Characterization of pyrgeometers and the accuracy of atmospheric long-wave radiation measurements

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Abstract
A detailed characterization is performed to calibrate pyrgeometers, using a newly developed apparatus that contains a blackbody radiation source and the means to vary the temperatures of the pyrgeometer under testing. Calibration measurements cover the parameter space of radiation and instrument temperatures that prevail during field measurements. Dome-temperature measurements, normally provided on pyrgeometers, are inadequate for accurate corrections of the dome emission. A new temperature measurement with three sensors inside the dome at 45° elevation is proposed and has been implemented on several test instruments. This modification and the detailed characterization measurements permit an improved evaluation, based on thorough analysis of the thermal balance of the instrument, leading to a sensitivity factor C and three correction factors, k 1,2,3. Test measurements demonstrate the substantial improvement achieved on the accuracy of atmospheric and terrestrial long-wave radiation measurements, down to ±2 W m−2.