Calibrations of filter radiometers for determination of atmospheric optical depth

Atmospheric optical depths are determined by relating ground-based measurem ents of direct solar radiation to the extraterrestrial value, I-0, that a f ilter radiometer would read outside the atmosphere. Usually I-0 is determin ed by the Langley extrapolation technique from a high-altitude site, where clear and highly stable atmospheric conditions may be found. Alternatively, I-0 can be measured in situ from a stratospheric balloon experiment. We ha ve employed both methods and found agreement to better than 1%. Filter radi ometers tend to change over time, especially when used operationally outdoo rs. Absolute calibrations in the laboratory are used to monitor the radiome tric stability of filter radiometers at the Physikalisch-Meteorologisches O bservatorium Davos (PMOD/WRC, Switzerland). A spectral calibration facility based on a calibrated trap detector from the Physikalisch-Technische Bunde sanstalt (PTB, Germany) is used to relate the filter radiometer to an accur ate and long-term traceable standard. An FEL-lamp-based standard, previousl y used for several years, was compared with the new trap standard via a fil ter radiometer at four wavelengths between 368 nm and 862 nm and revealed a systematic difference of the order of 5%. The link between radiometric and I-0 calibration is the value of the extraterrestrial solar spectrum at the filter radiometer wavelengths which can be determined from these two calib rations and compared with published values.