Continuous solar UV monitoring in Germany

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Journal of Photochemi:,try and Photobiology B: Biology 41 ' 1997) 181-187

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Continuous solar UV monitoring in Germany M. Steinmetz Federal Office for Radiation Protection. BJS. Salzgitter. Germany

Abstract Early in 1993, the Federal Office for Radiation Protection ( BfS, Salzginer ) together with the Federal Environmental Office ( UBA, Berlin) established an overall UV monitoring network for the continuous measurement of spectrally resolved UV radiation. Every 6 min the solar UV spectrum is measured by a Bentham DM 150 double monochromator system in a wavelength ranging from 290 to 450 rim. Every night, UV data are automatically transferred via modem to the reference station in Munich where they are quality coraro|led and then stored in a host compoter. Human health assessment of the exposure is documented in I/2 h MED ( minimal erythemad dose) v ~ . The selected sites of Zingst ( I n. 54°N. Baltic Sea), Offenbach ( I I0 m, 50°N, Rhine rift valley), Schauinsland ( 1205 m. 48°N, Black Forest) and Neuherberg (493 ,n. 4g°N, Munichj provide a good overview of the UV radiation situation in Germany and therefore an ideal supplement to more detailed biological effect research, especially of comparison measurements with biosensors under environmental conditions. Preliminary investigations have already been started. © 1997 Elsevier Science S.A.

1. Introduction

The sun is the main source of UV radiation. Terrestrial levels of UV radiation determine the impact on human health, marine organisms and plant life [ 1-41. Ozone depletion and associated increases in solar UV radiation reaching the earth's surface are therefore major environmental issues. Monitoring and modelling of the variability of these parameters are important for assessing the impact of these changes. Although short-term UV measurement campaigns to study the possible consequences of ozone depletion have already been conducted for many years at various sites, all of the data published so far on the net increase of UV radiation are inconclusive. For example, Scotto et al. [5] reported a UV decrease of 0.5% to !.1% per year from 1974 to !985 at mostly urban locations in the USA. Blumthaler and Ambach [61 reported a UV increase of 1.1 +0.4% per year between 1981 and 1989 measured above the polluted e~:mospheric boundary layer in the Swiss Alps. This might be due to compensatory effects in the atmosphere and to inadequate measuring techniques. The latter are primarily the reason for relatively limited UV measurement activity in Germany as well as in other countries. From measurement data of this nature, it is of course very difficult to derive a quantitative relationship between depleting ozone, increased UV irradiation and subsequent biological effects [7]. A prognosis of the expected UV radiation burd~.n, c~m .hardly be made on the basis of these sources. I011-1344/97/$17.00 © 1997ElsevierScienceS.A. All rights reserved 1-1344(96)07455-6

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Another problem is that measured data are not generally available.

2. Monitoring network Since early in 1993, the Federal Environmental Office (UBA, Berlin) together with the Federal Office for Radiation Protection (BfS, Salzgitter) has been operating an overall UV monitoring network for the continuous measurement of spectrally resolved UV radiation. The major tasks are: • to report on the level of current and future solar UV exposure; • to assess the effects of changed UV irradiation from a human health viewpoint; • to provide the population with guidelines for appropriate early protection. In addition, UV monitoring is designed to contribute to the research of biological effects [8]. Within the ecological framework, it will be possible to study the biological reactions to UV irradiation more closely. For this purpose, the. actual geographically and time-related distribution of UV radiation would flare to be correlated with radiation effects in spatially distributed populations. The actual network consists of a reference station in Munich-Neuherberg, quality control and quality assllrance of all data, and of three further measuring stations, located in Offenbach near Frankfurt, in Schauinsland it, the Black For-

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est, and in Zingst at the Baltic sea (Fig. I ). When deciding on these sites, the personnel and equipment-related internal structure of the participating institutions was taken into consideration as well as the different latitudes, climatic conditions and tropospheric environmental burdens. Zingst has a typical sea climate with only little air pollution, Offenbach within the Rhi.ne rift valley has a mild continental climate ,vith much sun and typical city air pollution. Schauinsland, as a high mountain station near the planetary boundary layer, has more extreme climatic conditions and air with less anthro-

pogenic pollution. Munich-Neuherberg with a continental climate influenced by the alps, represents an area with the type of air pollution found in big city peripheries. Every night, the accumulated measurement data are automatically transferred via modem to Munich where they are quality controlled, erythemally effectiveness assessed, published and subsequently stored in a host computer. The expected increase in UV radiation represents a global problem and requires the integration of the UV monitoring network into other national and international UV activities

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M. Steimnetz I Journal of Photochemistry a,d Photobiology t~: Biology 41 t 1997 ~ !81-187

and their subsequent support. A first step is to gain full cooperation from different national as well as f,:deral state/ research institutions.

trial spectra with sufficient accuracy, resolution and reproducibility. By system optimization it should be passib~ to limit deviations in the shortwave UV-B range to a ju~ifiab~ degree of less than 20~. The modular measuring system is ~ designed th~ ~asirive components (monochromator and electronic devices) can be operated in a closed room at stable lab~'a~'y c~'~itions. The system is optically connected by a 4 m tong Iigh~ fiber bundle with a waterproof teflon diffuser mounted on roof (Fig. 3). The diffuser is horizontally oriented a ~ l equipped with a ~lf-regulating small electric hea~er. Ai ~ e site where the diffuser is mounted, the surrounding skyline is below 5 °. Additionally, the whole global solar radia~km is continuously mea.,;ured by a pyranometer. A built-in temperature sensor provides in formalion on passible wavelength shifts. The selected bi-alkali photomuitiplier has an UV optimized wavelength lange; it is of strictly linear for 8 decades and has a very low dark current of 10-2 nA.

3. Measuring device High spectral resolution measurements are indispensible for a biologically effective evaluation of UV radiation [9]. The measured spectral irradiation together with the respective weighting functions form the product that yields the biological effectivity of solar UV radiation ( see Fig. 2). This makes high demands on the measurement technique, since the spectral distribution of solar UV radiation covers several orders of magnitude [ 10]. After a thorough evaluation in spectral meetings and equipment comparisons, a BENTHAM DM 150 double monochromator measuring system was selected [ i 1-13J. If well maintained, this system is capable of recording solar terres-

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tioning management. Furthermore, the reference station will participate in national and international field comparisons.

4. Measurement specifications

Fig. 3. Enmmceopticson the roof of the BfS buildingin Neuherberg. The electronics ccnsist essentially of a programmable current amplifier, a multiplexer for the exchange of entrance parameter signals, an integrated analog/digital converter with 14 bit resolution, a high voltage photomultiplier and a step-motor steering system (Fig. 4). During measurements, the electronic offset and the dark current of the photomuitiplier are automatically diverted from the measuring signal. For minimal scan time the integration time of the A / D converter is adjusted to the sensitivity range of the current ampli~er. Additionally, a semiprofessional weather station provides climatic data every ! / 2 hour, especially on temperature, humidity~ pressure and rain. A conventional personal computer (PC) controls and drives the whole system and serves as an intermediate storage medium for the measured data. From all measuring stations the daily erythemally effective 1/2 hourly dose rates are printed on a colour ink jet printer. For quality control and quality assurance, each individual measuring station is calibrated within 2-3 months by 1000 W quartz halogen lamps calibrated at the Federal Institute of Physics and Metrology (PTB, Braunschweig). Wavelength stability is controlled by comparing Fraunhofer lines in every measured solar spectrum. In order to achieve a high technical standard, continuous hardwale and software improvements are made on the equipment operated at the reference station. With reference to hardware, changes in the diffuser are currently being tested; for software, the automation of measuring input and malfunc-

Measurements are carried out in a wavelength range between 290 and 450 nm. The irradiance of shorter wavelengths is very low and contributes only slightly to the biological assessment of solar UV radiation. The bandwidth of 1 nm, considering the steep flanks of the solar spectrum at 300 nm, is still sufficiently accurate. In the range 290-320 nm, the step width is 0.5 nm, in the range 320--450 nm the step wiath is 5 nm. The relatively long step width is sufficient for later reconstruction with a radiation transfer model [ 14]. In Fig. 2, the markers ( × ) by one of the spectrum demonstrate the measuring points. To monitor a possible wavelength shift of the double monochromator, the region between 390 and 400 nm is highly resolved to detect the Fraunhofer line at 393.36 nm. The detectable limit of the measuring system is governed by the dark current of the detector or the remaining stray light, depending on which of the two is greater. The lower limit of irradiance amounts to about 10 - 6 w m -2 nm-~ in our system. To measure short-term UV radiation changes, as e.g. on cloudy days, the scan time should be very brief and the scan frequency should be high. The system in current use still requires about 90 s for a whole spectrum and works at 6 min intervals.

5. Data assessment All data measured during one day are put into a single data file very similar to the internationally accepted NASA Ames format [ 15 ]. With each respective morning start, a file header will be written with all site- and system-relevant information and data structure description. All subsequent measurements are appended to the header. So, each time, the complete todate irradiance situation can be called up. For a preliminary human health assessment, all 6 rain spectra within 1/2 h are added together into ! / 2 h mean spectra and subsequently weighted by the CIE response function [ 16]. Integration over a wavelength range between 290 and 400 nm and a time range of 30 min yields the effective exposure dose rate Jeff m -2 30 min-N. The division of the value 250 Jeff m -2 results in a dimensionless number that describes up to how many MEDs (l M E D = 2 5 0 J m -2, minimal erythemal dose for sensitive skin type I1, not protected against and not accustomed to light) the target is exposed to within 30 min. This is the so-called l / 2 hour MED. A time integration between i O.O0 and 16.00 hours, the average time for outdoor activities, results in the so-called 6 hour MED.

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is needed. At latitudes of nearly 50 °, this value can be reached and exceeded on sunny days between May and August. The summary below will give an overview of the daily radiation situation in the respective area. For better data handling, especially mailing, this report exists also in form of an ASCIIfile. To inform the public on the actual solar UV radiation and its risk, every Thursday BfS publishes a UV prognosis for the next three days. including sunburn times. A week-end forecast seems to be a well chosen time sequence because this is when most of the public outdoor activities take ~ e .

M. Steinmet: /Journal of Photochemistry and Photobiology B: Biology 41 (1997) 181-187

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Fig. 5. Daily MED-repon. TOP: I / 2 hour MED time for all 4 stations, t in UTC; values below dotted line: sunburn may occur in more than 2 hours, sun protection not needed, values between dotted line and dashed line: sunburn may occur within 2 hours and 30 min, sun protection is recomntended, values above dashed line: sunburn may occur in less than 30 rain. sun protection is required. BO'V['OM: integrated 6 hour MED for all 4 stations, same sequence as above.

M. Steinmet: ~Journal of Photochemistry and Photohioh~gy B: Biology 41 (19971 181-187 F o r p r o g n o s e s for northern, middle and s o u t h e r n G e r m a n y the n e w U V I n d e x ( U V t ) is used. T h e U V I is the 40-fold daily m a x i m u m o f e r y t h e m a l l y effective irradiation, a v e r a g e d o v e r a time period o f I0 to 30 m i n u t e s . A U V I o f ! c o r r e s p o n d s to a b o u t 0.2 o f the a b o v e d e s c r i b e d effective I / 2 h o u r M E D . T h e p r o g n o s i s is b a s e d on statistical e v a l u a t i o n s o f o u r o w n m e a s u r e d data at nearly identical s u n zenith a n g l e s and a t m o s p h e r i c conditions, especially c l o u d cover. N o r m a l s u m m e r o z o n e variations influence the U V I in the region by only less than 10% and need not to be c o n s i d e r e d on s u c h a r o u g h scale as the integer U V I . T h i s entire constellation is a g o o d o v e r v i e w o f the U V radiation situation in G e r m a n y and therefore is an ideal s u p p l e m e n t to m o r e detailed studies on o f biological effects. B i o s e n s o r c o m p a r i s o n m e a s u r e m e n t s h a v e already b e e n started.

References I I I International Agency for Research on Cancer. Solar and UV radiation, IARC Monographs on the evaluation of carcinogenic risks to humans. 55 I 1992 ). 121 World Health Organization. UV radiation. Environmental Health Criteria, 160 (1995), 13 ] M. Tevini, Molecular biological effects of UV radiation, in: M. Tenni ted.), UV-B radiation and ozone depletion: effects on humans, animals, plants, microorganisms, and materials, Lewis, 1993. pp. I15. 141 D.P. Hader and R.C. Worrest, Effects of enhanced solar UV radiation on aquatic ecosystems. Photochem. Photobiol.. 53 I 1991 ) 717-725.

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151 J. Scotto. G. Cotton. F. Urbach. D. Berger and T. Fe~.rs, Biok~gk:~ly effective UV radiation: Surface measurements in the United S~zae~, 1974 to 1985, Science. 239 11988) 762-764. [ 6 ] M. Blumthaler and W. Ambach. Indication of increasing sota, UV-B radiation flux in Alpine Regions. Science, 248 (1990) 2C~fi~--,-~. [ 7 ] G. Kelfkens. F.R. de Gruijl and J. ,,'an der Leon. Ozone depletkm and increase in annual carcinogenic ultraviolet dose. Photochem. Photobiol.. 52 11990) 819---823. [ F,] J.F. Frederick, Ultraviolet sunlight reaching tl;e earlh's surface: a review of recent research, Pholochem. PhotobioL. 157 ~ I l ~ 1993~ 175-178. 191 R.I. McKenzic, P.V. Johnston M. Kotkamp. A. Bittar and.I.D. Hart'din, Solar uhras'iolet spectroradiometry in New Zealand: instrumemaikm and sample results from 1990. Applied Optics. 31 { 1992) 65OI-6509. 1101 M. Nicolet. Solar spectral irradiances v,'ith their diversity hetween 120 and 901) nm, Placer. Space Sci.. 37 119891 1249-1289. I I I I B.G. Gardiner and J,K. Kitsch leds.I European intercomparison of UV-spectrometers, Panorama, Greece. 3-12 July, 1991. Relx~t to the Commission of the European Communities. (STEP Project 76) Brussels. 1992. [ 121 J.H G~b~on I ed. ), Justification and Criteria for the monitoring of UV r;~diation, Report of UV-B measurements workshop. Denver. CO. 1991. [ 131 M. Steinmetz, {ed. ) I. Arbeitsgespf.ich "terrestrisches solares UVMonitoring', 2.6.1992 in Munich. BfS/ISH-Bericht. 163, 1993. 1141W.E. Meador and W.R. Wever, Two-stream approximac,ions to radiative transfer in planetary atmospheres: A unified description of existing methods and a new improvement. J. Atmos. Sci., 37 (1980) 6311--643. 1151 S.E. Gaines and R.S. Hipskind. Format specification for dataexchange. ~ersion I.I. 1992. I 161 A.F. McKinlay and B.L. Diffey. A reference action spectrum for ultraviolet induced erythema in human skin. CIE J.. 6 ~ 19871 17-22.