Calibrations of filter radiometers for determination of atmospheric optical depth

Atmospheric optical depths are determined by relating ground-based measurements of direct solar radiation to the extraterrestrial value, I-0, that a filter radiometer would read outside the atmosphere. Usually I-0 is determined 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 have employed both methods and found agreement to better than 1%. Filter radiometers tend to change over time, especially when used operationally outdoors. Absolute calibrations in the laboratory are used to monitor the radiometric stability of filter radiometers at the Physikalisch-Meteorologisches Observatorium Davos (PMOD/WRC, Switzerland). A spectral calibration facility based on a calibrated trap detector from the Physikalisch-Technische Bundesanstalt (PTB, Germany) is used to relate the filter radiometer to an accurate and long-term traceable standard. An FEL-lamp-based standard, previously used for several years, was compared with the new trap standard via a filter 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 calibrations and compared with published values.