time across different regions [8].…”

“…The turbidity of a certain area is influenced by both the local emissivity, which includes contributions from natural (such as dust and clouds) and anthropogenic sources (such as emissions from cars, factories, etc. ), and the characteristics of air mass (e.g., continental vs. maritime), transporting the aerosol particles [14,16,17].…”

“…Generally, precise assessments of atmospheric turbidity require clear-sky spectral radiation data, which can be measured using spectroradiometers or multi-wavelength solar photometers. However, due to the limited availability and high cost of these sensors, turbidity is typically quantified, characterized, and estimated using broadband irradiance measurements, a more cost-effective method, and various turbidity factors and indices such as the Linke turbidity factor, the Ångström turbidity coefficient, the Schüepp turbidity factor, and the Unsworth-Monteith turbidity coefficient [14,15,20,35,38,39]. Among these, the most frequently used are the Linke turbidity factor (Linke, 1922) [40] and the Ångström turbidity coefficient [41].…”

“…Since the introduction of the Linke turbidity factor by [40] and the Ångström turbidity coefficient by [41], numerous researchers have proposed various models, including those by [22,45,[52][53][54][55][56][57][58]. These empirical equations have been used to quantify the impacts of air pollutants and aerosols on horizontal visibility degradation and the reduction in solar radiation received at the Earth's surface under different weather conditions and locations [4,6,8,11,[13][14][15]19,20,22,35,43,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74]. The extent of the application of all these indices at both regional and global scales remains unclear.…”

Study of Atmospheric Turbidity on Different Algerian Sites Using a Model and MSG Satellite Images

“…Various methodologies are available to study the turbidity at a ground site, either by using instruments dedicated to its measurements or from solar radiometric measurements and clear-sky models. The second methodology is adopted here as the previous study for the estimation of the Linke turbidity factor (T l ) in Tamanrasset (southern Algeria) [1]. It showed that the clear sky model ESRA2 was well suited to achieve this goal with solar radiometric measurements.…”