1. Solar UV measurements at the Agricultural University of Norway Cecilia Futsaether and Arne Auen Grimnes, Dept. of Agricultural Engineering, Agricultural University of Norway, N-1432 Ås, Norway.

2. Measurement overview A meteorological station was established at the Agricultural University of Norway at Ås (35 km south of Oslo) in 1870. The purpose of the station is to study the interaction between the weather/climate and agricultural production. During the early years, measurements were restricted to parameters such as air and soil temperature, humidity and wind velocity. Since approximately 1950, these measurements have been extended to include solar radiation. These meteorological data are currently used to develop weather forecasts which target agricultural activities such as irrigation and fertilizer and pesticide application. The station is located on a 1 hectar flat, grassy field. The grass is kept at a fairly constant length (5 cm) throughout the summer months. The instruments have been position such that interference (i.e. instrument shading) is avoided. No other obstacles (houses or trees) are located within 50 m of the perimeter of the field. A small area of the field is reserved for plant growth studies.

3. The meteorological station

4. Meteorological measurements

5. Solar UV measurements Solar radiation measurements have been conducted since 1950. Broadband ultraviolet measurements were commenced in 1977 in conjunction with a joint research project between the five Nordic countries [1]. The aim of the project was to chart the spectral distribution of solar radiation in the Nordic area in order to provide agronomists studying the effects of radiation on plant growth with data. Solar radiation (385-2800 nm) has since been measured using Eppley spectral pyranometers equipped with different glass filters. An Eppley TUVR radiometer which measures integrated irradiances over the wavelength range 290-385 nm has been used to measure UV radiation. The UV radiometer has been operating fairly continuously since 1977. The radiometer is logged 50 times per minute and the mean values for 10 min, 1 h and 24 h time intervals are stored. The data has been used to study UV effects on plant growth and the mechanical properties of materials as well as to assess atmospheric models of radiative transfer.

6. Solar irradiance measurements Radiation energy balance Global irradiance Eppley radiometer (UV, 295-385 nm) irradiance RG8 IR (695-2800 nm) irradiance RG2 (630-2800 nm) irradianceGG14 (495-2800 nm) Global irradiance

7. The Eppley radiometer

8. Norwegian intercomparison 1995 I The first Norwegian UV- and PAR intercomparison meeting [2] was held at the University of Oslo in the period 5-9 June 1995. Outdoor synchronous solar UV measurement as well as measurements of a portable lamp unit from the Swedish Radiation Protection Authority (SSI) were conducted. The Eppley radiometer was examined for stability during the measurement period. In addition, the lamp and sky measurements were compared to a Macam spectroradiometer (Norwegian Radiation Protection Authority). The spectroradiometer had been calibrated prior to the intercomparison. The Eppley radiometer performed satisfactorily. Instrument drift did not occur. The Eppley radiometer deviated by approximately 10% from the Macam spectroradiometer for both lamp and sky measurements.

9. Norwegian intercomparison 1995 II Eppley TUVR measurements of SSI’s 100 W and 150 W lamps. Ratio between the broadband irradiance provided by the Eppley TUVR and the Macam spectrometer for daylight (7.06.1995) and lamp measurements. Error 10%.

10. Further work In order to assure the reliability of the data, proper calibration routines for the Eppley radiometer must be established. These routines should include an “in the field” method. This method should be easy to conduct such that the radiometer can be frequently examined for abnormalities. The radiometer should also be calibrated in the laboratory once a year. The calibration factor should be modified accordingly. The Eppley radiometer has been operating fairly continuously since 1977. Valuable data has been collected during this period. Additional instruments i.e. a multiband filter radiometer or another single band filer radiometer should be purchased to provide overlap between the data. Further analysis of the data will be conducted. The databases will be also be made available on WWW.

11. References 1G. Kvifte, K. Hegg, V. Hansen. 1983. Spectral distribution of solar radiation in the Nordic countries. Journal of climate and applied meteorlogy, 22:143-152. 2 The 1995 intercomparison of UV- and PAR instruments at the University of Oslo. Strålevern Rapport 1997:7, Statens Strålevern. Editors Bjørn Johnsen and Merete Hannevik. Authors Bjørn Johnsen, Cecilia Futsæther, Arne Dahlback, Arne Auen Grimnes, Merete Hannevik, Trond Morten Thorseth, Berit Kjeldstad, Tor de Lange, Arvid Skartveit, Lars Opedal, Oddbjørn Mikkelborg, Gry Storsveen. ISSN 0804-4910.