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 were carried out on Tibetan Plateau (Lhasa, 3648 meters above sea level) with a multi-channel moderate bandwidth filter instrument in the period from June 24 to October 30, 1996. The analysis of preliminary data shows that hourly mean CIE weighted biologically effective UV dose rate at local noon can reach to as much as 390.5 mW/m2 with a average of 222.5 mW/m2; Due to the combined effect of cloud cover variation with ozone variation, the increasing trend of this UV dose rate exists during this period of measurements. The relation between the ratio of irradiance close to noon at 340nm to that at 305nm and ozone column amount shows that the variation of ozone amount may also causes the variation of UV radiation. In addition, UV dose rate is more sensitive to the variation of solar zenith angle than total global radiation.

Introduction

The importance of solar ultraviolet radiation and its impact on the human beings have been extensively discussed during last few decades [ Scotto et al, 1988; Blumthaler and Ambach, 1990], and 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 in Tibetan Plateau have been reported, where UV radiation is extremely important for the population and plants in this region because of the combined effect of high altitude with low latitude.

Tibetan Plateau is the highest plateau in the world with average height about 4,000 metres above sea level. Due to the strong solar radiation caused by shallow atmosphere and the special nature of surface, Tibetan Plateau becomes a strong heating source to the atmosphere, which can modify the atmospheric circulations on local, regional and global scales [Yeh et al, 1979]. Especially, in the meridional monsoon circulation, which becomes much stronger over Tibetan Plateau compared to other corresponding longitudes, such as the Rocky Mountains or the Andes Mountains. In addition, the works of Zhou et al.[ 1994] and Han Zou, [1996] showed that there is a region of low ozone existing over Tibetan Plateau in summer. They also found decreasing trend of ozone over Tibetan Plateau from TOMS 1978-1991 year-round data. Therefore, study of UV radiation on Tibetan Plateau has essential importance in mainly three aspects: 1) to obtain its spatial extents and temporal changes in high altitude area; 2) to understand the biological effect of UV radiation; 3) combined with radiative transfer model, to determine the total ozone concentration and trends and compare it with the results from satellite measurements.

In this paper, we report on the first ground-based solar UV radiation measurements over Tibetan Plateau. The characteristics of resulting CIE weighted biologically effective UV dose rate over Tibetan Plateau and the relation between the trend of UV dose rate and the trend of ozone column amount measured by Earth Probe TOMS are also discussed.

Experimental technique

A multi-channel filter instrument (NILUV) was installed on the roof of Institute of Tibetan Plateau Atmospheric and Environment Science Research (ITPAESR) in Lhasa (29°40'N, 90°08'E; 3648 m above sea level). This instrument was produced by Norwegian Institute of Air Research ( NILU), it consists basically a Teflon diffuser, interference filters and photo-diodes as detectors. It has three channels, 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). One minute interval was adopted in this measurements.

The effective UV dose rate is determined by a linear combination of the irradiances measured by 305nm channel and 340nm channel. The coefficients used in this linear combination are calculated with the radiative transfer model. The conclusion from the tests of sensitivity of these coefficients to ozone profile and surface albedo shows that only a single set of coefficients is needed to determine CIE-weighted UV dose rate [Dahlback,1996].

The above instrumental technique was constructed and tested by Dahlback [1996], and the comparison with high resolution spectral radiometer which was done during a one week period with variable cloudiness in San Diego, USA, showed that for solar zenith angle (SZA) <80°, the relative difference was 1.4˝3.2% under all sky conditions, and only 0.6˝1.5% under near clear sky condition; [Dahlback, 1996].

The hourly mean total global radiation used in this paper is observed simultaneously at Tibet Meteorological Observatory, which is about 100 metres from our UV measurements site. In addition, The ozone column amount data is obtained from the products of Earth Probe TOMS, which is available since July 25, 1996, and the average value of four pixels around location of UV radiation measurements, Lhasa, is taken as Ozone column amount over Lhasa.

Results and Discussion

Tibetan Plateau is located in middle latitude area (29°N 40°N), and its high altitude makes solar radiation penetrate through atmosphere in a shorter pass from the sun to the ground. The highest solar irradiation is received on ground level ( 1178.0 w/m2, maximum of hourly mean at local noon in this data set). According to the analysis of measurements from June 24 to November 30, 1996, the hourly mean of UV dose rate at local noon (06:00 UT) can reach to as much as 390.5 (mW/m2), and the average value is 225.4 (mW/m2).

Fig. 1. Time series of biological effective UV dose rates from Lhasa, Tibet in the period from June 24 to November 30, 1996 at solar zenith angle (SZA) 60° and ozone column amounts (DU) measured by Earth Probe TOMS from July 25 to November 30. The dashed line represents the fitted line for ozone column amount(DU). The solid line and dotted line are fitted lines for morning and afternoon UV dose rate at solar zenith angle 60° respectively . AM and PM represent morning measurements and afternoon measurements at the same solar zenith angle.

The intensity of solar UV radiation on ground level is strongly influenced by the concentration of ozone in atmosphere, equally important parameters are cloud cover and solar zenith angle. For eliminating the influence of solar zenith angle, the ten minutes average of UV dose rate are calculated at solar zenith angle 10°˝1°, 30°˝1°, 50°˝1° and 60°˝1° respectively. Figure .1. shows the time series of UV dose rate measured at solar zenith angle 60°and ozone column amount measured by Earth Probe TOMS. The decreasing trend of ozone column amount clearly exists during this measurement. Using linear regression analysis, a daily ozone trend was found to be -0.12˝0.09 DU/day with 95% confidence during period from July 25 to November 30. The same analysis was used to the corresponding UV dose rate time series at solar zenith angle 60°, and a daily trend of UV dose rate was found to be 0.15˝0.16 mW/m2/day( or 0.11˝0.18 mW/m2/day) for morning value ( or afternoon value). However, It is difficult to deduce that the corresponding increasing trend of UV dose rate is due to the decreasing of ozone amount, since the data set used here includes data under different cloud cover conditions, which cause different modulating effects to UV dose rate.

Fig.2. Time series of the ratio of irradiance at 340nm to that at 305nm with solar zenith angle 60° and ozone column amount (DU) in Lhasa from June 24 to November 30.

However, as scattering by clouds is essentially independent of wavelength, and the absorption of ozone decreases with the increase of wavelength almost in orders of magnitude[Dan, 1991], much of the variability associated with cloud cover can be cancelled in the ratio of irradiance at 340nm to that at 305nm. In order to assess the effect of ozone variation to UV radiation, the ratio of irradiance at 340nm to that at 305nm at certain solar zenith angle, which was measured during this experiment, is used in this paper(Fig.2). It shows that the corresponds of this ratio variation at solar zenith angle 60° to ozone column amount variation improve with time, and the correlation analysis between this ratio and ozone column also shows that linear correlation coefficient in the period from October 14 to November 30 (R=0.62,N=47) increases to 2 times of that in the period from July 25 to September 10 (R=0.31,N=47). The reason can be explained by the fact that the time when UV irradiances was measured is more close to the ozone observing time by satellite at the end of this time series. The ozone column amount is observed almost constantly by Earth Probe TOMS at about 11:20 am (LT) , but in the period of this UV measurement from July 25 to November 30, the time when solar zenith angle is equal to 60°, changes from 7:30 to 10:30 (LT). Therefore, according to the availability of data and the closeness of UV irradiance measurement time to ozone observation time, the ratio at solar zenith angle 30° is chosen to analyse the effect of ozone variation on UV radiation instead during the time period from July 25 to September 23, and the correlation coefficient between this ratio and ozone amount increases from 0.31 to 0.49. The scatter plot in Figure.3 shows that the positive correlation exists between this ratio and ozone column amount during these two periods, even if the standard deviation of ozone amount during these two periods are only 4.8 and 8.0 respectively. It indicates that the variation of ozone amount also has effect on the variation of UV radiation at Earth's surface during the period of measurements in Lhasa.

Fig.3. Scatter plot between ratio of irradiance at 340nm to that at 305nm and ozone column amount (DU) measured from Earth Probe TOMS. (a). Measurements from October.14 to November.30 at solar zenith angle is 60° . (b). Measurements from July 25 to September 22 at solar zenith angle is 30°.

Based on the hourly mean total global radiation measured at the same time, the hourly mean percentages of UV dose rate in the total global radiation are also calculated. Figure 4. shows the relation between hourly mean percentages and hourly mean solar zenith angles. It is clear that the percentage of UV dose rate in total global radiation generally decrease with the increasing of solar zenith angle, that is due to the long optical depth in the ozone layer at high solar zenith angle, the UV dose rate is more sensitive to the change of solar zenith angle than total global radiation. In addition, except the high values which appeared when solar zenith angle becomes high than 85° can be associated with the ratio between two small values, the high values also happen at other solar zenith angle, and a general increasing trend of these high values with solar zenith angle exists. It probably results from the difference between effects of cloudiness on UV dose rate and that on total global radiation, and It can be explained by the fact that the reducing effect of cloudiness is considerably greater on total global radiation than on UV dose rate, while it is also systematically greater at high solar zenith angles compared with lower solar zenith angles [Blumber.M, 1994]. On the other hand, some part of deviation from the decreasing trend with solar zenith angle , specially these lower values, can be attributed to the variation of ozone amount at same solar zenith angle during this time period, but Its effects are expected to be smaller compared to that of the cloudiness.

Fig.4. The change of the ratio of hourly mean UV dose rate to hourly mean total global radiation with hourly mean solar zenith angle from June 24 to October 30 in Lhasa.

The amount of UV radiation reaching to ground is mainly determined by solar zenith angle, ozone amount and cloud cover. Figure.5 shows the scatter plot between morning UV dose rates and afternoon UV dose rates at solar zenith angle 10°˝1°, 30°˝1° and 60°˝1°. The morning UV dose rate and afternoon UV dose rate are defined as UV dose rate measured at the same solar zenith angle in morning and at afternoon. The gradient and correlation coefficient of the regression line is 0.82˝0.27 and 0.78 respectively, indicating slightly higher UV dose rates exist in the morning compared with that at afternoon, which can mainly be attributed to the observed more frequently and heavily cloud cover at afternoon compared to prenoon during summer monsoon season.

Fig.5. Scatter plot between measured morning and afternoon UV dose rate with same solar zenith angle during time period from June 24 to November 30 in Lhasa. Fitted line are also shown(solid line).

Concluding remarks

The principal results obtained by these first ground-based measurements of UV radiation in Lhasa during time period from June 24 to November 30 may be summarized as follows:

Acknowledgement: This work was financially supported by Norwegian Department for Development (NORAD). The authors are grateful to our colleagues in Institute of Tibetan Plateau Atmospheric and Environmental Science Research (ITPAESR) in Lhasa for helping us install the instrument, specially Mr. Cuduo who is in charge of down loading data and maintaining this instrument.

References:

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.