Atmospheric aerosols are one of the main components driving solar radiation variability in clear sky conditions. In desert and arid regions, such as in Doha, Qatar, sand and dust storms happen frequently, holding a large amount of aerosols in the atmosphere and causing poor visibility and a significant reduction of incoming solar radiation; this occurrence is generally more intense towards the spring and summer seasons. In this contribution, we study and quantify the effect of atmospheric dust on solar radiation components: direct normal irradiance (DNI), global horizontal irradiance (GHI) and diffuse horizontal irradiance (DHI), during strong dust events. This analysis can help to understand and estimate the reduction in solar energy conversion systems under heavy-dust events in the region.
In order to quantify the impact of dust on solar radiation, the method consists of selecting days marked by strong dust events, and determining the difference between the measured irradiance during these days and an estimated reference irradiance calculated for the same day under sky conditions specific to Doha, excluding the contribution of extra aerosols coming from the dust storm. Measured irradiances are acquired by a high precision solar radiation monitoring station equipped with a sun tracker, one pyrheliometer for measuring DNI and two pyranometers (one of them shaded) for GHI and DHI measurements . The modeled irradiances are derived with SMARTS2 using predefined input parameters and aerosol types that correspond to the atmospheric conditions seen in Doha. These parameters will be defined based on a good agreement between measured and modeled irradiance profiles analysed during clear days. When available, the corresponding Aerosol Optical Depth (AOD) values are derived using multifilter rotating shadow band radiometer measurements. Ceilometer backscatter measurements are also used to help in the selection of clear and dusty days based on the visualization of the vertical structure of the atmospheric boundary layer as well as additional surface and elevated aerosol layers .
Figure 1 shows daily profiles of DNI calculated at ground level using SMARTS2 for different values of AOD. It can be seen that the maximum value of DNI can be reduced by about 61 % due to an increase in AOD from 0 to 1.6. Figure 2 shows a sample of daily profiles of DNI, GHI and DHI for three consecutive days in Doha, with a dust event striking on the third day. The reduction in DNI and increase in DHI during the dusty day are clearly seen.
Fig.1: Profiles of modeled Direct Normal Irradiance with different values of AOD.
Fig.2: Daily profiles of solar radiation in Doha, Qatar for three consecutive days.