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) P. Trisolino1*, A. di Sarra, D. Meloni, G. Pace, F. Anello, F. Monteleone and D. Sferlazzo ENEA, Laboratory for Observations and Analyses of Earth and Climate, Italy, 1Department of Ecological and Biological Sciences, Tuscia University, Italy *Corresponding Author: pamela.trisolino@enea.it M E A S U R E M E N T S I T E The Station for Climate Observations on the island of Lampedusa (35.5°N, 12.6°E; http:\\www.lampedusa.enea.it), is a long-term measurement site. Atmospheric parameters (radiation, aerosol, greenhouse gases, ozone, water vapour, clouds, etc) are continuously monitored. It is operational since 1997 and contributes to several international measurement networks. Lampedusa is in a strategic position in the central Mediterranean and is also important to evaluate the effect of Saharan desert dust. I N S T R U M E N T A T I O N M E A S U R E M E N T S A N D M E T O D O L O G Y This study uses measurements made with Multifilter Rotating Shadowband Radiometers (MFRSR) and with Photosynthetically Active Radiation (PAR) sensors. The MFRSR is a passive instrument that measures global and diffuse components of the the solar irradiance in six narrowband and one broadband channel. The PAR sensor is a passive instrument that measures the Photosynthetically Active Radiation, which is the solar irradiance between 400 and 700nm. It is the fraction of solar radiation available for photosynthesis. MFRSR measurements at Lampedusa are continuously calibrated on-site by using the Langley plot method. The MFRSR signals at the four bands falling within the PAR spectral interval were combined to infer the corresponding PAR: 𝑷𝑨𝑹 𝑴 𝒊 = 𝒄 𝟏 𝑴𝟏 𝒊 + 𝒄 𝟐 𝑴𝟐 𝒊 + 𝒄 𝟑 𝑴𝟑 𝒊 + 𝒄 𝟒 𝑴𝟒 𝒊 The coefficients of the linear combination of the bands were determined by minimizing the least squares differences in periods in which a fresh PAR calibration was available: 𝑫= 𝒊 𝑷𝑨𝑹 𝒊 − 𝑷𝑨𝑹 𝑴 𝒊 𝟐 By applying this method we obtain, from the long-term MFRSR measurements, a continuous PAR dataset with an accuracy of about 5%. The frequency of occurrence of relative variations between PAR measurements and PAR values calculated from MFRSR observations are shown in two histograms: red bars are for measures obtained in all atmospheric conditions, blue bars for clear-sky conditions. SD=0.047 SD=0.013 All conditions Clear-sky conditions R E S U L T S Curves of clear sky solar radiation at the mean Sun-Earth distance derived during partly cloudy days. The fit is obtained from the adaptation of the method proposed by Long and Ackerman for the shortwave radiation. The fitting function has the following form: F(θ)=A(cosθ)B PAR monthly average 2002-2011 The coefficient A represent the clear-sky irradiance for a zenith solar angle of 0° including the effects of the water vapour and aerosol atmospheric content at the mean Earth-Sun distance; coefficient B includes some effects as the cosine response of the instrument. Interannual variability of glabal PAR irradiance monthly averages in all atmospheric conditions (solid line) and in clear sky conditions (dashed line). Diffuse PAR and aerosol optical depth monthly averages 2002-2011 The main drivers of the interannual variability of PAR in the Mediterranean are clouds and aerosols. The aerosol effect comes out evidently when cloudy data are removed. Giugno r²=0.13 r²=0.27 r²=0.29 r²=0.76 r²=0.004 r²=0.04 r²=0.08 r²=0.93 Diffuse PAR irradiance monthly averages obtained from MFRSR data in all atmospheric conditions (green solid line) and in clear sky conditions (green dashed line), and aerosol optical depth (AOD) at 500 nm monthly averages (orange solid line). December The results are in accordance with modelling obtained with MODTRAN* Black line: Global radiation Blue line: Direct radiation Green line: Diffuse radiation Model in clear-sky conditions Model in cloudy conditions Model in presence of aerosol *MODTRAN (MODerate resolution atmospheric TRANsmission)