3) and to determine the total ozone concentration with its trends in order to
compare with in-situ satellite measurements.
In this paper, we initiate and focus our attention on the the above points by
presenting the first ground-based measurements of solar UV radiation
from the Tibetan Plateau. The characteristics of the resulting CIE weighted biologically
effective UV dose rate together with the effect of cloud cover and the relation to the
corresponding total ozone column as measured by Earth Probe TOMS are discussed.
A multi-channel filter instrument (NILUV) was installed on the roof of the Institute
of Tibetan Plateau Atmospheric and Environment Science Research (ITPAESR) in Lhasa
(29ø40'N, 90ø08'E; 3648 m above sea level). The instrument was produced by the Norwegian
Institute of Air Research (NILU). The NILUV consists basically of a large transmitting
Teflon diffusor as front optics.
Each channel has a broad-band interference filter and a photo-diode as detector.
The UV-B channel is centred at 305nm, while the UV-A channels are centred at 320nm and 340nm.
The filters have 10nm full width at half maximum (FWHM). The integration time was set
to one minute.
The effective UV dose rate was determined by a linear combination of the irradiances
measured by the 305nm channel and the 340nm channel.
The instrumental filter functions together with a corresponding set of calibrated
coefficients are needed to determine CIE-weighted UV dose rate.
The above instrumental technique was constructed and tested by Dahlback .
The hourly mean total global radiation used in this paper is measured simultaneously
from the Tibet Meteorological Observatory in Lhasa.
The total ozone column data is obtained from the Earth Probe TOMS, which was
available July 25, 1996.
The average value of four pixels is taken as representative for the total ozone
column over Lhasa.
Results and Discussion
The maximum solar irradiation received on ground level
during the period of measurements from June 24 to December 1, 1996, was
1178.0 w/m2. Local noon is at 06:00 UT.
The corresponding biological effective UV dose rate peaked at 390.5 (mW/m2) with
an average value of 225.4 (mW/m2).
Fig. 1.The time series of the biological effective UV dose rate from Lhasa, Tibet,
in the period from June 24 to December 1, 1996.
The corresponding total ozone column measured by Earth Probe TOMS are also shown.
The dashed line represents a fit to the total ozone column.
The solid- and the dotted line are fits for
morning and afternoon UV dose rates, respectively.
The solar zenith angle (SZA) was 60 degrees both for the morning (AM) and the
afternoon (PM) measurements.
The intensity of the solar UV radiation on ground level is strongly
influenced by the concentration of ozone in atmosphere.
Equally important parameter are the cloud cover and solar zenith angle.
To eliminate the influence of the sun elevation,
ten minutes average of the UV dose rate are calculated at solar zenith angles
10, 30, 50 and 60 degrees both in the morning and afternoon.
Figure 1. shows the time series of the UV dose rate obtained
at solar zenith angle 60 degree.
Also shown is the total ozone column obtained by the Earth probe TOMS.
We see an increase in the dose rate with a decrease of the total ozone column
during the period of measurements.
The daily ozone decrease was found to be -0.12ñ0.09 DU per day with a 95% confidence.
The same regression analysis was used on the UV dose rate.
The dose rate increased 0.15ñ0.16 mW/m2 per day in the morning and
0.11ñ0.18 mW/m2 per day in the afternoon.
It may seem difficult to deduce whether this increasing trend of the UV dose rate is due
to ozone alone or if its equally affected by the cloud cover.
Figure 2. shows the hourly mean percentages of the UV dose rate in the total global
radiation as a function of solar zenith angle.
It is clear that the percentage of the UV dose rate in the total global
radiation generally decreases with increasing solar zenith angle, which is
due to the long optical depth in the ozone layer at high solar zenith angles.
The UV dose rate is also more sensitive to a change in solar zenith angle than
the total global radiation.
Fig.2. The ratio between the hourly mean UV dose rate and the
total global radiation as a function of solar zenith angle
from June 24 to December 1, 1996, from Lhasa.
The amount of UV radiation pentrating to ground level is mainly influenced by the solar zenith angle,
the total ozone column and the cloud cover.
Figure 3. shows a scatter plot between the morning UV dose rates and the afternoon UV dose rates at
corresponding solar zenith angles 10, 30 and 60 degrees.
The UV dose rate is slightly higher in the morning compared to afternoon.
This effect is mainly due to the more frequently and havily cloud cover observed
in the afternoon compared to prenoon during the summer monsoon season.
Fig.3. Scatter plot between measured morning and afternoon UV dose rates at the same solar zenith
angles during time period from June 24 to December 1, 1996, in Lhasa.
The principal results obtained by the first ground-based measurements of UV radiation in Lhasa during
time the period from June 24 to December, 1996, may be summarized as follows:
Acknowledgement: This work was financially supported by the Norwegian Department for Development (NORAD).
The authors are grateful to our colleagues at the Institute of Tibetan Plateau Atmospheric and Environmental Science
Research (ITPAESR) in Lhasa for helping us installing the instrument.
Especially, Mr. Cuduo who is in charge of downloading data and maintaining the instrument.
- 1. The UV dose rate was extremely high at local noon. The maximum value was
close to 390 mW/m2 with a total average of 225 mW/m2.
- 2. The biologically effective UV dose rate increased 0.13ñ0.17 mW/m2 per day.
- 3. The total ozone column over Tibet from TOMS decreased 0.12ñ0.09 DU per day.
- 4. The UV dose rate in the morning is slightly more intense compared to afternoon.
- 5. Further studies of the spectral irradiance are needed to verify the seasonal and
long-term trends of UV radiation and the total ozone column over the Tibetan Plateau.
Ambach,W., M. Blumthaler, G.Wendler, A comparison of ultraviolet radiation measured at an arctic and an alpine
site, Solar Energy(2), 121-126,1991.
Bais, A.F., C. S. Zerefos, C. Meleti, I. C. Ziomas and K.Tourpali, Spectral measurement of solar UVB
radiation and its relations to total Ozone, So2 and clouds, J. Geophys. Res., 98, 5199-5204,1993.
Blumthaler,M., and W. Ambach, Indication of increasing solar ultraviolet-B radiation flux in Alpine regions,
Science, 248, 206-208, 1990.
Blumthaler, M., W. Ambach and M. Salzgeber, Effects of cloudiness on global and diffuse UV irradiance in
a high- mountain area, Theor. Appl. CLimatol.,50, 23-30, 1991
Dahlback. A., Measurements of biological effective UV-doses, total ozone abundance and cloud effects with
multi-channel moderate bandwidth filter instruments, Appl.optic. Vol. 35, No. 33, 1996.
Dan Lubin, The ultraviolet radiation environment of the Antarctic Peninsular: The role of ozone
and cloud cover, Journal of applied Meteorology, Vol.30, 1991.
Frederick, J. E., P. F. Soulen, S. B. Diaz, I. Smolskaia and C. R. Booth, Solar ultraviolet irradiance observed
from southern Argentina: September 1990 to March 1991, J. Geophys.Res.,98, 8891-8897, 1993.
Han Zou, Seasonal variation and trends of TOMS ozone over Tibet, Geophys. Res. Let., 23, 1029-1032, 1996.
Mckinlay,A.F. and B.L.Diffey,
A reference action spectrum for ultra-violet radiation: Risks and Regulation,Elsevier, Amsterdam, 83-87, 1987.
Scotto,J., G. Cotton, F.Urbach, D. Berger, and T.Fears,
Biologically effective ultraviolet radiation: surface measurements in the United States,
1974 to 1985, Science, 239,762-764,1988.
Yeh, T. C and Y. X. Gao, Meteorology of Tibetan Plateau ( in Chinese ), lpp.,
Scientific Press, Beijing, China, 1979.
Zhou, Xiuji and Chao Luo, Ozone valley over Tibetan Plateau, Acta Meteorologica Sinca, 8(4), 505-506, 1994.