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Measurement of solar ultraviolet radiation at a temperate and a tropical site using polysulphone film
MEASUREMENT OF SOLAR ULTRAVIOLET RADIATION AT A TEMPERATE AND A TROPICAL SITE USING POLYSULPHONE FILM
A. Davis, B. V. HOWES, K. J. LEDBURY & P. J. PEARCE
Propellants, Explosices and Rocket Motor Establishment, Waltham Abbey, Essex, Great Britain (Received: 22 June, 1978)
ABSTRACT
Measurements of solar ultraciolet radiation ( < 3 2 0 n m ) have been made with polysulphone film at a temperate and a tropical site and compared mean monthly values of the fraction of ultraviolet in solar radiation are calculated which allow an estimate of ultraviolet dose to be made fronz a knowledge of total solar radiation. While the data demonstrate the marked effect of sun elevation on the ultraviolet fraction of solar radiation they also indicate the effect of seasonal cariation in the thickness of the ozone laver aboce the temperate site. It is also shown that whereas sunlight makes by far the major contribution to total solar radiation, skylight is of prime importance as regards ultraciolet radiation.
INTRODUCTION
Information as to the weathering stability of a material is generally obtained from exposure trials carried out in a limited number of climates. The interpretation of such information in terms of other climates is best done by comparing those features of the weather which determine the breakdown of the material. For many organic materials the ultraviolet (UV) portion of the solar spectrum is of primary importance in determining their useful lifetimes and hence worldwide data on this portion of the solar spectrum is essential. In the medical field the need for this information has long been recognised. Realisation of the importance of UV to Man as a source of vitamin D and as a cause of skin cancer and other dermatological complaints dates back to the last century, t However, over the past few years this interest has intensified and has become the concern of governments as well as dermatologists. The possibility that the nitrogen oxides in the exhausts of 121 Polymer Degradation and Stabilio" (1) ( 1979)-- C Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
122
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
supersonic aircraft could reduce the ozone in the stratosphere which protects man from excessive UV doses was the question which stimulated a wider interest in solar UV. Although. as a result of extensive research, it is concluded that the number of supersonic aircraft flying in the next decade could cause only a minor reduction in stratospheric ozone, the increasingly detailed knowledge of the chemistry of the stratosphere led to the realisation that the chlorofluorocarbons released extensively to the atmosphere by their use as propellants in aerosols, etc. might provide a significant source of chlorine atoms in the upper atmosphere and pose a more serious threat to the ozone mantle. 2'3 In addition, it has recently been argued that nitrous oxide, resulting from the biological denitrification of the soil, may have an impact on the ozone because of the increasing use of fertitisers. "~ It is seen, therefore, that because of its detrimental effects on organic polymers, be it Man or his materials, there is a need for a better definition of the UV environment of the world's major climates. To meet this requirement electronic UV sensors have been developed. The instrument designed by Robertson 5 to measure the portion of the spectrum which causes erythema ( < 3 t 0 n m ) is currently being used to monitor UV radiation continuously at six sites in the Unites States. 6 Also, the Building Research Establishment: has developed sensors which are now commercially available s and which monitor at three narrow wavebands (centred on 315+ 350 and 400 nm). Recently it has been shown that films of the thermoplastics polyphenylene oxide (PPO) + and potysulphone t° can be used to monitor solar UV radiation. Both these polymers show marked increases in UV absorption when exposed to UV and this phenomenon has been quantitatively related to dose. Polyphenylene oxide, which is sensitive to wavelengths up to about 400 nm, is being used to continuously monitor the solar UV incident on a horizontal surface at more than thirty sites throughout the world. Many of these sites have been running for more than three years and results obtained have been published. 9 Polysulphone has a high sensitivity to wavelengths less than 315 nm and is thus attractive as a monitor of the portion of solar UV which induces ervthema in human skin. t° The simplicity of the technique has enabled it to be used as a personal dosimeter and so to measure the amount of UV radiation received by different groups of people--valuable information when one is trying to relate to occupation the incidence of medical complaints which are deemed to be dependent on UV. For example, using potysulphone the doses received by laboratory workers. gardeners and hospital-bound geriatric patients were compared and related to the m a x i m u m available UV dose possible, t t A similar experiment has been carried out on a population of office workers, t 2 Other uses of the polysulphone monitor include the measurement of the UV dose falling on different parts of the body t3 and the measurement of UV at different depths in sea water.~ t Although it can be argued that electronic sensors are more suited to continuous
MEASUREMENT OF SOLAR U V RADIATION USING POLYSULPHONE FILM
123
monitoring of solar UV, the film technique, because of its simplicity, can readily be used and permits, if required, a range of situations to be monitored simultaneously and cheaply. In this paper the results of monitoring solar UV radiation ( < 3 2 0 n m ) with polysulphone film at the Propellants, Explosives and Rocket Motor Establishment (PERME), Waltham Abbey, Essex, Great Britain, for 1976 are presented and discussed along with a limited amount of data obtained at the Joint Tropical Trials and Research Establishment (JTTRE). Innisfail, Queensland, Australia, a tropical exposure site run jointly by the Ministry of Defence in Great Britain and the Australian Department of Supply.
METHOD
The preparation of the polysulphone film has already been described. ~o The basis of the method is that the absorbance of the film at 330 nm increases in proportion to the incident dose of ultraviolet radiation below 320 nm. As the sensitivity varies with wavelength, to quantify a dose of heterogeneous ultraviolet radiation such as solar UV it is convenient to express the dose in terms of an equivalent dose of monochromatic radiation--in this case 305 nm. Evidence will be presented which indicates that this equivalent dose of 305 nm radiation agrees reasonably well with the absolute dose of solar radiation below 320 nm. The procedure is to measure the absorbance of the film before and after exposure and by means of a calibration curve convert the change in absorbance (AA~30 rim) into the dose of 305 nm monochromatic radiation which would produce the same AA33 o. Films were exposed at two locations: (i) on the roof of a single storey building at PER M E which is in a semi-rural area 20 miles north east of London and (ii) 4 ft from the ground in a tropical forest clearing at JTTRE. In general measurements were made within 24 h of exposure. If films could not be measured within this period they were stored in a refrigerator prior to measurement as this eliminated a slight dark reaction. Measurement of the daily dose of global UV ( t ) - - t h a t is, the UV in skylight plus sunlight--and of the UV in skylight--that is, diffuse UV (d)--was the prime purpose of the programme. Global UV was monitored by exposing a film horizontally 7 mm above a metal plate painted matt-black. The number of films required per day varied with season: at PER M E in winter one film per day was sufficient whereas in mid-summer six were required. To reduce the number of films required, a neutral density filter in the form of a dome of perforated metal was used for a period in summer to reduce the intensity by 50 30. To obtain a measure of the diffuse component of solar UV a film was exposed horizontally under a shadow ring.~5
124
~,. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
Total global radiation (T) was measured with a pyroheliometer. For a period, the diffuse component of global solar radiation (D) was also measured at PERME, The direct component of U V - - t h a t is. the UV in sunlight--incident on a horizontal surface was estimated for PERME from the difference between global and diffuse UV. An estimate of the direct component of total radiation was obtained in a similar manner.
RESULTS AND DISCUSSION
Acerage monthly UV doses at PERME Daily measurements of global UV (t) for each month of 1976 were obtained for P ERME along with a limited number of measurements of the diffuse component of global UV. Details of these measurements are given in reference 16; in general only average daily values are presented here (Table 1, Fig. 1).
TABLE 1 PERME UV
Month
Acerage
daih" global UV (wh m ' - )
Acerage Estimate of daily acerage diffuse UV daily ( w h r m ' - ) direct UV ( wh/m'- )
Acerage daily global solar
radiation (wh/m 2)
Acerage daily diJfase solar radiation (wh/m:)
Estimate of acerage
daily direct solar radiation
(wh/m z ) January February March April May June July August September October November December
0.39 0.68 1-60 2-71 4-41 9-12 8-19 5.89 3.18 1.33 0-55 0-26
3-80 7.30 7.33 5-08 2-40 1.09
0.61 1.82 0-86 0.81 0-78 0.24
607 951 2291 3772 4612 5990 5540 4480 2586 1335 860 654
2730 2810 2640 2106
1882 3180 2900 2377
During the year the average daily UV (?) and total radiation ( ~ levels reflect the seasonal variation in the elevation of the sun. However, the wide range of daily values observed demonstrates the effect of day to day variations in the weather. There is approximately a thirty-fold increase in the average daily dose of global UV between winter and summer, The average daily dose shows a maximum in midsummer but the spring--early summer and the late summer-autumn doses are not symmetrical about this peak; readings were significantly higher for the latter period
MEASUREMENT OF SOLAR U V RADIATION USING POLYSULPHONE FILM I
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15 L
l
l
l
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125
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---,10
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5
or" J Fig. 1.
FMAMJ
J ASON 1976
D
Monthly average UV per day at PERME (temperate), O, and JTTRE (tropical), ©.
(Fig. 1). While this can be partly explained by the finer weather of this period. demonstrated by the higher levels of global total solar radiation, an additional difference between these two periods is also suggested. From the limited number of measurements made it is seen that of the two components of global UV, namely direct and diffuse, the latter is by far the greater. accounting for about 85 0/o of the total. From the limited measurements made no seasonal variation in the relative contributions of the diffuse and direct components could be detected. The UV content of total solar radiation at P E R M E While the diffuse component of global UV averages out at about 85 °o for the months May to August, the diffuse component of global total solar radiation is only about 50 ~o-That is, a unit of radiation fram the sky on average contains almost six times more UV ( < 320 nm) than a unit of direct sunlight. Table 2 and Fig. 2 show the average daily UVcontent of total solar radiation (~ 7-) falling on a horizontal surface for each month of 1976. It is seen that there is approximately a threefold increase in ?/T between winter and summer. It is also
126
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
TABLE 2
Month
Acerage UVJ~'action of global solar radiation × 103
January February March April May June July August September October November December
A cerage UVfraction of diff'use solar radiation :~ 103
Acerage U Vj?action of direct solar radiation × 10"~
1-39 2-60 2.78 2.41
0"32 0.57 0"30 0.34
0-64 0.71 0-70 0.72 0.96 1.52 1.48
1.31 1"23 1-00
0.63 0.40
4
'3
.2
I
i
I
I
I
I
I
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r
l
l
J F M A M J J A $ 0 N O
1976 Fig. 2.
Monthly average UV content of total solar radiation at PERME, O, and JTTRE. ©.
MEASUREMENT OF SOLAR U V RADIATION USING POLYSULPHONE FILM
127
noticeable that the ratios of the spring months are lower than those of the autumn months. This is probably because the ozone layer, which absorbs UV radiation up to about 330 nm, varies in thickness with season: in temperate latitudes in the northern hemisphere the thickness of the layer shows a maximum in the spring and a minimum in the autumn. This is clearly seen from the measurements made by the Meteorological Office at Bracknell (Table 3). TABLE 3 AVERAGE MONTHLY STRATOSPHERIC OZONE (cm ..~T STP). BRACKNELL, 1976
Ozone
Jan,
Feb.
),larch
April
May
June
July
Aug.
Sept.
Oct.
No~.
Dec.
0-354
0.358
0.373
0.383
0.374
0-350
0.344
0-359
0.273
0"263
0"260
0"330
There is no obvious correlation between individual daily UV and ozone measurements. This is not considered surprising as the ozone, which can var.v by as much as 30 00 in 24h, is a spot measurement. To investigate the nature of the relationship between UV ( < 3 2 0 n m ) and thickness of the ozone layer both measurements are now being made side by side at Bracknell. Average values of the UV content of global solar radiation obtained by Coblentz ~7 for Washington over an eight-year period show the same seasonal features, namely higher UV fractions in the autumn than in the spring. The i T fractions observed at P E R M E are in general higher than those observed in Washington. This is probably because polysulphone is sensitive up to about 320 nm whereas the photocell-filter combination used by Coblentz only responded to wavelengths less than 315 nm. Luckiesh ~s showed that at mid-day in Cleveland in summer the UV content of solar radiation tess than 320nm is about 3 x 103 . From polysulphone measurements made at mid-day on a clear summer's day at P E R M E a ratio o f t T o f 2.3 × 103 was obtained. Allowing for the 10 ° difference in latitude between Cleveland and P E R M E , this agreement between the two ratios suggests that a reasonable estimate of the absolute dose of solar UV radiation less than 320 nm is obtained by expressing the solar UV dose incident on the polysulphone in terms of 305 nm monochromatic radiation.
Estimation of UV From the ratios given in Table 2 it is possible to obtain, for a temperate site in the northern hemisphere, a reasonable estimate o f t for a particular month if Tis known. However, estimates of the dose on individual days based on these ratios and a knowledge of T are much less reliable for at least two reasons. First, while the average daily thickness of the ozone layer tends to vary systematically from month to month there can be wide fluctuation, of the order of + 15 ~o, from one day to the next. Secondly, as already shown, t/T is dependent on the level of T: while the
128
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
absolute amount of t is generally reduced with increasing cloud cover, i.e. reducing 7". the fraction t/Ttends to increase. The explanation is that cloud does not attenuate all regions of the solar spectrum equally. Compared with the UV and visible portions of the solar spectrum a higher proportion of the infrared portion of the solar spectrum is absorbed by cloud. For example, the relationship between t/T and T for February and June is illustrated in Fig. 3 in which the daily data are plotted. It is clear that the dependence i
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4 6 8 TOTAL rodn (k w h m -2 cl-I)
Daily UV content of total solar radiation for February. O. and June, I . 1976, at PERME.
oft/Ton Tis more pronounced in winter than in summer. No explanation can yet be offered for this difference. Figure 3 indicates that for a constant value of T t h e ratio oft/Tcan vary by as much as 100%. it is probable that much of this scatter can be attributed to variation in the thickness of the ozone layer. However, additional factors besides variations in the ozone and T levels are likely to be involved. The nature and height of any cloud cover may be important. Undoubtedly pollutants including fog, haze, etc., can contribute to variation in t and T. Barton and Robertson, -'° using a photocell which has a spectral response similar to that of polysulphone, demonstrated a relationship between UV and vertical ozone but although their analysis was restricted to relatively well defined periods. namely clear half-hour periods around mid-day, the correlation was far from ideal and suggested that other variables are involved. The highest daily t obtained in 1976 was 15-56 wh/m 2 and this was for a clear day
MEASUREMENT OF SOLAR U V
R A D I A T I O N U S I N G P O L Y S U L P H O N E FILM
129
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Diurnal variation in hourly dose o f global (t) and diffuse (d) U V solar radiation tor a clear mids u m m e r ' s d a y at P E R M E .
in mid-summer (30 June). This is just over forty times greater than the dose measured for a relatively clear day in mid-winter. For the mid-summer's day the diffuse and direct UV falling on a horizontal surface increased as the elevation of the sun increased (Fig. 4). Both reached a maximum at noon before falling off'again in the afternoon as the elevation of the sun became less. In the early morning and late afternoon the diffuse component made the major contribution to t. As Luckiesh t8 found from measurements at Cleveland in mid-summer, it is only around mid-day that the direct matches the diffuse component. The contributions ofthe diffuse and the direct components of total solar radiation for this particular day are shown in Fig. 5. While the direct component reflects the variation of sun elevation during the course of the day the diffuse component
130
a . DAVIS, B. V. HO~,'ES, K. J. LEDBURY, P. J. PEARCE
1000
7-¢..C v
t"
500 0
D
6 Fig. 5.
NOON
6
Diurnal variation in hourly dose of global (7") and diffuse (D) total solar radiation for a clear mid-summer's day at P E R M E .
remains at a relatively constant low level throughout the day and overall only contributes about 18 °,il to the global total solar radiation dose. Over the day the average fraction of UV in the diffuse component is 6 x 10 - 3 while for the direct component the fraction is 1 x 10 - 3 . The marked difference in these fractions highlights once again the important UV contribution of skylight. UP" measurentents at J T T R E and comparison with P E R M E duta
During the second half of 1976 daily UV was monitored at the Joint Tropical Testing and Research Establishment, lnnisfail, Queensland, Australia. From the limited number of measurements made (5-10 per month) an average daily dose of UV was calculated for each month. The values obtained for Innisfail (latitude 17 °S) demonstrate its tropical location (Table 4, Fig. 1). The agreement between the mid-winter value for JTTRE and the summer values for PERME (latitude 52 ~N) reflects the transitional similarity in the elevation of the sun at the two sites. At J T T R E there is a two-fold increase in global UV in passing from mid-winter to mid-summer. Although the average daily UV
MEASUREMENT OF SOLAR U V
RADIATION USING POLYSULPHONE FILM
131
TABLE 4 1976
JTRE L V
Momh
June July August September October No~ember
,4 cerage daily global UV (~ttl rtl a)
A cerage UV fraction of direct solar radiation x 103
7.0 -9.5 13.5 15.3 14.4
1-9 -2-2 2.4 2-4 2-6
dose at J T T R E in mid-summer was matched on one occasion at P E R M E (Fig. 4) it is about one-and-a-half times greater than the average daily global UV at PERME. Comparing the two sites on an annual basis the data indicate that J T T R E receives about three-and-a-half times more UV than PERME. The same two sites have been compared previously using P P O film as a means of monitoring solar UV. It was found that over a year the tropical site received about two-and-a-half times more UV' than the temperate site. The disparity in these two factors is probably because P P O film responds to.wavelengths up to about 370 nm and polysulphone is only- affected by wavelengths below about 325 nm and the shorter wavelengths suffer greater attenuation at lower sun elevations--i.e, higher latitudes. The attenuating effect of the earth's atmosphere at wavelengths < 320 nm is also demonstrated by the 50"o increase in the UV content of total global radiation between mid-winter and mid-summer at J T T R E (Table 4 and Fig. 2). There is a gradual increase in the UV fraction with elevation at lnnisfail which ~as not observed with the PER M E data. This is probably because whereas there is a marked seasonal variation in the thickness of the ozone layer at P E R M E the thickness of the layer at the tropics is fairly constant.
CONCLUSION
The results of this investigation confirm the relative importance of diffusion as opposed to direct solar U V. Thus. to maximise the effect o)'solar U\" in an exposure. trial specimens should be exposed to maximise diffuse U V - - t h a t is horizontally and not at the conventional angle of 45 ° t'acing the equator. A major objective in weathering studies on materials is to estimate relatively how long they will last in different climates. Ira material is primarily photo-degraded and has a spectral sensitivity which is significant beyond 400 rim, e.g. Kevhtr. then it is reasonable to make comparisons based on the relevant total global radiation data. However, many materials have spectral responses which only partly overlap the UV
132
A. DAVIS, B. V. HOVCES, K. J. LEDBURY, P. J. PEARCE
portion of the solar spectrum. For these materials a significant error, ~vhich would be greater the less the overlap, would be introduced in making a comparison based on total global radiation. Thus it is essential in such comparisons that where possible specific UV data should be employed. The relative stability of materials which are affected by wavelengths < 320 nm can be compared for a variety of locations by making use of the information given in Tables 2 and 4.
REFERENCES I. H. F. BLU.',I, Radiation hiology (Ed. Hotlaender. A.). Vol. II, New York, McGraw-Hill (1955). 2. M. A. A. CLVNE, Nature. 263, 28 (1976). 3. ANON.. Department of the Environment, Central Unit of Environmental Pollution. Pollution Paper No. 5. 1976, London. HMSO. 4. S. I. R.~,SOOL. Nature. 264, 115 (1976). 5. D. F. ROaERTSON,"The biological ¢[]'ects O/UV radmtion (Ed. Urbach. F.). Pergamon. Oxford, 436 (1969). 6. ANON.. Measurement of UV radiation in the United States aml comparison with skin cancer data, US Dept. of Health, National Institute of Health DH EW No. 76-1029 (1975). 7. P. B. H~,RRES..J. Sci. hist., 1(2). 1007 (1968). 8. Fleming Inst., Stevet~age, Herts. UK. 9. A. D.,vrs. G. H. W. DEANE, D. GORDON. G. V. HOWELL and K. J. LI~DBURY.J..4ppl. Polym. Sci.. 20, 1165 (1976). 10. A. DAVlS, G. H. W. DE,-XNEand B. L. DIEFEY. Nature, 261, 169 (1976). 11. A. V. J. CaALLOYER. D. CORLESS. A. DAVIS. G. H. W. DEANE, B. L. DI~EV. S. P. GUPTA and I. A. MAGNUS, Clinical and Experimental Dermatology. 1, 175 (19"7,6). 12. A. DAVIS. G. H. DE.aYE, J. F. LEACH, V. E. McLEoD and A. R. PINGSTONE. Clinical amt Experimental Dermatology, 3, 77 (1978). I3. B. L. DIFFEY and A. Daws, Phys. Med. Biol.. 22, 1014 (1977). 14. A. DAVIS. G. H. W. DE.-',NEand B. V. HOWES. in preparation. t 5. A. J. DRU~,I~,IOYD, Arch. Meteorol. Geophys. Biochem.. I, 413 (1956). 16. A. D,xvls. B. V. HOWES, K. J. LEDBURV and P. J. PEARCE, Propellant Explosives and Rocket Motor Establishment, Report No. 52. 17. W. W. COBLENTZ, Bull. Am. Met. Soc., 33, 158 (1952). 18. M. LL'CKIESH, Germicidal. erythemal and inJi'arcd energy. New York, D. Van Nostrand Co. (1946). 19. A. DAvIs. G. H. W. DEANE and K. J. LEDBUR','. Plastics and Rubber Inst. Symposium BI. 1. (June 1976). 20. I. J. B.~,RTON and D. F. ROBERTSON, Nature, 258, 68 (1975).
A. Davis, B. V. HOWES, K. J. LEDBURY & P. J. PEARCE
Propellants, Explosices and Rocket Motor Establishment, Waltham Abbey, Essex, Great Britain (Received: 22 June, 1978)
ABSTRACT
Measurements of solar ultraciolet radiation ( < 3 2 0 n m ) have been made with polysulphone film at a temperate and a tropical site and compared mean monthly values of the fraction of ultraviolet in solar radiation are calculated which allow an estimate of ultraviolet dose to be made fronz a knowledge of total solar radiation. While the data demonstrate the marked effect of sun elevation on the ultraviolet fraction of solar radiation they also indicate the effect of seasonal cariation in the thickness of the ozone laver aboce the temperate site. It is also shown that whereas sunlight makes by far the major contribution to total solar radiation, skylight is of prime importance as regards ultraciolet radiation.
INTRODUCTION
Information as to the weathering stability of a material is generally obtained from exposure trials carried out in a limited number of climates. The interpretation of such information in terms of other climates is best done by comparing those features of the weather which determine the breakdown of the material. For many organic materials the ultraviolet (UV) portion of the solar spectrum is of primary importance in determining their useful lifetimes and hence worldwide data on this portion of the solar spectrum is essential. In the medical field the need for this information has long been recognised. Realisation of the importance of UV to Man as a source of vitamin D and as a cause of skin cancer and other dermatological complaints dates back to the last century, t However, over the past few years this interest has intensified and has become the concern of governments as well as dermatologists. The possibility that the nitrogen oxides in the exhausts of 121 Polymer Degradation and Stabilio" (1) ( 1979)-- C Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
122
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
supersonic aircraft could reduce the ozone in the stratosphere which protects man from excessive UV doses was the question which stimulated a wider interest in solar UV. Although. as a result of extensive research, it is concluded that the number of supersonic aircraft flying in the next decade could cause only a minor reduction in stratospheric ozone, the increasingly detailed knowledge of the chemistry of the stratosphere led to the realisation that the chlorofluorocarbons released extensively to the atmosphere by their use as propellants in aerosols, etc. might provide a significant source of chlorine atoms in the upper atmosphere and pose a more serious threat to the ozone mantle. 2'3 In addition, it has recently been argued that nitrous oxide, resulting from the biological denitrification of the soil, may have an impact on the ozone because of the increasing use of fertitisers. "~ It is seen, therefore, that because of its detrimental effects on organic polymers, be it Man or his materials, there is a need for a better definition of the UV environment of the world's major climates. To meet this requirement electronic UV sensors have been developed. The instrument designed by Robertson 5 to measure the portion of the spectrum which causes erythema ( < 3 t 0 n m ) is currently being used to monitor UV radiation continuously at six sites in the Unites States. 6 Also, the Building Research Establishment: has developed sensors which are now commercially available s and which monitor at three narrow wavebands (centred on 315+ 350 and 400 nm). Recently it has been shown that films of the thermoplastics polyphenylene oxide (PPO) + and potysulphone t° can be used to monitor solar UV radiation. Both these polymers show marked increases in UV absorption when exposed to UV and this phenomenon has been quantitatively related to dose. Polyphenylene oxide, which is sensitive to wavelengths up to about 400 nm, is being used to continuously monitor the solar UV incident on a horizontal surface at more than thirty sites throughout the world. Many of these sites have been running for more than three years and results obtained have been published. 9 Polysulphone has a high sensitivity to wavelengths less than 315 nm and is thus attractive as a monitor of the portion of solar UV which induces ervthema in human skin. t° The simplicity of the technique has enabled it to be used as a personal dosimeter and so to measure the amount of UV radiation received by different groups of people--valuable information when one is trying to relate to occupation the incidence of medical complaints which are deemed to be dependent on UV. For example, using potysulphone the doses received by laboratory workers. gardeners and hospital-bound geriatric patients were compared and related to the m a x i m u m available UV dose possible, t t A similar experiment has been carried out on a population of office workers, t 2 Other uses of the polysulphone monitor include the measurement of the UV dose falling on different parts of the body t3 and the measurement of UV at different depths in sea water.~ t Although it can be argued that electronic sensors are more suited to continuous
MEASUREMENT OF SOLAR U V RADIATION USING POLYSULPHONE FILM
123
monitoring of solar UV, the film technique, because of its simplicity, can readily be used and permits, if required, a range of situations to be monitored simultaneously and cheaply. In this paper the results of monitoring solar UV radiation ( < 3 2 0 n m ) with polysulphone film at the Propellants, Explosives and Rocket Motor Establishment (PERME), Waltham Abbey, Essex, Great Britain, for 1976 are presented and discussed along with a limited amount of data obtained at the Joint Tropical Trials and Research Establishment (JTTRE). Innisfail, Queensland, Australia, a tropical exposure site run jointly by the Ministry of Defence in Great Britain and the Australian Department of Supply.
METHOD
The preparation of the polysulphone film has already been described. ~o The basis of the method is that the absorbance of the film at 330 nm increases in proportion to the incident dose of ultraviolet radiation below 320 nm. As the sensitivity varies with wavelength, to quantify a dose of heterogeneous ultraviolet radiation such as solar UV it is convenient to express the dose in terms of an equivalent dose of monochromatic radiation--in this case 305 nm. Evidence will be presented which indicates that this equivalent dose of 305 nm radiation agrees reasonably well with the absolute dose of solar radiation below 320 nm. The procedure is to measure the absorbance of the film before and after exposure and by means of a calibration curve convert the change in absorbance (AA~30 rim) into the dose of 305 nm monochromatic radiation which would produce the same AA33 o. Films were exposed at two locations: (i) on the roof of a single storey building at PER M E which is in a semi-rural area 20 miles north east of London and (ii) 4 ft from the ground in a tropical forest clearing at JTTRE. In general measurements were made within 24 h of exposure. If films could not be measured within this period they were stored in a refrigerator prior to measurement as this eliminated a slight dark reaction. Measurement of the daily dose of global UV ( t ) - - t h a t is, the UV in skylight plus sunlight--and of the UV in skylight--that is, diffuse UV (d)--was the prime purpose of the programme. Global UV was monitored by exposing a film horizontally 7 mm above a metal plate painted matt-black. The number of films required per day varied with season: at PER M E in winter one film per day was sufficient whereas in mid-summer six were required. To reduce the number of films required, a neutral density filter in the form of a dome of perforated metal was used for a period in summer to reduce the intensity by 50 30. To obtain a measure of the diffuse component of solar UV a film was exposed horizontally under a shadow ring.~5
124
~,. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
Total global radiation (T) was measured with a pyroheliometer. For a period, the diffuse component of global solar radiation (D) was also measured at PERME, The direct component of U V - - t h a t is. the UV in sunlight--incident on a horizontal surface was estimated for PERME from the difference between global and diffuse UV. An estimate of the direct component of total radiation was obtained in a similar manner.
RESULTS AND DISCUSSION
Acerage monthly UV doses at PERME Daily measurements of global UV (t) for each month of 1976 were obtained for P ERME along with a limited number of measurements of the diffuse component of global UV. Details of these measurements are given in reference 16; in general only average daily values are presented here (Table 1, Fig. 1).
TABLE 1 PERME UV
Month
Acerage
daih" global UV (wh m ' - )
Acerage Estimate of daily acerage diffuse UV daily ( w h r m ' - ) direct UV ( wh/m'- )
Acerage daily global solar
radiation (wh/m 2)
Acerage daily diJfase solar radiation (wh/m:)
Estimate of acerage
daily direct solar radiation
(wh/m z ) January February March April May June July August September October November December
0.39 0.68 1-60 2-71 4-41 9-12 8-19 5.89 3.18 1.33 0-55 0-26
3-80 7.30 7.33 5-08 2-40 1.09
0.61 1.82 0-86 0.81 0-78 0.24
607 951 2291 3772 4612 5990 5540 4480 2586 1335 860 654
2730 2810 2640 2106
1882 3180 2900 2377
During the year the average daily UV (?) and total radiation ( ~ levels reflect the seasonal variation in the elevation of the sun. However, the wide range of daily values observed demonstrates the effect of day to day variations in the weather. There is approximately a thirty-fold increase in the average daily dose of global UV between winter and summer, The average daily dose shows a maximum in midsummer but the spring--early summer and the late summer-autumn doses are not symmetrical about this peak; readings were significantly higher for the latter period
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D
Monthly average UV per day at PERME (temperate), O, and JTTRE (tropical), ©.
(Fig. 1). While this can be partly explained by the finer weather of this period. demonstrated by the higher levels of global total solar radiation, an additional difference between these two periods is also suggested. From the limited number of measurements made it is seen that of the two components of global UV, namely direct and diffuse, the latter is by far the greater. accounting for about 85 0/o of the total. From the limited measurements made no seasonal variation in the relative contributions of the diffuse and direct components could be detected. The UV content of total solar radiation at P E R M E While the diffuse component of global UV averages out at about 85 °o for the months May to August, the diffuse component of global total solar radiation is only about 50 ~o-That is, a unit of radiation fram the sky on average contains almost six times more UV ( < 320 nm) than a unit of direct sunlight. Table 2 and Fig. 2 show the average daily UVcontent of total solar radiation (~ 7-) falling on a horizontal surface for each month of 1976. It is seen that there is approximately a threefold increase in ?/T between winter and summer. It is also
126
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
TABLE 2
Month
Acerage UVJ~'action of global solar radiation × 103
January February March April May June July August September October November December
A cerage UVfraction of diff'use solar radiation :~ 103
Acerage U Vj?action of direct solar radiation × 10"~
1-39 2-60 2.78 2.41
0"32 0.57 0"30 0.34
0-64 0.71 0-70 0.72 0.96 1.52 1.48
1.31 1"23 1-00
0.63 0.40
4
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1976 Fig. 2.
Monthly average UV content of total solar radiation at PERME, O, and JTTRE. ©.
MEASUREMENT OF SOLAR U V RADIATION USING POLYSULPHONE FILM
127
noticeable that the ratios of the spring months are lower than those of the autumn months. This is probably because the ozone layer, which absorbs UV radiation up to about 330 nm, varies in thickness with season: in temperate latitudes in the northern hemisphere the thickness of the layer shows a maximum in the spring and a minimum in the autumn. This is clearly seen from the measurements made by the Meteorological Office at Bracknell (Table 3). TABLE 3 AVERAGE MONTHLY STRATOSPHERIC OZONE (cm ..~T STP). BRACKNELL, 1976
Ozone
Jan,
Feb.
),larch
April
May
June
July
Aug.
Sept.
Oct.
No~.
Dec.
0-354
0.358
0.373
0.383
0.374
0-350
0.344
0-359
0.273
0"263
0"260
0"330
There is no obvious correlation between individual daily UV and ozone measurements. This is not considered surprising as the ozone, which can var.v by as much as 30 00 in 24h, is a spot measurement. To investigate the nature of the relationship between UV ( < 3 2 0 n m ) and thickness of the ozone layer both measurements are now being made side by side at Bracknell. Average values of the UV content of global solar radiation obtained by Coblentz ~7 for Washington over an eight-year period show the same seasonal features, namely higher UV fractions in the autumn than in the spring. The i T fractions observed at P E R M E are in general higher than those observed in Washington. This is probably because polysulphone is sensitive up to about 320 nm whereas the photocell-filter combination used by Coblentz only responded to wavelengths less than 315 nm. Luckiesh ~s showed that at mid-day in Cleveland in summer the UV content of solar radiation tess than 320nm is about 3 x 103 . From polysulphone measurements made at mid-day on a clear summer's day at P E R M E a ratio o f t T o f 2.3 × 103 was obtained. Allowing for the 10 ° difference in latitude between Cleveland and P E R M E , this agreement between the two ratios suggests that a reasonable estimate of the absolute dose of solar UV radiation less than 320 nm is obtained by expressing the solar UV dose incident on the polysulphone in terms of 305 nm monochromatic radiation.
Estimation of UV From the ratios given in Table 2 it is possible to obtain, for a temperate site in the northern hemisphere, a reasonable estimate o f t for a particular month if Tis known. However, estimates of the dose on individual days based on these ratios and a knowledge of T are much less reliable for at least two reasons. First, while the average daily thickness of the ozone layer tends to vary systematically from month to month there can be wide fluctuation, of the order of + 15 ~o, from one day to the next. Secondly, as already shown, t/T is dependent on the level of T: while the
128
A. DAVIS, B. V. HOWES, K. J. LEDBURY, P. J. PEARCE
absolute amount of t is generally reduced with increasing cloud cover, i.e. reducing 7". the fraction t/Ttends to increase. The explanation is that cloud does not attenuate all regions of the solar spectrum equally. Compared with the UV and visible portions of the solar spectrum a higher proportion of the infrared portion of the solar spectrum is absorbed by cloud. For example, the relationship between t/T and T for February and June is illustrated in Fig. 3 in which the daily data are plotted. It is clear that the dependence i
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Daily UV content of total solar radiation for February. O. and June, I . 1976, at PERME.
oft/Ton Tis more pronounced in winter than in summer. No explanation can yet be offered for this difference. Figure 3 indicates that for a constant value of T t h e ratio oft/Tcan vary by as much as 100%. it is probable that much of this scatter can be attributed to variation in the thickness of the ozone layer. However, additional factors besides variations in the ozone and T levels are likely to be involved. The nature and height of any cloud cover may be important. Undoubtedly pollutants including fog, haze, etc., can contribute to variation in t and T. Barton and Robertson, -'° using a photocell which has a spectral response similar to that of polysulphone, demonstrated a relationship between UV and vertical ozone but although their analysis was restricted to relatively well defined periods. namely clear half-hour periods around mid-day, the correlation was far from ideal and suggested that other variables are involved. The highest daily t obtained in 1976 was 15-56 wh/m 2 and this was for a clear day
MEASUREMENT OF SOLAR U V
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129
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Diurnal variation in hourly dose o f global (t) and diffuse (d) U V solar radiation tor a clear mids u m m e r ' s d a y at P E R M E .
in mid-summer (30 June). This is just over forty times greater than the dose measured for a relatively clear day in mid-winter. For the mid-summer's day the diffuse and direct UV falling on a horizontal surface increased as the elevation of the sun increased (Fig. 4). Both reached a maximum at noon before falling off'again in the afternoon as the elevation of the sun became less. In the early morning and late afternoon the diffuse component made the major contribution to t. As Luckiesh t8 found from measurements at Cleveland in mid-summer, it is only around mid-day that the direct matches the diffuse component. The contributions ofthe diffuse and the direct components of total solar radiation for this particular day are shown in Fig. 5. While the direct component reflects the variation of sun elevation during the course of the day the diffuse component
130
a . DAVIS, B. V. HO~,'ES, K. J. LEDBURY, P. J. PEARCE
1000
7-¢..C v
t"
500 0
D
6 Fig. 5.
NOON
6
Diurnal variation in hourly dose of global (7") and diffuse (D) total solar radiation for a clear mid-summer's day at P E R M E .
remains at a relatively constant low level throughout the day and overall only contributes about 18 °,il to the global total solar radiation dose. Over the day the average fraction of UV in the diffuse component is 6 x 10 - 3 while for the direct component the fraction is 1 x 10 - 3 . The marked difference in these fractions highlights once again the important UV contribution of skylight. UP" measurentents at J T T R E and comparison with P E R M E duta
During the second half of 1976 daily UV was monitored at the Joint Tropical Testing and Research Establishment, lnnisfail, Queensland, Australia. From the limited number of measurements made (5-10 per month) an average daily dose of UV was calculated for each month. The values obtained for Innisfail (latitude 17 °S) demonstrate its tropical location (Table 4, Fig. 1). The agreement between the mid-winter value for JTTRE and the summer values for PERME (latitude 52 ~N) reflects the transitional similarity in the elevation of the sun at the two sites. At J T T R E there is a two-fold increase in global UV in passing from mid-winter to mid-summer. Although the average daily UV
MEASUREMENT OF SOLAR U V
RADIATION USING POLYSULPHONE FILM
131
TABLE 4 1976
JTRE L V
Momh
June July August September October No~ember
,4 cerage daily global UV (~ttl rtl a)
A cerage UV fraction of direct solar radiation x 103
7.0 -9.5 13.5 15.3 14.4
1-9 -2-2 2.4 2-4 2-6
dose at J T T R E in mid-summer was matched on one occasion at P E R M E (Fig. 4) it is about one-and-a-half times greater than the average daily global UV at PERME. Comparing the two sites on an annual basis the data indicate that J T T R E receives about three-and-a-half times more UV than PERME. The same two sites have been compared previously using P P O film as a means of monitoring solar UV. It was found that over a year the tropical site received about two-and-a-half times more UV' than the temperate site. The disparity in these two factors is probably because P P O film responds to.wavelengths up to about 370 nm and polysulphone is only- affected by wavelengths below about 325 nm and the shorter wavelengths suffer greater attenuation at lower sun elevations--i.e, higher latitudes. The attenuating effect of the earth's atmosphere at wavelengths < 320 nm is also demonstrated by the 50"o increase in the UV content of total global radiation between mid-winter and mid-summer at J T T R E (Table 4 and Fig. 2). There is a gradual increase in the UV fraction with elevation at lnnisfail which ~as not observed with the PER M E data. This is probably because whereas there is a marked seasonal variation in the thickness of the ozone layer at P E R M E the thickness of the layer at the tropics is fairly constant.
CONCLUSION
The results of this investigation confirm the relative importance of diffusion as opposed to direct solar U V. Thus. to maximise the effect o)'solar U\" in an exposure. trial specimens should be exposed to maximise diffuse U V - - t h a t is horizontally and not at the conventional angle of 45 ° t'acing the equator. A major objective in weathering studies on materials is to estimate relatively how long they will last in different climates. Ira material is primarily photo-degraded and has a spectral sensitivity which is significant beyond 400 rim, e.g. Kevhtr. then it is reasonable to make comparisons based on the relevant total global radiation data. However, many materials have spectral responses which only partly overlap the UV
132
A. DAVIS, B. V. HOVCES, K. J. LEDBURY, P. J. PEARCE
portion of the solar spectrum. For these materials a significant error, ~vhich would be greater the less the overlap, would be introduced in making a comparison based on total global radiation. Thus it is essential in such comparisons that where possible specific UV data should be employed. The relative stability of materials which are affected by wavelengths < 320 nm can be compared for a variety of locations by making use of the information given in Tables 2 and 4.
REFERENCES I. H. F. BLU.',I, Radiation hiology (Ed. Hotlaender. A.). Vol. II, New York, McGraw-Hill (1955). 2. M. A. A. CLVNE, Nature. 263, 28 (1976). 3. ANON.. Department of the Environment, Central Unit of Environmental Pollution. Pollution Paper No. 5. 1976, London. HMSO. 4. S. I. R.~,SOOL. Nature. 264, 115 (1976). 5. D. F. ROaERTSON,"The biological ¢[]'ects O/UV radmtion (Ed. Urbach. F.). Pergamon. Oxford, 436 (1969). 6. ANON.. Measurement of UV radiation in the United States aml comparison with skin cancer data, US Dept. of Health, National Institute of Health DH EW No. 76-1029 (1975). 7. P. B. H~,RRES..J. Sci. hist., 1(2). 1007 (1968). 8. Fleming Inst., Stevet~age, Herts. UK. 9. A. D.,vrs. G. H. W. DEANE, D. GORDON. G. V. HOWELL and K. J. LI~DBURY.J..4ppl. Polym. Sci.. 20, 1165 (1976). 10. A. DAVlS, G. H. W. DE,-XNEand B. L. DIEFEY. Nature, 261, 169 (1976). 11. A. V. J. CaALLOYER. D. CORLESS. A. DAVIS. G. H. W. DEANE, B. L. DI~EV. S. P. GUPTA and I. A. MAGNUS, Clinical and Experimental Dermatology. 1, 175 (19"7,6). 12. A. DAVIS. G. H. DE.aYE, J. F. LEACH, V. E. McLEoD and A. R. PINGSTONE. Clinical amt Experimental Dermatology, 3, 77 (1978). I3. B. L. DIFFEY and A. Daws, Phys. Med. Biol.. 22, 1014 (1977). 14. A. DAVIS. G. H. W. DE.-',NEand B. V. HOWES. in preparation. t 5. A. J. DRU~,I~,IOYD, Arch. Meteorol. Geophys. Biochem.. I, 413 (1956). 16. A. D,xvls. B. V. HOWES, K. J. LEDBURV and P. J. PEARCE, Propellant Explosives and Rocket Motor Establishment, Report No. 52. 17. W. W. COBLENTZ, Bull. Am. Met. Soc., 33, 158 (1952). 18. M. LL'CKIESH, Germicidal. erythemal and inJi'arcd energy. New York, D. Van Nostrand Co. (1946). 19. A. DAvIs. G. H. W. DEANE and K. J. LEDBUR','. Plastics and Rubber Inst. Symposium BI. 1. (June 1976). 20. I. J. B.~,RTON and D. F. ROBERTSON, Nature, 258, 68 (1975).