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Calibration Hierarchy




Sunphotometers can be calibrated by several methods: i) classical extrapolation through the atmosphere (Langley method), ii) comparison with an irradiance standard lamp and the solar spectrum, and iii) in situ measurements from stratospheric balloons or rockets (Schmid and Wehrli, 1995; Schmid et al., 1998).

The 12 WMO standard wavelengths for sunphotometers are: 368, 412, 450, 500, 610, 675, 719, 778, 817, 862, 946, and 1024 nm (primary wavelengths underlined).

  • Standard instruments are calibrated at high altitude stations (Jungfraujoch, Mauna Loa, Izana) by atmospheric extrapolation methods (Langley calibration method).

  • Stability of standard instruments is monitored by spectral comparison with a metrologically traceable absolute detector. The spectral irradiance scale is based on a cryogenic radiometer at PTB (Berlin) transferred via a calibrated trap detector. The radiometric response is determined every 6 months by a spectral comparator facility.

  • Field instruments are calibrated by comparison with WORCC standards at Davos or Langley calibration at Jungfraujoch.

  • Station instruments are linked to WORCC standards by travelling standards, exchange of sensors or re-calibration at Davos.

  • Performance is tested through intercomparison of field instruments with other, co-located AOD networks e.g. AERONET (McArthur et al., 2003).



         

Jungfraujoch station
(3580 m asl)

    

Mauno Loa station
(3397 m asl)

    
WORCC, Davos
(1596 m asl)



References

McArthur, L.J.B., D.H. Halliwell, O.J. Niebergall, N.T. O'Neill, J.R. Slusser, and C. Wehrli, Field comparison of network sunphotometers, J. Geophys. Res., 108, D19, 4596, 2003.

Schmid, B., and C. Wehrli, Comparison of sun photometer calibration by use of the Langley technique and the standard lamp, Applied Optics 34, 4500-4512, 1995.

Schmid, B., P.R. Spyak, S.F. Biggar, C. Wehrli, J. Sekler, T. Ingold, C. Mätzler, and N. Kämpfer, Evaluation of the applicability of solar and lamp radiometric calibrations of a precision sun photometer operating between 300 and 1025 nm, Applied Optics 37, 18, 3923-3941, 1998.


WORCC, PMOD/WRC, Dorfstrasse 33, Davos Dorf
CH-7260, Switzerland, Tel. +41 58 467 5137

 

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