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									Spectral UV observations at the Royal Meteorological Institute in Uccle

Hugo De Backer, Anne Cheymol, René Lemoine and Andy Delcloo
Royal Meteorological Institute of Belgium, Uccle, Belgium

  Abstract. UV spectral measurements at the RMI             another standard during the Brewerworkshop in
(Uccle, Belgium) date back as far as 1989. Over the         Halikidiki in 1998 [WMO, 1998]. Some lamps were
long time period 1989-2004 the frequency of                 damaged during the transport. New lamps with
observations and calibration checks has changed.            calibration data were acquired the later that year. It
A description of the software tools developed to            was noticed that the calibration of the new lamps was
manage the different calibration levels and the re-         consistent with the calibration level in use in
analysis of the whole set will be explained. The            Thessaloniki. The calibration files of the old lamps
methodology takes the temperature of the instrument         were adjusted to match the same level. In 2003 the
into account, and in this way the estimated error on        calibration was checked with two 1000 W NIST
the measurements is less than 5%. This reprocessed          traceable lamps during a calibration and maintenance
dataseries of spectral data is used to construct a data     visit. It turned out that there was no need for change
set of UV indices. The resulting time series will be        of the calibration level. Therefore all calibration data
examined for the temporal evolution, in relation with       was generated assuming the corresponding
the concurrent ozone observations and other ancillary       calibration for the 50W lamps as they were
data at the same site.                                      established in 1998.
                                                               Brewer #178 is a double monochromatior (MK III)
                                                            and its spectral range extends from 286 to 363 nm.
Introduction                                                The calibration level was established with the same
                                                            50W lamps as Brewer #016. It was also checked and
   UVB intensities received at the Earth’s surface are
                                                            confirmed with the 1000W-lamp system during the
closely related with the ozone content of the
                                                            calibration in 2003.
atmosphere [Bais et al, 1993]. Although it is not the
change in ozone in the stratosphere, but the possible       Data reduction
change in UVB radiation at the surface that has
impact on biosystems, there are much more long-term            The calibration of both instruments is established
data sets on ozone than on UVB.                             in almost the same way. For each observation (scan
   Also in Uccle (near Brussels in Belgium, 50º48'N,        of global irradiation, or scan of a calibration lamp, the
4º21'E, 100m) we have total ozone observations since        following calculations are performed. The raw photon
1971 with a Dobson spectrophotometer. In 1983               counts are first corrected for the dark count and the
Brewer #016 was installed. Initially it was only used       dead time of the instruments. Then they are converted
for total ozone observations. In 1989 the instrument        to counts per second. Finally the temperature
was automated and equipped with the UVB dome.               correction is applied.
Since that time we started the UVB measurements. In            Further processing depends on the purpose of the
2001 Brewer 178 was installed, and since that time          observation.
both Brewers operate side-by side. This offers the             If a calibration with a lamp is performed (ul or xl
opportunity to compare the reliability of both              routine for the single and the double monochromator
instruments. We can also have a look at the long-term       respectively), the corresponding lamp spectrum is
evolution of the UV intensities at the site. Before we      read. Normally this is done with a set of scans on
can do so we have to take care of the changes in the        different lamps. From these data a fifth order function
observation and calibration practices during this           of the wavelength is fitted by least square regression
period.                                                     to obtain a wavelength dependent response curve. For
                                                            the double monochromator Brewer, this is done
Instrumentation and calibration                             twice, once for the wavelength range 286-350nm (slit
                                                            1) and once for the wavelength range 350-363nm (slit
   As mentioned in the introduction we have two
Brewers. Brewer #016 is a single monochromator
                                                               If a scan of the global irradiation from the dome is
(MKII). His observational range is 290-325nm in
                                                            treated, then the corresponding responsefile is applied
steps of .5 nm. However before May 1991 the
                                                            to convert the counts per seconds into Watt per
software performed only scans in the range 290-315
                                                            square meter per nm. In this case the mean of the
nm. A lamp unit and five 50W lamps were used to
                                                            measured values at the 5 shortest wavelengths is
keep track of the calibration level of the instrument.
                                                            considered as a measure of the internal stray-light,
The instrument participated in 1994 in the laboratory
                                                            and is subtracted from the whole spectrum. This is
campaign of CAMSUM at IASB [A. Webb, 1997]. In
                                                            most important for the single monochromator.
1998 the calibration of the lamps was compared with
                                                                 offset and the correlation between the two data sets
                                                                 (not shown here). The differences at the lowest
                                                                 wavelengths are probably due to stray light problems
                                                                 in the single monochromator instrument.
                                                                 The UVI time series
                                                                   For the calculation of the UVI the measured
                                                                 spectrum is multiplied with the CIE action spectrum
                                                                 [CIE, 1987] and then integrated between 290 and
                                                                 400nm. Since the instruments do not measure the
                                                                 entire UV-A region we have to estimate the
                                                                 contribution at wavelengths above the highest
 Figure 1. Relative deviation of the response at 5 wavelengths
                                                                 measured 315 or 325 nm for the single
 of Brewer #016 from the response curve in use at the            monochromator and 363 for the double). This is done
 corresponding time.                                             by completing the spectrum with an artificial
                                                                 spectrum that is matched with the mean value of the
   To account for changes in the response of the                 observations at the five longest observed
instrument due to ageing or interventions, different             wavelengths. Figure 3 shows the time series of the
response files are used for different times. Between             daily maximum values of the UVI’s obtained in this
these times linear interpolation with time is                    way. The seasonal cycle arising from the change in
performed between the calibration files.                         the elevation of the sun is obvious. Trends will be
   To get an idea of the remaining variation, Fig. 1             calculated for fixed solar zenith angles.
shows the relative deviation of the response of
Brewer 016 determined from individual lamp scans
and the response spectrum in use at the corresponding
time. It can be seen that the stability of the instrument
obtained in this way better than +/- 5% (using 12
responsefiles over the 16 years). For the double
mononochromater the results are even slightly better
(+/- 3% with 2 calibration files over 3 years, not
shown). Note that these comprise all different lamps
that were used (seven different 50W as well as two
1000W lamps).
Comparison of instruments
  A comparison of the spectra obtained with Brewer
#016 (single monochromator) and Brewer #178                      Figure 3. Daily maximum of the UVI at Uccle as measured
(double monochromator) is shown in Fig. 2. It is seen            with Brewer #016 (red asterisks) and Brewer #178 (green
that both instruments compare very well above about
297nm. This was confirmed by the calculation of the              Acknowledgements Part of this work was supported by the
                                                                 Belgian Federal Science Policy Office within the ESAC grant
                                                                 number II project EV/34/3B.

                                                                 Bais, A. F., C. S. Zerefos, C. Meleti, I. C. Ziomas, and K.
                                                                   Tourpali, Spectral measurements of solar UVB radiation and
                                                                   its relation to total ozone, SO2, and clouds, J. Geophys. Res.
                                                                   (98), (D3) 5199-5204, 1993.
                                                                 CIE, A reference action spectrum for ultraviolet induced
                                                                   erythema in Human skin, CIE-Journal,6:17-22, 1987.
                                                                 Webb, A, Advances in solar ultraviolet spectroradiometry, Final
                                                                   report, European commission, Luxembourg, ISBN 92-828-
                                                                   0990-0, 1997.
                                                                 WMO, The fifth biannual WMO consultation on Brewer Ozone
                                                                   and UV spectrophotometer operation, calibration and data
                                                                   reporting, edited by T. McElroy and E. Hare, WMO-GAW
                                                                   report nr 139, 1998.
 Figure 2. Mean ratio of the intensities measured within 1
 minute of time with Brewer 178 and Brewer 016 between
 October 2001 and April 2004. The black line represents two
 standard deviations of the mean.

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