Articles | Volume 5, issue 2
Geosci. Instrum. Method. Data Syst., 5, 531–540, 2016

Special issue: Multi-disciplinary research and integrated monitoring at the...

Geosci. Instrum. Method. Data Syst., 5, 531–540, 2016

Research article 22 Nov 2016

Research article | 22 Nov 2016

In search of traceability: two decades of calibrated Brewer UV measurements in Sodankylä and Jokioinen

Anu Heikkilä1, Jakke Sakari Mäkelä1,a, Kaisa Lakkala2, Outi Meinander1, Jussi Kaurola1, Tapani Koskela3, Juha Matti Karhu2, Tomi Karppinen2, Esko Kyrö2, and Gerrit de Leeuw1,4 Anu Heikkilä et al.
  • 1Finnish Meteorological Institute, Climate Research, P.O. Box 503, 00101 Helsinki, Finland
  • 2Finnish Meteorological Institute, Arctic Research Center, Tähteläntie 62, 99600 Sodankylä, Finland
  • 3independent researcher: Helsinki, Finland
  • 4University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Helsinki, Finland
  • acurrently at: University of Jyväskylä, Jyväskylä, Finland

Abstract. The two Brewer spectrophotometers of the Finnish Meteorological Institute at Jokioinen and Sodankylä have been operated according to the highest levels of the WMO∕GAW (World Meteorological Organization∕Global Atmosphere Watch) recommendations with rigorous quality control and quality assurance. The calibration of the instruments is based on annual recalibrations of primary standard lamps in the VTT MIKES Metrology National Standards Laboratory in Finland and an exhaustive measurement program with measurements of standard and working lamps in the on-site optical laboratories. Over the years, the maintenance of the calibration has produced data sets of approximately 2000 lamp scans for both instruments. An extensive re-examination of the lamp measurements and the response of the spectrophotometers was carried out. The primary standard lamps were found to age on an average rate of 0.3 % per burn. The responsivity at wavelength 311 nm was found to exhibit both long-term and short-term changes. The overall long-term change was declining. In addition, abrupt changes of as large as 25 % were detected. The short-term changes were found to fluctuate on time frames shorter than the interval between the measurements of the primary standard lamps. This underlines the importance of the use of more frequently measured working standard lamps.

Short summary
Lamp measurements used for the UV irradiance calibration of two Brewer spectrophotometers operated for 20 years in Jokioinen and Sodankylä, Finland, were examined. Temporal development of the responsivity after fixing the irradiance measurements into a specific scale was studied. Both long-term gradual decrease and abrupt changes in responsiveness were detected. Frequent-enough measurements of working standard lamps were found necessary to detect the short-term variations in responsiveness.