Measurement of the middle ultraviolet at durban

Atmospheric Enrironment Vol. 12. I1 19-l 123. @ Pergamon Press Ltd. 1978. Printedin Great

MEASUREMENT

WC14-69S1/78/0501-lll9

$02.00/O

Britain.

OF

THE MIDDLE DURBAN

ULTRAVIOLET

AT

C. J. KOK, A. N. CHALMERS* and F. HENGSTBERGER National Physical Research Laboratory, C.S.I.R., P.O. Box 395, Pretoria, Oool South Africa (First receiued 27 June and infinalform

20 August 1977)

Abstract-A project to measure the U.V.and visible spectral irradiance of global and diffuse daylight and direct sunlight over a one year period was undertaken at Durban. Special attention was given to the 295 to 315 nm region which is of biological interest. One of the goals of the project was to establish how well measured values agree with those obtained semi-empirically by computation. In spite of the many variables and some uncertainties, concerning the different parameters, fairly good agreement has been obtained with the results of Dave and others. -

possible to do such measurements all over the world. It would therefore be ideal if a theoretical or empirical model(s) could be found which would yield accurate computable values for all places on earth and for all possible atmospheric conditions. Some excellent work has been done in this field, but direct measurements made at as many different locations as possible all over the globe are necessary to prove general validity. To our knowledge very few measurements, especially concerning the U.V.spectrum, have been made ,in the Southern Hemisphere and at places relatively close to the equator. The Pretoria measurements were largely concentrated on the visible part ofthe spectrum (Winch et al., 1966), but an attempt has been made to report U.V.spectral irradiance values for air masses 1 and 2 (Boshoff and Kok, 1969; Kok, 1972a,b). For shorter wavelengths (< 350 nm) reasonably good agreement was obtained with the values of Green et al. (1974) for air mass 2, but for air mass 1 the Pretoria values in this region below 330 nm appear to be low. It was therefore decided to make more measurements of a similar nature in South Africa, but this time at sea level, and to give special attention to the U.V.region. The site used was the roof of the Electrical Engineering building of the University of Natal in Durban. It has an unobstructed view down to the horizon in all directions. Durban is at 30” latitude and 30” longitude and is at sea level.

INTRODUCTION

irradiance of global and diffuse skylight was measured at Pretoria (Winch et al., 1966) for about 7 months during 1964-65. The spectral region involved was 300-775 nm. These results, when normalized at 560nm for reasons of direct comparison, showed average values for the ultraviolet which were considerably higher than some of those obtained in Europe and North America from direct experimental measurements (Henderson and Hodgkiss, 1964; Tarrant, 1968; Judd et al., 1974). Some of the reasons for these differences are : (i) The average ozone content could be considerably lower for Pretoria (normally < 3 atmosphere mm). (ii) The altitude (1400 m) and latitude (26”) of Pretoria would give a considerably lower average air mass throughout the year than for places at sea level and at higher latitudes. (iii) Aerosol concentration and albedo could also influence the results. In spite of all these factors the above mentioned differences were still considered unrealistically high. The most probable explanation for the big differences in the ultraviolet values was the different methods of sampling; in some cases flat receivers were used in tilted orientations to reflect/transmit the sky radiation into the spectrum analyser. This could lead to serious errors, especially at shorter wavelengths because part of the sky, with relatively high shortwave radiation, is completely or partially eliminated. In addition, flat surfaces are normally not good cosine corrected receivers. For this reason integrating spheres with good cosine correction were used for both the Pretoria and Durban measurements. Direct experimental measurements of this kind are difficult to perform accurately and it would be imThe spectral

APPARATUS The

*A. N. Chalmers is with the Department of Electrical Engineering, University of Natal, King George V Ave, Durban.

apparatus was designed to measure the spectral irradiance ofuv. and visible daylight received on a horizontal surface obeying Lambert’s cosine law. Measurements were made for global radiation, diffuse skylight and south sky. A schematic diagram of the layout-of the apparatus is shown in Fig. 1. A is a 150 W quartz-iodine spectral irradiance sub-standard lamp which was calibrated at regular intervals against an NRC and an Eppley spectral irradiance standard. B is a 250mm dia. integrating sphere having a 25 mm dia. collecting aperture. The sphere could be turned round a horizontal axis to collect either sky radiation or

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A

B

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CHALMERS

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Fig. 1. Schematic diagram of the measurement equipment. A : Standard lamp; B: Integrating sphere; C: Radiation screen ; D: Monochromator; E : Photomultiplier; F: d.c. Power supply ; G : Digital voltmeter; H : Wavelength discs; I : Micro-switches.

and F. HENGSTBERGER radiation from.the comparison or standard lamp. The part of the sphere wall opposite the entrance slit was screened so that no radiation from first reflection could enter the monochromator. Tests with a goniophotometer showed that the sphere obeyed the cosine law to within about 3%. The internal surface ofthe sphere was coated with BaSO, paint which gave high reflectance in the U.V.and visible. For low irradiances a second 250mm dia. sphere with a 50mmdia. receiving aperture was used. M~surements showed that the latter sphere obeyed the cosine law almost as closely as the 25 mm aperture one. C is a radiation screen to shield the north sky when south sky measurements are being made. A Carl Leiss double monchromator (D) with quartz prisms was used as dispersing system. Over the wavelength range in which readings were taken (295-775 nm), the dispersion of the monochromator varies by a factor twelve, being greatest in the U.V.The slits were set to yield a bandpass of approx 1 nm at 295 nm. A camera shutter was mounted immediately in front of the entrance slit of the monochromator. This shutter was closed to measure the dark current of the photomultiplier immediately prior to and after a set of measurements. A small circular radiation screen (mounted on a thin stem) having a dia. just bigger than the receiving aperture of the sphere was placed at about 50 cm from the sphere aperture to shield direct radiation from the sun when measurements were made on diffuse sky. The detector, E, is an EM1 955SQ photomultipIier (PMT) with quartz window. F is a stabilized dc power supply (Fluke Model 412B). A digital voltmeter, G, type DM2020, was used for measuring the signal from the PMT in terms of the voltage across a 100 kti load resistance. Semi-automatic equipment

Fig. 2. Global spectral irradiance in the erythema region as measured on a horizontal plane for cloudless conditions and different solar zenith angles.

Measurement of middle ultraviolet at Durban in the form of three perspex discs with radial studs (H) and three micro-switches I, was installed to command readout signals to the DVM at discrete wavelengths. Because of the extreme variations in temperature, the PMT housing and monochromator were water cooled in addition to lagging with polystyrene. A reflecting aluminium screen was mounted on top of the apparatus. OBSERVATIONS Measurements on the spectral irradiance of daylight were started in April 1976 and continued until May 1977 with some stoppages due to breakdowns and holiday periods. During the active periods, sets of measurements were made one to two times per day on an average of three days per week. Each set of observations comprised calibration with the spectral irradiance standard at every 1 nm from 295 to 315 nm, at every 5 nm from 320 to 700 nm and at 10 nm intervals up to 770 nm. Similar sets of measurements were made for global, diffuse sky and south sky when circumstances allowed. Each set of measurements occupied about 5-6min. Fortunately, atmospheric conditions were usually quite stable and it was assumed that very little change would occur during a single run. These measurements were taken at different times during the day throughout the seasons to yield values for various sun heights. During such measurements notes were made of the time, cloud cover, visibility, degree of pollution and other relevant information on weather conditions. RESULTS AND DISCUSSlON

Although measurements were made for the U.V.and visible regions of the spectrum, this paper will deal

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only with the U.V.and in particular the erythema region which is of biological interest. Figure 2 shows typical global spectral irradiance curves for global radiation in the 295-315 nm region on a horizontal plane for a few solar zenith angles. The effect of some of the Fraunhofer Fe lines, e.g. those round 302, 305 and 3 10 nm, can clearly be seen. No attempt was made to fold these into the “erythema curve” as it appears as if different people use different erythema curves. Figures 3 and 4 show the change of global and diffuse sky irradiance in the 295-315 nm band with air mass for cloudless conditions (crosses). The black dots represent values for low cloud cover (<2/10). Figures 5 and 6 show a comparison of the Durban results for global and diffuse sky irradiances on a horizontal plane in the 295-315 nm band with those of Dave and Halpern (1976) for approximately similar ozone content. Unfortunately, the ozone values for Durban are not measured on a routine basis; it can, however, be assumed from routine ozone measurements at Pretoria (which is 26” South and 28” East), that the values for Durban would vary between 2.5 and 3 atmosphere mm. The graphs for the values of Dave and Halpern are for fixed ozone amounts (3 atm. mm) and albedos (0 and 0.3) while for the Durban results these two parameters would vary throughout the seasons, which make direct comparisons difficult. Even so, it can be seen that although the graphs have somewhat different shapes, they lie quite close to each other. The differences in shapes could partly be attributed to the seasonal variation of the ozone content and to a lesser extent, the variation of albedo for the Durban results. Experimental accuracies are limited by the follow-

Fig. 3. Variation of global irradiance in the 295-315 nm band on a horizontal surface with air mass.

C. J. KOK,

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CHALMERS and F. HENGSTBERGER

AR YASS

Fig. 4. Variation

of diffuse sky irradiance on a horizontal

factors : (i) Daylight irradiance values are relatively low at wavelengths below 330nm and are therefore difficult to measure spectroradiometrically. (ii) The accuracies of the calibration values of spectral irradiance standards are not very high at shorter wavelengths ( -c 350 nm). (iii) Incorrect sampling may give rise to errors. Although integrating spheres might be superior to other kinds of receivers, they also have shortcomings which could cause errors. (iv) Because the apparatus is exposed to the elements, extreme ing

plane in the 295-315 nm band with air mass.

temperatures have to be dealt with. In spite of watercooling and lagging, it is difficult to keep temperatures constant to better than 2-3°C through the day ; temperature variations have an influence on the sensitivity of the photomultiplier and could also influence the wavelength accuracy. (v) It is difficult to obtain “standard values” because of the great many variables and uncertainties to be considered, e.g. changes in ozone content, albedo, aerosol concentrations, cloud cover, etc.

SO-

SOLAR

ZENITH

ANGLE

Fig. 5. Comparison of global irradiance in the 295-3 15 nm band as measured at Durban with that computed by Dave and Halpem for different solar zenith angles. A : Dave and Halpem : 0, concentration 3 atm. mm. Albedo (R) = 0.3. B : Durban. C: Dave and Halpem : 3 atm. mm Os, R = 0.

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Measurement of middle ultraviolet at Durban

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obtained models.

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Acknowledgements-We

wish to thank the University of Natal for -permission to use the roof of the Electrical Enaineerine Building. We are also indebted to R. Turner for heliful dis&ssion and J. Turner for his assistance with the computing. REFERENCES Boshoff M. C. and Kok C. J. (1969) Spcctroradiometric measurements of ultraviolet radiation in daylight at Pretoria. Plastics, Paint, Rubber, March/April, 16-17. Dave J. V. and Halpern P. (1976) Effects of changes in ozone amount on the ultraviolet radiation received at sea level of a model atmosphere. Atmospheric Environment 10, SOLAR

ZENITH

ANGLE

Fig. 6. Similar to Fig. 5 but for diffuse sky irradiance.

It would therefore be unreasonable to claim accuracies of better than about 10% for measurement in the middle U.V.region below 330nm

CONCLUSIONS

There is a fairly close general agreement between the computed values of Dave and Halpern (and others) for the 295-315 nm region and the present experimental results. It therefore appears as if a reasonable amount of confidence can be placed on computed results

547-555.

Green A. E. S., Sawada T. and Shettle E. P. (1974) The middle ultraviolet reaching the ground. Photochem. Photobiol. 19, 251-259. Henderson S. T. and Hodgkiss D. (1964) The spectral energy distribution of davlinht. J. Phvs. D: aDDI. Phvs. 15. . _ 947-952.

Judd D. B., MacAdam D. L. and Wyszecki G. (1964) Spectra1 distribution of typical daylight as a function of correlated color temperature. J. opt. Sot. Am. 54, 1031-1039. Kok C. J. (1972b) Spectral irradiance of daylight for air mass 2. J. Phvs. D: appl. Phvs. 5, L85-88. Tarrant A: W. S. ii968) The spectral power distribution of J. Phys. D: appl. Phys. 5, L85-88. Tarrant A. W. S. l(968) The spectral power distribution of daylight. pans. I/h& En&. Sot. i3, 75-82. Winch G. T.. Boshoff M. C.. Kok C. J. and du Toit A. G. (19661 Spectrorahiometric and calorimetric characteristics ~0; daylight in the Southern Hemisphere: Pretoria, South Africa, J. opt. Sot. Am. 56.456-464.