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A non-scanning UV spectroradiometer for long-term measurements in polar regions

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Citation:
Wuttke, S. , Schrems, O. , Hanken, T. and Wiencke, C. (2005): A non-scanning UV spectroradiometer for long-term measurements in polar regions , 6th Workshop on Ultraviolet Radiation Measurements, 20-21 October, Davos, Switzerland. .
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Abstract:

Spectral UVB irradiance (280-320 nm) is being monitored at the NDSC primary site at Ny-Alesund, Spitsbergen (78.9° N, 11.9° E) and at the German Antarctic Neumayer Station (70.65° S, 8.25° W, complementary NSDC site) by the Alfred Wegener Institute for Polar and Marine Research (AWI). A third polar site operated by AWI, where spectral UV irradiance is monitored, is the Dallmann Laboratory/Jubany Station at the Antarctic Pensinsula (62° 14 S, 58° 40W).The UVB-spectroradiometer is based on a Bentham DTM150 double monochromator and a Microchannel Photomultiplier Plate with 32 channels. Additional instruments were installed at Ny-Alesund in 2000 and at Neumayer station in 2001 to cover also the UVA range (320-400 nm) of the solar spectrum. This instrument contains a single monochromator as the dynamic range is low in the UVA compared to the UVB. The detector is a photodiode array with 256 detection channels. Single spectra can be taken every second. In routine operation the spectra are stored as 5-minute averages.Currently, the UV spectroradiometer is in the NDSC research mode. Since Ny-Alesund is a primary NDSC site and Neumayer station a complementary NDSC site, it is also desirable for the measurements of spectral UV irradiance to comply with the NDSC spectral UV specifications. Thus, the main focus is directed to the characterization of the instrument and its performance.Procedures for quality control have been developed and applied to check the data quality. At the Arctic site in Ny-Alesund, the UV spectroradiometer is returned every winter to the home laboratory in Bremerhaven, Germany, for recalibration. Until 2004, the recalibration was performed in the radiation laboratory with a 1000 W FEL lamp traceable to PTB. In 2005 the main standard has been replaced by a horizontal DXW lamp, also traceable to PTB. This change in lamp type was necessary due to the design of the instrument. The instruments at the two Antarctic sites, Neumayer and Dallmann, are also exchanged on a regular basis for recalibration. During operation at the polar sites, the sensitivity and wavelength stability of the instrument is checked regularly by the station staff.Sensitivity changes due to transport are monitored with a mobile calibration unit. This calibration unit includes a 150 W tungsten halogen lamp which is contained inside a lamp housing. The lamp housing was especially designed for our instruments. It is cylindrical with a diameter of about 15 cm and a height of 35 cm. The inner surface is coated with a non-reflecting black matt varnish. During operation this lamp housing is ventilated from outside to prevent overheating and thus an instability of the lamp. The portable power supply has an internal data storage, where the voltage across the lamp and the current through the lamp can be recorded. This way, the performance of the mobile calibration lamp can be monitored as well.Time series of daily doses of UVB, UVA, and erythemal irradiance will be shown for all three polar sites. Differences in the UV radiation climate will be pointed out. At two of these sites the measured UV irradiance are not only important with respect to establishing a UV climatology. They are also important for studies of UV effects on biological systems. At Ny-Alesund and Dallmann, effects of UV radiation on different species of algae are investigated by a biological research group of AWI. This group at our institute further underlines the importance and need to conduct measurements of spectral UV radiation in polar regions.

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