Ground-based Measurements of Solar Ultraviolet Radiation in Tibet: Preliminary Results

Pu Bu Ci Ren (1) Fred Sigernes (2) Yngvar Gjessing (1)
(1) Geophysical Institute, University of Bergen, Allegaten 70, N-5007, Bergen, Norway
(2) University Courses on Svalabard, P.O. Box 156, N-9170, Longyearbyen, Norway
Abstract: Measurements have been carried out on the Tibetan Plateau (Lhasa, 29N 90E; 3648 meters above sea level) with a multi-channel moderate bandwidth filter instrument in the period from June 24 to December 1, 1996. The analysis of the preliminary data shows that the hourly mean CIE weighted biologically effective UV dose rate at local noon can reach as high as 390.5 mW/m2. The corresponding average value was 222.5 mW/m2. In situ measurements of the total ozone column by the Earth Probe TOMS are discussed in relation to the measured increase in the UV dose rate at ground level throughout the whole time period.

Introduction

The importance of solar ultraviolet radiation and its impact on human beings have been extensively discussed during the last few decades [Scotto et al, 1988; Blumthaler and Ambach, 1990]. Both broadband and spectral measurements of UV radiation have been performed at several sites around the world [Blumthaler et al, 1991; Frederick et al, 1993; Bais et al, 1993]. However, neither long-term broadband nor long-term spectral measurements of UV radiation on the Tibetan Plateau have been reported.

The Tibetan Plateau is the highest plateau in the world with an average height of about 4,000 metres above sea level. Due to the strong solar radiation caused by the shallow atmosphere and the special nature of the surface, the Tibetan Plateau becomes a strong heating source to the atmosphere, which may modify the atmospheric circulation on local, regional and global scales [Yeh et al, 1979]. Especially, in the meridional monsoon circulation, which becomes much stronger over the Tibetan Plateau compared to other corresponding longitudes, such as the Rocky Mountains or the Andes Mountains. In addition, Zhou et al.[ 1994] and Han Zou, [1996] showed that there is a region of low ozone existing over the Tibetan Plateau during summer. Also, based on measurements of the total ozone column from TOMS 1978-1991 year-round data, a decreasing trend of ozone have been found over the Tibetian Plateau.
Therefore, a study of UV radiation on the Tibetan Plateau has essential importance in mainly three aspects, namely:
  • 1) to obtain its spatial extent and temporal change,
  • 2) to understand its biological effect,
  • 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.

    Experimental technique

    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 (2940'N, 9008'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 [1996].

    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 -0.120.09 DU per day with a 95% confidence. The same regression analysis was used on the UV dose rate. The dose rate increased 0.150.16 mW/m2 per day in the morning and 0.110.18 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.

    Concluding remarks

    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.

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