Determination of Photosynthetically Active Radiation from multi-filter rotating shadowband measurements: Method and validation based on observations at Lampedusa (35.5°N, 12.6°E)

Solar radiation plays an important role in several terrestrial, atmospheric and biogeochemical processes, and in climate. This study focusses on the verification of a method for the determination of PAR, the Photosynthetically Active Radiation. The method is based on the use of measurements made with Multifilter Rotating Shadowband Radiometers (MFRSR). The MFRSR measures global and diffuse components of the solar irradiance in six narrowband and one broadband channel; four of the MFRSR bands fall within the spectral range of PAR. This study is based on measurements made at the Station for Climate Observations, on the island of Lampedusa, in the central Mediterranean. MFRSR measurements at Lampedusa are continuously calibrated on-site by using the Langley plot method. The MFRSR signals in the four bands within the PAR spectral interval are linearly combined to infer the corresponding PAR. The method is verified by comparing PAR determinations in days with very different aerosol and cloud conditions. A very good agreement between measured and estimated PAR is found in all conditions, confirming that the method has the capability to accurately retrieve PAR irradiances. The variability of the PAR diffuse component appears also well described.