Armospherk Emhmmmt Vol. 23, No. 7, pp. 1573-1579, 1989. ooo4-6981/89 s3.oo+o.oo Printed in Great Britain. 0 1989 Maxwell Pergamon Macmillan plc

MEASUREMENTS OF SOLAR U.V., VISIBLE AND NEAR I.R.

IRRADIANCE AT 78” N

K. HENRIKSEN, K. STAMNES* and P. OSTENSEN

The Amoral Observatory, University of Tromso, Tromsnr, Norway

(First received I October 1988 and received for publication 6 January 1989)

Abstract-Spectral measurements of solar radiation in the Arctic during the summer of 1987 were carried out with a double monochromator calibrated in absolute units. Integrated irradiances for parts of the UVB (~~ooA-~IsoA), UVA (3150A-4000A) and the whole measured spectral range (BOO A-8000 A) are tabulated. No irradiance is detected below 3050 A. The measurements show that the diurnal variation in the UVB is a factor 7 stronger than in the UVA and a factor 6 stronger than for the total measured spectral range. We also 6nd that in the ultraviolet part of the spectrum the diffuse radiation is the dominant component of global irradiance. Under clear sky conditions the measured spectral irradiances are closely reproduced by radiative transfer calculations.

Key word index: U.V. irradiances, diurnal variations, ozone density, radiation measurements.

INTRODUCTION

Light has a crucial effect on microorganisms and plants. Thus, the production of algae and plankton in the sea depends sensitively on the spectral composi- tion of the solar radiation penetrating into the ocean. This in turn has a major impact on the fish population since these microorganisms constitute the basic food supply for all small fish and animals in the sea. Similarly, plants depend on light for growth, and changes in the spectral composition might steer their life cycle, i.e; spectral changes of the available radi- ation may trigger their biological clock, initiating and closing the diurnal and seasonal growing periods. Ultraviolet light is important for vitamin D synthesis (Giese, 1976), but increased levels of UVB radiation [caused either by ozone depletion or decrease in cloud cover (Frederick and Lubin, 1988)] may have deleter- ious health effects (Dahlback et al., 1989). It is there- fore important to investigate the spectral distribution of solar radiation reaching the biosphere.

In this paper we present the results of spectral, global horizontal measurements of solar irradiance at Longyearbyen, Svalbard carried out from 23 July to 4 August 1987. The sun was above the horizon both day and night, and the solar elevation during this period varied between 31.8” and 29.0” at noon and between 8.2” and 5.3” at midnight. The measurements were carried out for 168 h, providing diurnal spectral variations for each day, and the data are used to construct the avera e diurnal irradiance for the spec- tral regions 2900 f -315OA (UVB), 3150&4OOObi (UVA) and 2900A400A.

l Geophysical Institute and Department of Physics, Uni- versity of Alaska, as of 1 July 1988.

These measurements were initiated by an inter- disciplinary group consisting of biologists, chemists, dermatologists, geophysicists and meteorologists to satisfy their respective needs for spectral irradiance data. There is a much closer agreement between various spectral it-radiance measurements obtained at the top of the atmosphere (cf. Iqbal, 1983; WMO, 1985) than at the ground, which is a consequence of the variability of the atmospheric optical conditions. The aim of the spectral measurements is (1) to deter- mine the amount of solar radiation reaching the ground in the u.v., visible and near i.r. spectral regions, (2) to quantify the diurnal variation, and (3) to investi- gate how atmospheric conditions affect the spectral distribution.

INSTRUMENTATION

The measuring device is a double monochromator (HlOD u.v.) manufactured by Jobin Yvon) with two spherical gratings having 10 cm focal lengths. The spectral range of the monochromator extends from 2000 to 8OOOA. It is operated in the first order with entrance and exit slits of 0.5 mm width, resulting in a spectral resolution of 20A. In order to prevent over- filling of the field of view a baffling system is built in front of the entrance slit, reducing the optical accept- ance angle to 5”. To measure global irradiances, we used an integrating sphere, constructed and described by Hisdal(l986). The sphere is placed in front of the entrance baffling system, as illustrated in Fig. 1. The global horizontal solar irradiance was measured by keeping the acceptance area of the integrating sphere horizontal.

The detector is an U.V. sensitive photomultiplier, R446 Hamamatsu, interfaced to an integrated PAD- HV unit, produced by Research Support Instruments.

1573

1574 K. HENRIKSEN el al.

25 SPECTRAL IRftADIANCE OF

9 DXW LAMP AT A DISTANCE

“5 2om

OF 50 cm

“-1 /I

/’

0 ,*.I-./ I

300 400 500 600 700 I

WAVELENGTH (nm)

30 0

Fig. 2. Spectral irradiance of the Eppley DXW lamp used for absolute irradiance calibration of the

double monochromator.

Fig. I. Diagram of the principle of HIOD Jobin Yvon spectrometer with baffling system and integrating sphere in front of the entrance slit. At the exit slit detector and

pre-amplifier are mounted.

By using this unit the signal appears as transistor- transistor logic (TTL) pulses from the spectrometer.

The spectrometer is operated with a microcompu- ter, steering the stepping motor which turns the gratings. The pulse counting intervals are synchro- nous with the grating steps. Each step shifts the wavelength by 1 A. The data are stored on diskettes, and each spectrum is displayed on a monitor screen in real time, yielding a ‘quick-look’ of the data. In addition, on-line stack plots appear on a printer, to provide an overview of the stored data.

Irradiance absolute calibration is carried out with an Eppley calibration source, a DXW quartz-iodine 1000 W lamp, which is currently in general use as a secondary standard of spectral irradiance. Its spectral irradiance is given in Fig. 2. The spectral irradiances of the lamp is more than an order of magnitude less than solar irradiances and absolute measurements are therefore obtained by extrapolation. The calibration curve is shown in Fig. 3. The hump at 3700A and dip at 5000 A are due to grating anomalies. The low values below MOO A reflect the high sensitivity in the U.V. and part of the visible spectral regions, whereas the in- creasing values towards longer wavelengths indicate decreasing sensitivity. At the longer wavelengths the calibration curve is less reliable, and the uncertainty is estimated to be 20% above 7000A.

MEASUREMENTS

The measurements were carried out in Adventdalen close to Longyearbyen, Svalbard (78.2”N, 15.6” E; geographic coordinates). Mountains towards the north and south reached between 5” and lo” above the horizon, but only about 1” towards the east and west.

The ground in the surroundings consists mostly of brown sandy soil with scanty polar vegetation provid- ing a relatively low ground albedo. The open integrat- ing sphere prevented continuous operation in rain, since water will destroy the BaSO, diffuse inner surface of the sphere. Otherwise the observations were carried out both in clear and overcast weather.

The global irradiance observations were inter- rupted two times when the sky was clear. Then direct solar measurements were taken, and these data were used to derive the total column 0, density. The data analysis is reported elsewhere (Stamnes et al., 1988a).

RESULTS AND DWUSSION

To satisfy diverse needs for spectral solar radiation data by different scientific communities and because of instrument limitations, we measured the global hori- zontal spectral irradiance for the wavelength region from 2900 A to 8ooo A. We did not record any radi- ation at wavelengths shorter than 305OA, even if the scanning range extended down to 2900 A.

The calibration function of the instrument (Fig. 3), however, does not indicate any major change in sensitivity around 3050 A. The count rate of the dark current was equivalent to 0.03 mW m-2. A in the U.V. Below 3050 A the count rate of the signal had dropped below 0.003 mW rn-‘. A and therefore hard to distin- guish from the inherent count rate produced by the dark current.

The i.r. radiation beyond 8OOOA cannot be meas- ured with this U.V. sensitive instrument. The sensitivity of the monochromator decreasea dramatically as the measurements approach 8000A (see Fig. 3).

In Fig. 4 we show a spectrum obtained under partly cloudy skies of the global horizontal irradiance for the spectral region 3000 A-WO A. In ttis spectral region there are no distinct absorption feature-s due to water vapor, but attenuation due to cloud droplet scattering

Measurements of irradiance at 78”N 1575

O?~‘~~~‘~‘.r’~~~l’~~‘I”~‘I”‘~“~~~””~””””~~”’~~’ 3000 4000 5000 6000 7000 8000

WAVELENGTH iA1

Fig. 3. Calibration curve of the double mon~hromator equipped with the integrating sphere in front of the entrance slit.

T

I 3000 4000 6000 6000

WAVELENQTH (h

Fig. 4. Global horizontal spectrum obtained in part- ly cloudy weather. This spectrum is the average of single spectm measured during 1 h from 1104 UT. Conspicuous Fraunhofer lines and the atmospheric

NaD absorption line are indicated.

and absorption is prevalent. Below 3300 A the irradi- ante drops rapidty due to strong absorption by at- mospheric 0,. Some of the most prominent Fraun- hofer lines are identified in Fig. 4 by their absorption species in the solar chromosphere and corona. There are additional absorption features in the atmosphere by minor species such as NO,, NOs, CH,, etc., but in order to quantify these absorption features higher resolution is required.

The data are divided into three spectral ranges 2~A-8~A, 29tJOA-31SOA and 31SOA-4UOOA. Averages for each hour are calculated and given in Tables 1,2 and 3. The units for tbe 2900A-8000 A and

3lSOA~A ranges are Wms2, but mWmm2 for the UVB range, 29OOA-315OA.

The sampling times per grating step were 100 and 200 ms, for high and low sun elevations, respectively. Thus, the accumulation time of each complete spec- trum was normally 10 min during the day and 20 min during the night. The shorter accumulation time, used during the day, was necessary to prevent over8ow of the pulse co,unters which had a capacity of 16 bits.

The irradiance values of the three spectral ranges were calculated for each observed spectrum and plot- ted in Figs S,6 and 7 for the entire observation period. When the global irradiance observations were stop- ped either to make direct solar m~su~rnen~ or due to rain, these intervals are seen as data gaps in Figs S,6 and 7.

The average diurnal irradiance values obtained from these measurements are plotted in Fig. 8. For the UVB range (29OOA-3150A), the diurnal variation ranges from 4 to 168 mWmm2; for UVA (31~0 Aa A), the contending numbers are 3 and 19 Wmm2; and for the total range (2~A-8~A), 35 and 249 W mP2. The strongest diurnal variation occurs for the UVB radiation which shows a factor 42 increase from midnight to noon, whereas UVA and the total irradiance vary only by factors of 6 and 7, respectively.

When our spectra are compared with spectral irra- diances, obtained by radiative transfer calculations using the extraterrestrial solar radiation spectra, 0s absorption and Rayleigh scattering cross-sections re- commended by WMO (1985) as input (cf. Fig. 9), there is a close agreement between calculations and meas- urements in the U.V. below 3300A where adoption by 0, is the dominant mechanism affecting the trans- mission. At longer wavelengths the observed irradia- nce values are about 10% lower than the calculated

1576 K. HENRIKSEN er al.

ones. We attribute this discrepancy to atmospheric aerosols ignored in the computations. From the re- sults of Rao et al. (1983) we infer that the aerosol content in Svalbard is < lOa particles per cm’, since the visibility normally exceeds 70 km, and therefore the air in Svalbard can be considered as clean for all practical purposes. The multiple scattering calcu- lations were performed by a method described by Stamnes et al. (1988b). More details about our model calculations are provided elsewhere (Stamnes et al., 1989). Preliminary calculations for clear sky condi- tions indicate that the observed diurnal variation of the spectral distribution is in basic agreement with theoretical estimates. For UVA and visible radiation, this variation will be sensitive to changes in cloud amount and surface albedo, while the transmitted UVB radiation is expected to depend strongly on column 0, content as well.

The spectrometric equipment was also used to measure the global and diffuse irradiance during a clear day in Tromse (69.7”N, 18.7”E; geographic co- ordinates) with a solar elevation of 20”. The diffuse irradiance was obtained by using a circular patch to prevent the direct attenuated solar beam from enter- ing the integrating sphere, and the resulting irradian- ces are shown in Fig. 10. At wavelengths < 3300 A the direct irradiance becomes insignificant compared with the diffuse component, which is confirmed by our radiative transfer calculations. The relative magnitude of the diffuse component decreases with increasing wavelength.

Similar global irradiance measurements were per- formed for several years during summer in Ny-Ales- und, Svalbard at 79” N (Hisdal, 1986). The irradiance values found by Hisdal are within 10% in the U.V. but differ more towards longer wavelengths. The devi- ation is most probably due to difficulties associated with absolute calibration. Recent measurements were also carried out at Bedford, Massachusetts and Golden, Colorado (Bird et al., 1982), but these authors do not give the solar depression angle. This angle must be known before meaningful comparisons can be made between measurements at different sites.

1 6

: 7 2

~ \ ‘8 29 30 31 1 2 3 4 5

AUGUST ,987

Fig. 5. Integrated global horizontal irradiana of the wave- length region 29oA-31SOA (UVB) for the observation period. The minimum irradiana during night is about 2

orders of magnitude less than the maximum at noon.

Measurements of irradiance at 78”N 1577

Table 2. Values in W mm2 of integrated global horizontal irradiance for the wavelength region 3150-4000 A

\ MD H 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

07.23 _ _ _ _ _ _ _ _ _ _ - 17 15 20 - - - - - - - - - 3

07.24 33444 5 - 9 10 11 12 18 22 15 15 11 14 13 13 10 - - - - 07.25 ---44 3 811 6 12 19 24 24 23 20 17 15 13 13 9 7 6 - 5 07.26 4 4 5 6 7 10 12 14 13 - - - - 25 23 20 19 16 12 11 9 6 5 - 07.27 2 2 1 1 4 5 8 7 11 12 8 10 19 23 20 7 - - - - - - - - 07.28 _ _ _ _ _ _ _ - 11 14 15 10 11 12 13 14 10 - - - 6 5 4 4 07.29 44445 6 - 11 11 14 12 9 10 7 - - - - - - 6 5 3 - 07.30 - - - - _ - _ - _ - _ - - - - - - - - - - 4 3 3

07.31 - - - - - 6 5 8 9 10 8 - 14 - - - - - - - - - 5 4 08.01 33456 811 - 17 18 21 21 21 18 - - 18 16 - 10 8 6 5 4 08.02 3-l_-_-_-_-_-_-_- ---- ---

08.03 - - - - - - - - - 14 20 18 9 - - 11 12 11 10 10 - - - - 08.04 2 2 3 4 5 7 9 9 11 13 - - 21 22 18 15 14 13 11 9 - - - -

Aver. 3 3 3 4 5 6 9 10 10 13 14 16 17 19 19 15 14 14 12 10 8 5 4 4

Table 3. Values in mW m-* of integrated global horizontal irradiance for the wavelength region 2900-3150 A

012345678 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

07.23 _ - _ _ _ _ _ _ _ _ _ 148 143 179 - - - - - - - - - 5 07.24 5 4 7 10 11 15 - 51 65 75 101 153 175 147 143 102 109 96 74 47 - - - - 07.25 - - - 10 13 12 35 62 41 110 156 214 230 219 191 166 115 92 68 40 26 18 - 8 07.26 7 7 9 11 21 33 50 71 78 - - - - 220 208 176 138 103 67 45 28 16 11 - 07.27 4 2 2 2 10 17 38 43 70 90 75 93 169 193 158 52 - - - - - - - - 07.28 _ _ _ _ _ _ _ - 74 97 115 87 100 100 102 111 74 - - - 16 9 6 6 07.29 5 5 7 9 13 17 - 59 69 104 101 72 89 60 - - - - - - 16 11 5 - 07.30--_-_-------

102

----_-__,064

07.31 - - - - - 15 17 35 47 64 52 - - - - - - - 7 5 08.01 3 3 4 6 12 21 35 - 96 109 140 155 165 157 - - 115 85 1 40 25 1; 7 4 08.02 3 _ 1 _ _ _ _ _ _ _ _ 08.03 _ _ _ _ _ _ _ _ _ 100 134 132 68 I Z 82 81 70 47 40 I I Z I

08.04 2 2 3 5 10 21 30 43 67 99 - - 176 167 149 120 92 76 61 38 - - - -

Aver. 4 4 5 8 13 18 34 51 64 93 112 138 148 168 164 125 104 88 66 43 25 13 7 5

01” k j ” a a a n s 0 232425262728293031 12 3 4 5 23 24 25 26 27 28 29 30 31 1 2 3 4 5

JULY 1987 AUGUST 1987 JULY 1987 AUGUST ,987

Fig. 6. Integrated global horizontal irradiance of UVA, the wavelength region from 3150A to 4000 A. Diurnal vanations

Fig. 7. Integrated global horizontal irradiance of the wave- length region from BMIA to 8oooA. Diurnal variations as

can be as much as a factor of 10. much as a factor of 10 occurs.

CONCLUSIONS by changing weather conditions, notably varying cloud amount, which significantly alter the atmos-

We have presented spectral measurements of the pheric opacity. By integrating the h-radiances over the solar radiation in the Arctic during the summer of 2-week observation period, we find that the diurnal 1987. The measured spectra show systematic diurnal variation of the irradiance ranges from 4 to variations as well as large irregular variations caused 168 mWm_’ in the UVB (29ooA-315Odj), from 3 to

AE 23:7-5

1578 K. HENRIKSEN et af.

250

200

cc+ E

g 160

6

5 z 100

60

0

LOCAL TIME (hour)

Fig. 8. Average global irradiances at Longyearbyen for the period 23 Jul to 4 August 1987.

x - x - x 29OOkNOO x (scale to the left) + - + - -+ 3150 k4OOO A (scale to the left) ~...O_O 2900 k315Ok (scale to the right).

0.6

0.5

0.4

E

a; 0.3

5

0.2

0.1

0.c 2500 9000 3600 4000 4500 6000 5600 8000

WAVELENGTH, i

Fig. 9. Comparison of calculated (dashed line) and observed (solid line) irradiance spectra. The calculations were done for surface a&do of 0.3 and total ozone amount of 300 DU obtained from

spectral me~urements of the direct irradiance.

19Wm-2intheUVA(31S0k4000~),andfrom35 to 249 W m- * in the total measured spectral range (2!NOA-8NlOA). Thus, the strongest diurnal varia- tion, occurring in the UVB, is a factor 6 and 7 larger than the corresponding diurnal variation in UVA and the total measured irradiance, respectively. Model calculations indicate that these diurnal variations are within the range expected on theoretical grounds. In the ultraviolet part of the spectrum (below 4000 A) the

diffusely transmitted radiation is larger than the direct component. Comparison with detailed radiative transfer calculations taking into account absorption by O,, scattering by atmospheric molecules and reftec- tion by the underlying surface reveals close agreement between measured and computed spectra.

Our observations show that there are large diurnal and day-to-day variations in the U.V. and visible radiation reaching the surface of the earth. Although

Measurements of irradiance at 78”N 1579

22 SEPT. 1967

visible radiation at the ground. In an attempt to alleviate this situation, we intend to continue these measurements and build a data base for solar radi- ation in the Norwegian sector of the Arctic.

Acknowledgements-This work was supported by the Royal Norwegian Council on Science and Humanities, and the Air Force Office of Scientific Research under Grant AFOSR-86- 0327.

REFERENCES

Bird R. E., Hulstrom R. L., Kliman A. W. and Eldring H. G. (1982) Solar spectral measurements in the terrestrial en- vironment. Appl. Opt. 21, 143&1436.

Dahlback A., Henriksen T., Larsen S. H. H. and Stamnes K. (1989) Depletion of the ozone layer and UV-doses. Photo-

/ I I I biol. Photochem. (in press). 3000 4000 6000 6000 Frederick J. E. and Lubin D. (1988) The budget of bio-

WAVELENGTH (A) logically active ultraviolet radiation in the earth-atmos- phere system. J. geophys. Res. 93, 3825-3832.

Fig. 10. Horizontal global (I) and diffuse (II) irradiance at Giese A. C. (1976) Living with Our Sun’s Utrauiolet Rays,

Tromse with a solar elevation of 20” in clear weather. Below p. 188. Plenum, New York. 4800 A the diffuse it-radiance becomes the dominant compo- Hisdal V. (1986) Spectral distribution of global and diffuse

nent of the global irradiance. solar radiation in Ny-Alesund, Spitsbergen. Polar Re- search 5, l-27.

Iqbal M. (1983) An Introduction to the Solar Radiation, pp. 380-381. Academic Press, New York.

radiative transfer calculations for clear sky conditions Rao C. R. N., Takashima T., Bradley W. A. and Lee T. Y. indicate basic agreement between observed spectral (1983) Near ultraviolet radiation at the earth’s surface:

variations and theoretical predictions, further studies measurements and model calculations. Tellus 368,

are required to unravel how changes in cloud cover, 286-293.

surface albedo and column ozone content affect the Stamnes K., Henriksen K. and 0stensen P. (1988a) Simul-

taneous measurement of UV radiation received by the spectral distribution. In view of the fact that the solar biosphere and total ozone amount. Geophys. Res. Lett. 15,

radiation is of fundamental importance to life on 784787.

earth, it is important to quantify these spectral vari- Stamnes K., Tsay S.-C., Wiscombe W. J. and Jayaweera K.

ations when studying the effects of solar radiation on (1988b) Numerically stable algorithm for discrete-ordina- te-method radiative transfer in multiple scattering and

the biosphere. Furthermore, current concerns about emitting layered media. Applied Optics 27, 2502-2509.

decreasing ozone amounts underscore the necessity to Stamnes K., Tsay S.-C. and Henriksen K. (1989) UV radi-

start measuring the amount of potentially harmful ation in the Arctic: implications of potential ozone deple-

U.V. radiation reaching the ground. There is, however, tions. (In preparation.)

an acute lack of measurements, documenting the World Meteorological Organization (1985) Global ozone

research and monitoring project. Report No. 16, Atmos- spectral, temporal and spatial variation of U.V. and pheric Ozone 1985, Vol. 1, pp. 355-362.