Estimate of the atmospheric turbidity from three broad-band solar radiation algorithms. A comparative study

摘要

Atmospheric turbidity is an important parameter for assessing the airpollution in local areas, as well as being the main parameter controlling theattenuation of solar radiation reaching the Earth's surface under cloudlesssky conditions. Among the different turbidity indices, the Ångströmturbidity coefficient β is frequently used. In this work, we analyse theperformance of three methods based on broad-band solar irradiancemeasurements in the estimation of β. The evaluation of the performance of the models was undertaken by graphical and statistical (root mean square errorsand mean bias errors) means. The data sets used in this study comprisemeasurements of broad-band solar irradiance obtained at eight radiometricstations and aerosol optical thickness measurements obtained at oneco-located radiometric station. Since all three methods require estimates ofprecipitable water content, three common methods for calculating atmosphericprecipitable water content from surface air temperature and relativehumidity are evaluated. Results show that these methods exhibit significantdifferences for low values of precipitable water. The effect of thesedifferences in precipitable water estimates on turbidity algorithms isdiscussed. Differences in hourly turbidity estimates are later examined. Theeffects of random errors in pyranometer measurements and cloud interferenceson the performance of the models are also presented. Examination of theannual cycle of monthly mean values of β for each location has shown that all three turbidity algorithms are suitable for analysing long-term trends andseasonal patterns.