ELDONET — European Light Dosimeter Network hardware and software

https://doi.org/10.1016/S1011-1344(99)00102-5Get rights and content

Abstract

A three-channel dosimeter has been developed to measure solar radiation in the UV-B (280–315 nm), UV-A (315–400 nm) and PAR (400–700 nm) wavelength bands. A total of 31 instruments have been installed in Europe from Abisko in Northern Sweden to Gran Canaria, covering most light-climate zones. In addition, instruments are installed in India, Africa, New Zealand and South America. Seven of the instruments have been installed under water (each in conjunction with a terrestrial instrument), and two instruments are located in high mountain locations (Zugspitze, Germany and Sierra Nevada, Spain). The instruments use an integrating Ulbricht sphere and silicon photodiodes in combination with custom-made filters. All instruments are carefully calibrated to ensure a high standard of quality control and documentation. The software records all data (three light channels, external and internal temperature and depth for the underwater instruments) at 1 min intervals, displays them on a PC, stores the data on disk and transmits the data to the central server in Pisa (http://power.ib.pi.cnr.it:80/eldonet), where all available data can be seen in graphical form and downloaded as ASCII files.

Introduction

After the discovery of stratospheric ozone depletion caused by anthropogenic pollution 1, 2, scientists became concerned about what effects the resulting increased UV-B radiation might have on the biota, including humans [3], wild and crop plants [4]as well as aquatic ecosystems [5]. Because of the enormous size of these ecosystems, any substantial loss in biomass production will have serious consequences for the intricate food web as well as the global climate.

Scientists studying the effects of solar radiation need reliable measurements of the irradiation [6]. To satisfy these needs, instruments were developed to determine the irradiance of solar radiation 7, 8. Later systematic monitoring of solar radiation was established 9, 10, 11. Motivated by the finding that specific wavelength bands affect organisms in different ways, instrumentation was developed to measure spectrally resolved data 12, 13. This was not too difficult for most of the solar spectrum, with the exception of the short-wavelength band (UV-B, 280–315 nm) [14]. This wavelength band is characterized by high biological efficiency but low fluence rates [15]. The solar emission spectrum shows a sharp drop below 300 nm over several orders of magnitude. Therefore low signals need to be measured against a background about 106 times higher. One important network is the Brewer network, which uses spectroradiometers 16, 17.

Alternatively, solar radiation can be measured by broad-band filter instruments that cover a wide band of wavelengths such as the PAR (photosynthetic active radiation, 400–700 nm) band. The third alternative is to measure at several wavelengths using narrow-band interference line filters.

A number of networks monitoring solar radiation have already been installed or are in the planning stage 18, 19. However, most of these measurements aim at the needs of physicists and do not satisfy the needs of the scientists who are interested in the biological effects of solar radiation [20]. In addition, to our knowledge there is no network that continuously measures the light penetration into the water column.

Some networks aim specifically at the UV-B band to monitor stratospheric ozone depletion. The Robertson–Berger (R–B) network has been measuring UV-B irradiance at eight stations in the USA since 1974 [21]. The wavelength range (290–330 nm) covers that associated with erythemal activity, which does not coincide with the CIE (Commission Internationale d'Eclairage) definition of UV-B (280–315 nm). There was a long discussion about why the R–B meter network showed a consistent decrease in solar UV radiation over the years, while satellite data indicated a gradual ozone depletion [22]. This apparent contradiction was eventually explained by the fact that most R–B meters were installed at meteorological stations at or near airports, so that the increasing atmospheric pollution due to higher air traffic offsets the increase in UV-B reaching the ground. As a consequence the R–B readings were in contradiction to the predicted UV-B and ozone trends, both in their numerical values and even in the sign of the trend [23]. In addition, in recent years a significant temperature sensitivity of the R–B meters as well as drifts both in wavelength accuracy and amplitudes due to sensor aging were found, which more than offset the actual increases in UV-B reaching the ground [24]. Several other UV-measuring networks have been installed worldwide using both terrestrial and satellite-based instruments 25, 26, 27.

In contrast to the networks described above, the aim of the European Light Dosimeter Network (ELDONET) was to develop an instrument that satisfies the specific interest and needs of the scientific community involved in the study of the effects of solar radiation.

Section snippets

Background

Given the high cost and demanding maintenance of dozens of spectroradiometers, the ELDONET network was designed to consist of broad-band filter instruments [28]spread over Europe. This network allows the quantitative determination of both long-term and episodic changes in solar irradiance, which is a prerequisite to finding a correlation with relevant responses of terrestrial and aquatic ecosystems as well as potential threats to human health. For this purpose accurate and reliable, but

Hardware

The entrance optics of the ELDONET instrument consist of a 10 cm integrating (Ulbricht) sphere with internal BaSO4 coating [31]. A baffle blocks the direct entrance of the radiation to the detectors. It is positioned in such a way that the spot directly irradiated by the sun never falls on the baffle or the gap between baffle and sphere. To ensure this condition, the instrument is oriented with respect to true North. The horizontal orientation of the instrument can be checked by means of a

Software

The software package WinDose for the ELDONET instruments has been developed in Visual Basic (Microsoft, Redmond, WA, USA) and runs under Windows 95/98. At the beginning of a measurement the serial connection is tested and the type of instrument determined automatically. The measurements start 1 h before local sunrise and stop 1 h after local sunset. For the calculation of start and end of data collection, the latitude and longitude of the location, the time offset to universal standard time and

Calibration

The use of an Ulbricht sphere greatly reduces the cosine error that is characteristic for many filter instruments. Each instrument is accurately calibrated against a 1000 W quartz halogen calibration lamp operated with a highly stabilized power supply (SL 1000 W, Powertronic Lab. 710 D). In addition, the temperature dependence of the dark value is calibrated for each instrument between 20 and 60°C. The temperature-correction factors for the dependence are stored in the individual .INI file. Due

Discussion

A high-precision but low-cost three-channel filter dosimeter has been developed. The resolution and detection limits of the instrument are <0.1 W m−2 for PAR, <0.01 W m−2 for UV-A and <0.0005 W m−2 for UV-B. All currently available tools for quality control and documentation are used to ensure a high standard of precision. Under field conditions the deviation from integrated values measured by means of calibrated spectroradiometers was found to be less than 10% for UV-B, 5% for UV-A and 2% for

Acknowledgements

This work was supported by the European Union (Environment programme, EV5V-CT94-0425; DG XII).

References (35)

  • B.L Duigan et al.

    Surface UVB irradiance measurements at Durban during 1993

    South African J. Sci.

    (1995)
  • G Seckmeyer et al.

    UV-B in Germany higher in 1993 than in 1992

    Geophys. Res. Lett.

    (1994)
  • A.E.S Green et al.

    Solar spectral irradiance in the visible and infrared regions

    Photochem. Photobiol.

    (1988)
  • A.R Webb

    Solar ultraviolet radiation in Southeast England: the case for spectral measurements

    Photochem. Photobiol.

    (1991)
  • A.R. Webb, E.C. Weatherhead, Current status of UV measurements, in: C.S. Zerefos, A.F. Bais (Eds.), Solar Ultraviolet...
  • A.R. Webb, UVB Instrumentation and Applications. Gordon and Beach Science Publishers, New York,...
  • B.I. Diffey, Measurement and Trends of Terrestrial UVB in Europe, OEMF spa., Milan,...
  • Cited by (0)

    View full text