Determination of global and diffuse photosynthetically active radiation from a multifilter shadowband radiometer.

A method to estimate the photosynthetically active radiation from multifilter shadowband radiometer (MFRSR) measurements was developed and tested from observations carried out on the island of Lampedusa in the central Mediterranean. Calibrated irradiances in the four MFRSR bands within the PAR spectral range were combined linearly to estimate PAR. The coefficients of the linear combination were derived with the least squared method for different sky conditions. The analysis shows that global PAR irradiance may be estimated with an overall uncertainty of 4%-6%. The applicability of the method was tested by using radiative transfer simulations of the diffuse irradiance spectrum for different aerosol and cloud conditions. The diffuse PAR irradiance can be estimated with an overall accuracy of less than 9%. The application of this method allows us to obtain continuous and long-term calibrated measurements of global and diffuse PAR; in addition, information on the spectral dependency of PAR can be derived from the signals in the four bands.