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UV-B radiation at Penang
Atmospheric Research 51 Ž1999. 141–152
UV-B radiation at Penang Mohammad Ilyas ) , Arunasala Pandy, Syed Idris Syed Hassan Astronomy and Atmospheric Science Research Unit, UniÕersity of Science Malaysia, 11800 Penang, Malaysia Received 12 March 1998; received in revised form 6 August 1998; accepted 21 January 1999
Abstract A new series of measurements including global and UV radiation was initiated in 1994 at Penang. These high quality data were used to study diurnal and seasonal variations. The mean daily total UV-B and global radiation is about 1.43 = 10 4 J and 1.77 = 10 7 J, respectively. Maximum radiation values for both the UV-B and global radiation are received in March and September. The daily total global radiation shows a bigger seasonal variation than UV-B radiation. On clear days, the erythemal UV radiation flux is in the high or extreme range for about 5 h beginning at around 1030 hours throughout the year. Penang Ž5.38N. and Natal Ž68S. are found to exhibit similar daily maximum erythemal UV irradiance values. A simple empirical relationship for the noontime UV-B flux and global irradiance in the equatorialrtropical regions has been established. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Radiation; Ultraviolet; Global; Erythemal; Equator
1. Introduction Atmospheric ozone is the primary absorber of solar UV-B radiation. Recent changes to the stratospheric ozone levels have attracted strong interest from the scientific and environmental communities as well as from the policy makers. Ozone column amount in the tropics is lower than in the mid and higher latitudes. As a result, the tropical zone is richer in the shorter wavelength of the UV spectrum. Enhanced UV-B radiation at the ground level has the potential to cause adverse biological and environmental effects ŽUNEP, 1989.. Long-term exposure of human skin to UV radiation could induce skin cancer Žbasal and squamous cell carcinoma., cause ocular effects Žcataract, photokeratitis, lens capsule deformation and ocular melanoma. )
Corresponding author
0169-8095r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 Ž 9 9 . 0 0 0 0 5 - 8
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M. Ilyas et al.r Atmospheric Research 51 (1999) 141–152
and decrease body immune functions ŽHenriksen et al., 1990; Kripke, 1992.. Tremendous variability exists among plants in their sensitivity to UV-B radiation. Various responses have been reported including changes in leaf secondary chemistry Žflavonoid accumulation., alterations in leaf anatomy and morphology, reductions in net carbon assimilation capacity Žphotosynthesis. and changes in biomass allocation and growth ŽSullivan, 1992.. On aquatic ecosystems, experiments have demonstrated that UV-B radiation could cause damage to fish larvae and juveniles, shrimp larvae, crab larvae and plants essential to the marine food web ŽWorrest, 1992.. The amount of UV-B radiation received at the ground level depends on various temporal, spatial and meteorological factors such as time of day, season, latitude, altitude, clouds, surface albedo, ozone, air particulates and aerosols ŽFrederick et al., 1989.. Any small increases in UV-B flux could be offset by other competing factors. In fact, some studies ŽBerger and Urbach, 1982; Scotto et al., 1988. have shown lower levels of UV-B radiation reaching the ground. These decreases have been attributed to scattering and absorption by pollutant gases and dust particles ŽGrant, 1988., and by tropospheric ozone. In urban areas, pollutants like NOx and hydrocarbons, through photodissociation by the absorption of UV radiation, produce increasing amounts of atomic oxygen, thus increasing the tropospheric ozone levels. Tropospheric ozone is generally less than 15% of the total ozone ŽWang and Lenoble, 1994.. According to Bruhl and Crutzen Ž1989., tropospheric ozone absorbs scattered radiation more effectively than the direct beam. However, recent studies have shown increases in UV-B radiation at the surface level ŽBlumthaler and Ambach, 1990; Kerr and McElroy, 1993; Zheng and Basher, 1993; Feister and Grewe, 1995.. During the passage of the Antarctic ozone hole over Punta Arenas ŽChile. in October 1992, the erythemal UV radiation increased from about 70 to 180 mW my2 ŽKirchhoff et al., 1997b.. Although solar UV-B irradiance data collection for the higher latitudes have been carried out intensively at many locations for many years now, it is not the same for the equatorialrtropical regions. At the tropical latitudes, the erythemally-weighted irradiance far exceeds that incident at higher latitudes ŽFrederick and Erlick, 1994.. At the Antarctic latitudes, even during the occurance of the ozone hole, direct transmission of UV radiation of wavelength higher than 290 nm remains lower than at the tropics ŽDavies, 1993.. At Penang, measurements of surface level solar ultraviolet radiation were started in 1978 ŽIlyas and Barton, 1983; Ilyas et al., 1988; Ilyas, 1993.. In this paper, we present the diurnal and seasonal variations of solar UV-B radiation as measured at Penang Ž5.348N, 100.308E. during a new series of measurements initiated in 1994. The UV data during the first 2-year period ŽSeptember 1994 to August 1996. have been used in this study.
2. Experimental The site area is situated in an urban environment close to the sea at an elevation of about 50 m above sea level. Penang has an equatorial climate and its humidity is high
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throughout the year, about 85% ŽIlyas et al., 1981.. The solar radiation measuring instruments were mounted on a raised platform on the tower block at the Astronomy and Atmospheric Science Research Unit ŽAASRU. building at the University of Science Main Campus, Penang Ž5.348N, 100.308E.. The solar UV-B radiation was measured using a UVB-1 ultraviolet pyranometer ŽYankee Environmental Systems, 1991.. The instrument measures global Ždirect and scattered. broadband ultraviolet irradiance of wavelengths between 280 and 330 nm from the entire hemisphere of the sky. Coloured glass filters and a UV-B sensitive phosphor are used to block all of the sun’s visible light and convert the UV-B light into green light. The intensity of the green light is then measured by a solid state photodiode and the instrument output is a 0–5 V dc signal. The instrument output signal Žin volts. can be converted to the effective UV-B irradiance Ž280–330 nm. using a calibration factor of 0.5076 W my2 Vy1 Žor 1.97 V s 1 W my2 .. As shown in Fig. 1, the spectral response of the instrument is very similar to the International Commission of Illumination ŽCIE. human erythemal action spectra ŽDi-
Fig. 1. Relative spectral response of the UVB-1 instrument compared with the CIE ŽDiffey. and Parrish erythemal action spectra.
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chter et al., 1993.. It must be noted that the spectra of any broadband pyranometer varies strongly with ozone levels, aerosols and solar zenith angle. In the UV-B range, the spectral response of the UVB-1 instrument drops at the same rate as the CIE action curve. However, due to the shift of the UVB-1 instrument spectrum toward the longer wavelength by 10 nm, it is less sensitive to changes in ozone. The spectral error of the instrument will also change with the solar zenith angle. This is due to the change of the UV portion of the solar spectrum with zenith angle at the earth’s surface. However, this error is not expected to be more than 2% for effective UV-B irradiance and 4% for erythemal irradiance ŽDichter et al., 1993.. The UVB-1 instrument employs thermally regulated optics that eliminates temperature dependent wavelength problems. Calibration of the instrument is done at half-yearly intervals on clear days using a reference unit. At the time of installation, the difference between the instrument and reference unit was 1.8%. Stability of the instrument for the first year was very good, as the difference from the reference unit came up to only 2.1% at the end of its first year in operation. At the end of the second year in operation this difference increased slightly to 3.8%. These differences are not included in the analysis of the UV-B values. The global Ždirect and diffuse. solar radiation was measured using a CM-11 Pyranometer ŽKipp & Zonen Delft. which has a spectral response with 95% points over 335–2200 nm, and 50% points over the wavelength range 305–2800 nm. The UV-B and global irradiances are sampled every 5 s and averaged at the end of 30-min intervals. The irradiances were measured from about 1 h before sunrise Ž0600 Malaysian Standard Time. to about 1 h after sunset Ž2000.. All measurements are of radiation incident on a plane horizontal surface. The sensors were connected to a Datalogger and the data is retrieved regularly using a Personal Computer. The same sensors were used throughout the measurement period. The sensor domes were cleaned on a regular basis to prevent soiling which could seriously affect the radiation data.
3. Results The seasonal and diurnal variations for the UV-B and global radiation are shown in Fig. 2. The maximum flux is received between 1230 and 1300 Malaysian Standard Time. For each type of radiation, in the month of September Penang receives the lowest maximum radiation level, the highest radiation level being in March; this represents an increase from September to March of about 27% for the UV-B radiation and 50% for the global radiation. The lower maximum flux in September and October were influenced by the occurrence of haze and increased cloud cover. The bigger seasonal variation shown by the total global radiation means that it is more strongly attenuated by clouds. The absorptivity of the global radiation increases in the presence of clouds due to multiple scattering which increases the photon pathlength. According to Forster Ž1995., UV radiation is not absorbed much by water or ice clouds. Whenever cloud and pollution particles are present, UV-B radiation undergoes strong Mie scattering in the forward direction. The effect of clouds on the UV irradiance
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Fig. 2. Ža. Seasonal diurnal variation for measured global UV-B radiation at Penang Ž5.348N, 100.38E. for the period from September 1994 to August 1996. The curve is symmetrical about solar noon. Žb. Seasonal diurnal variation for measured global radiation at Penang. The curve broadens after solar noon due to attenuation by clouds.
depends on its amount. Feister and Grewe Ž1995. have shown that at Potsdam ŽGermany., the presence of lower than normal cloud amount further enhanced the biologically effective solar UV irradiance at the surface.
4. Discussion Long term cloud cover data at Penang ŽIlyas et al., 1981. shows lower cloud cover in February and March, and higher cloud cover in September and October. The maximum average daily total UV-B radiation received at Penang is in April Ž1.74 = 10 4 J. with the minimum in November Ž1.36 = 10 4 J.. For the total global radiation, the maximum average daily total is in March Ž2.19 = 10 7 J. and the minimum in September Ž1.57 = 10 7 J.. The UV-B radiation data appears to be very symmetrical around the local solar noon. Cloud variability could cause large temporal changes in the instantaneous irradiance. The total global radiation curve shows a broadening after the solar noon. In
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tropicalrequatorial regions, clouds normally build up in the afternoon. This implies that clouds attenuate the UV-B lesser than global irradiance. Even though we have measured the basic UV-B irradiance as presented earlier, the practical consideration requires us to relate this data to erythemal UV-B dosage for the McKinlay–Diffey spectrum. This would allow us to evaluate the degree of danger to human health due to the UV-B radiation as well as comparisons with measurements made at other locations which are generally presented in erythemal dosages. In order to reduce our data to Erythemal Dosage, we can establish the direct inter-conversion between UV-B irradiance and Erythemal Dosage with the help of calibration information given by the manufacturer as follows. Effective UV-B irradiances 0.5076 W my2 = signal in volts Erythemal Dosages 0.141 W my2 = signal in volts
Ž i. Ž ii .
Hence, for the same signal we can write, Erythemal dosage
0.141
s 0.27778 Effective UV-B irradiance 0.5076 For zenith angles smaller than 658, the accuracy of the calibration factor is within 4% ŽYankee Environmental Systems, 1991.. Bodhaine et al. Ž1998. have shown that the calibration of the instrument depends strongly on total ozone for smaller solar zenith angles ŽSZA. Ž- 658. and also a strong SZA dependence at larger SZA’s. According to the manufacturers, the Model UVB-1 pyranometer was subjected to three types of absolute calibration, details of which are available in the instruction manual. The calibration was done for total ozone values of about 322 Dobson units ŽDU.. The mean daily total ozone for 1994 as measured by the Total Ozone Mapping Spectrometer ŽTOMS. on the satellite Meteor-3 is 256 DU for Petaling Jaya, Malaysia, Ž3.108N, 101.658E. which is close enough to Penang. Measurements by Bodhaine et al. Ž1997. at Mauna Loa ŽHawaii. with a Yankee UVB-1 broadband pyranometer suggest that the calibration as provided by the manufacturer was within about 2% of spectroradiometer measurements for effective UV-B irradiance. Errors as high as 10% can occur if the effects of total ozone are not taken into account in the calibration of the instrument ŽBodhaine et al., 1998.. However, this error is expected to be very small for low latitudes due to the small seasonal variability in total ozone. Using the Canadian UV Index model ŽWilson, 1993., where a UV Index value of 1 represents approximately 25 mW my2 of erythemal UV flux reaching a horizontal plane on the ground, we found that on clear days the erythemal flux at Penang is in the high or extreme for 5 h a day, between 1030 and 1530 Malaysian Standard Time ŽFig. 3.. During this time of the day, many outdoor activities are carried out. Distribution of the predicted UV Index will enable individuals to take precautionary measures Žusing hats, proper clothing, sunglasses, sunscreens.. Comparison of the Penang erythemal UV radiation data with another equatorial site which has similar climatic and weather conditions, Natal ŽBrazil. Ž68S, 358W., shows that there is not much difference between them in the daily maximum readings with both sites showing an average of 0.30 W my2 ŽKirchhoff et al., 1997a. ŽTable 1.. The Natal s
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Fig. 3. Variation of the erythemal UV-B flux with time of day on a typical clear day in March. The UV Index is based on the Canadian AES scale.
data used is the CIE weighted UV-B radiation integrated for each day. The daily maximum erythemal UV irradiance at Stockholm Ž59.48N. in July is 0.14 W my2 ŽWester, 1992. and at Leba Ž54845X N, 17832X E., a Baltic coastal city in Poland, the maximum during the summer months is 0.12 W my2 ŽLitynska, 1994.. This means that tourists from the high latitudes will be exposed to more than twice the erythemal UV irradiance they receive at home. It is therefore of utmost importance that sun seekers from the high latitudes be made aware of overexposure to solar UV radiation.
Table 1 Daily maximum erythemal UV irradiance at different equatorial and high latitudes Location
Latitude
Daily maximum erythemal UV flux ŽW my2 .
Penang ŽMalaysia. Natal ŽBrazil. Leba ŽPoland. Stockholm ŽSweden. Punta Arenas ŽChile.
5.38N 6.08S 54.88N 59.48N 53.08S
0.30 0.30 0.12 0.14 0.07
At Punta Arenas ŽChile. the value was 0.18 W my2 during the occurance of the ozone hole in October 1992. This value is only half the flux received at the equatorial region.
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We also present the daily total UV-B and global solar radiation for the experimental period under discussion ŽFig. 4.. The daily total radiation is calculated by multiplying the daily summation of the half hourly irradiance by 1800 s. Large variations are observed for the daily totals, due mainly to cloud, tropospheric pollutants and seasonal ŽEarth–Sun distance. factors. The upper boundary represents the maximum radiation that would be received on a clear sky day. The total ozone levels shown in Fig. 4a are measurements from TOMS on Meteor-3. It can be seen that the total ozone levels has two peaks. A lower peak in April and a higher one in late September and early October. The higher maximum in September is due to the equinox effect ŽTeh, 1998.. The mean daily total UV-B radiation during the measurement period is 1.43 = 10 4 J and for global radiation is 1.77 = 10 7 J. The daily total radiation reaches maximum
Fig. 4. Ža. Seasonal variation of the total column ozone for the period 1991–1994 as measured by TOMS. Žb. Daily total global solar UV-B radiation as measured at Penang. The maximum radiation is received in March and September. Žc. Measured daily total global radiation at Penang. The large variations are mainly due to clouds.
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values in March and September with a value of 2.3 = 10 4 J and 2.8 = 10 7 J for the daily total UV-B and global radiation, respectively. This corresponds to the time of the year when the path length of the solar radiation through the atmosphere is at its shortest. The mean daily total UV-B and global radiation during September and October would have been higher if not for the heavier cloud cover, increased ozone levels and perturbation due to the local hazy period Žfrom mid-September to mid-October 1994. believed to be caused by widespread forest fires in Sumatra and Kalimantan ŽIndonesia. ŽSTAR, 1994.. During the haze period, both types of radiation were reduced appreciably. We found that the daily total UV-B radiation was reduced by 12% and the global radiation by 9% compared to the 1995 values during the same period. The enhanced absorption of the UV-B radiation was probably due to increased levels of tropospheric NO x and ozone, through photodissociation and oxidation processes ŽIlyas et al., 1996.. The relationship between 1r2 hourly UV-B and global radiation about local noon time Žbetween 1300 and 1330. for all sky conditions are shown in Fig. 5. At Penang, we find that close to the solar noon, usually there are almost no clouds or only some broken
Fig. 5. Relationship between the measured noontime irradiance of UV-B and global radiation. A strong linear correlation can be observed with a mean noontime UV-B to global irradiance ratio of 1.05=10y3 .
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and scattered clouds. There appears to be a strong linear relationship between the two types of radiation with most of the data being within the 95% confidence interval. The stray points are caused by the occasional clouds and perturbations on ozone levels. The mean noontime UV-B to global irradiance ratio is 1.05 = 10y3 . Regression of the UV-B irradiance on the corresponding global irradiance gives a linear relation of the form: I v s aIg q b where Iv and Ig are the UV-B and global noontime irradiance, and, a s 0.00105, b s 0.05. Although the regression coefficients may vary slightly for measurements at different tropicalrequatorial locations, it could form a good basis for estimation. In places where facilities for UV-B radiation measurements are not available, this kind of empirical relationship could be used for general risk-assessments, e.g., in determining the noontime UV index using global radiation measurements.
5. Conclusions The maximum daily total UV-B and global radiation values are highest in March and September with mean values of 1.43 = 10 4 J and 1.77 = 10 7 J, respectively. Seasonally, the total global radiation shows a larger variation than UV-B radiation due to its stronger attenuation by clouds. Based on the Canadian UV Index scale, the surface level solar erythemal radiation is high or extreme for about 5 h, beginning at 1030 Malaysian Standard Time. A good linear relationship exists between the UV-B and global irradiance which enables estimation of UV-B flux in tropicalrequatorial areas where facilities for UV-B measurements are not available but global radiation flux data are available. The erythemal UV radiations are of the same magnitude at Penang and Natal ŽBrazil., both equatorial cities but in different hemispheres.
Acknowledgements The authors wish to thank the staff of the Astronomy and Atmospheric Science Research Unit who were involved in the maintenance of the solar radiation measuring system. Part of this work is supported through an IRPA ŽRM 6. funded project under the Ministry of Science, Technology and Environment, Malaysia.
References Berger, D.S., Urbach, F., 1982. A climatology of sunburning ultraviolet radiation. Photochem. Photobiol. 35, 187–192. Blumthaler, M., Ambach, W., 1990. Indication of increasing solar ultraviolet-B radiation flux in alphine regions. Science 248, 206–208.
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Bodhaine, B.A., Dutton, E.G., Hofmann, D.J., McKenzie, R.L., Johnston, P.V., 1997. UV measurements at Mauna Loa: July 1995 to July 1996. J. Geophys. Res. 102, 19265–19273. Bodhaine, B.A., Dutton, E.G., Hofmann, D.J., McKenzie, R.L., Johnston, P.V., 1998. Calibrating broadband UV instruments: ozone and solar zenith angle dependence. J. Atmos. Oceanic Technol. 15, 916–925. Bruhl, C., Crutzen, P.J., 1989. On the disproportionate role of tropospheric ozone as a filter against solar UV-B radiation. Geophys. Res. Lett. 16 Ž7., 703–706. Davies, R., 1993. Increased transmission of ultraviolet radiation to the surface due to stratospheric scattering. J. Geophys. Res. 98 ŽD4., 7251–7253. Dichter, B.K., Beaubien, A.F., Beaubien, D.J., 1993. Development and characterization of a new solar ultraviolet-B irradiance detector. J. Atmos. Oceanic Technol. 10 Ž3., 337–344. Feister, U., Grewe, R., 1995. Higher UV radiation inferred from low ozone levels at northern mid-latitudes in 1992 and 1993. Global and Planetary Change 11, 25–34. Forster, P.M. de F., 1995. Modeling ultraviolet radiation at the earth’s surface: Part I. The sensitivity of ultraviolet irradiances to atmospheric changes. J. Appl. Meteor. 34, 2412–2425. Frederick, J.E., Erlick, C., 1994. The ultraviolet radiation environment of the biosphere. In: Report of the Workshop on UV-B for the Americas, WMO Global Atmosphere Watch, No. 11, Annex D, 11. Frederick, J.E., Snell, H.E., Haywood, E.K., 1989. Solar ultraviolet radiation at the earth’s surface. Photochem. Photobiol. 50, 443–450. Grant, W.B., 1988. Global stratospheric ozone and UV-B radiation. Science 242, 111. Henriksen, T., Dahlback, A., Larsen, S.H.H., Moan, J., 1990. Ultraviolet radiation and skin cancer. Effect of an ozone layer depletion. Photochem. Photobiol. 51, 579–582. Ilyas, M., 1993. UV radiation in the tropics. In: Frontiers of Photobiology. Excerpta Medica, Amsterdam, pp. 523–526. Ilyas, M., Barton, I.J., 1983. Surface dosage of erythemal solar ultraviolet radiation near the equator. Atmos. Environ. 17 Ž10., 2069–2073. Ilyas, M., Pang, C.Y., Chan, A.W., 1981. Effective temperature comfort indices for some Malaysian towns. Singapore J. Trop. Geog. 2, 27. Ilyas, M., Damla, A.A., Tajuddin, M.R., 1988. Medically important solar ultraviolet A—radiation measurements. Int. J. Dermatol. 27, 315–318. Ilyas, M., Pandy, A., Jaafar, M.S., 1996. The 1994 Malaysian haze episode: effect on the surface level solar ultraviolet radiation. In: Proc. Malaysian Science and Technology Congress, 22–25 August 1995, Kuala Lumpur, pp. 113–118. Kerr, J.B., McElroy, C.T., 1993. Evidence for large upward trends of ultraviolet-B radiation linked to ozone depletion. Science 262, 1032–1034. Kipp & Zonen Delft. Instruction Manual—Pyranometer CM 11, Delft, Holland. Kirchhoff, V.W.J.H., Casiccia, C.A.C., Zamorano, F., 1997a. The ozone hole over Punta Arenas, Chile. J. Geophys. Res. 102 Ž7., 8945–8953. Kirchhoff, V.W.J.H., Zamorano, F., Casiccia, C., 1997b. UV-B enhancements at Punta Arenas, Chile. Photochem. Photobiol. 38, 174–177. Kripke, M., 1992. Effects on human health. In: UV-B Monitoring Workshop: A Review of the Science and Status of Measuring and Monitoring Programs. Washington, DC, pp. C58–C63. Litynska, Z., 1994. First results of UV-B monitoring network of the Institute of Meteorology and Water Management. In: Report of the WMO Meeting of Experts on UV-B Measurements, Data Quality and Standardization of UV Indices, Les Diablerets, Switzerland, 25–28 July 1994, pp. 59–61. Scotto, J., Cotton, G., Urbach, F., Berger, D., Fears, T., 1988. Biologically effective ultraviolet radiation: surface measurements in the United States, 1974–1985. Science 239, 762–764. STAR, 1994. Star Newspaper, Penang, September 17, 1994, p. 3. Sullivan, J., 1992. Effects on terrestrial plants. In: UV-B Monitoring Workshop: A Review of the Science and Status of Measuring and Monitoring Programs, Washington, DC, pp. C64–C82. Teh, K.P., 1998. MSc Thesis, University of Science Malaysia, Penang, Malaysia Žto be presented.. UNEP, 1989. In: van der Leun, J.C., Tevini, M., Worrest, R.C. ŽEds.., Environmental Effects Panel Report. UNEP, Nairobi. Wang, P., Lenoble, J., 1994. Comparison between measurements and modeling of UV-B irradiance for clear sky: a case study. Appl. Opt. 33 Ž18., 3964–3971.
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Wester, U., 1992. Swedish Radiation Protection Institute: an authority for protection of the public from radiation hazards. In: UV-B Monitoring Workshop: A Review of the Science and Status of Measuring and Monitoring Programs, Washington, DC, pp. C281–C296. Wilson, L.J., 1993. Canada’s UV index—How it is Computed and Disseminated. Environment Canada, Atmospheric Environment Service. Worrest, R., 1992. Effects on aquatic ecosystems. In: UV-B Monitoring Workshop: A Review of the Science and Status of Measuring and Monitoring Programs, Washington, DC, pp. C83–C88. Yankee Environmental Systems, 1991. Instruction Manual—Model UVB-1 Ultraviolet Pyranometer, Turners Falls, USA. Zheng, X., Basher, R.E., 1993. Homogenisation and trend detection analysis of broken series of solar UV-B data. Theor. Appl. Climatol. 47, 189–203.
UV-B radiation at Penang Mohammad Ilyas ) , Arunasala Pandy, Syed Idris Syed Hassan Astronomy and Atmospheric Science Research Unit, UniÕersity of Science Malaysia, 11800 Penang, Malaysia Received 12 March 1998; received in revised form 6 August 1998; accepted 21 January 1999
Abstract A new series of measurements including global and UV radiation was initiated in 1994 at Penang. These high quality data were used to study diurnal and seasonal variations. The mean daily total UV-B and global radiation is about 1.43 = 10 4 J and 1.77 = 10 7 J, respectively. Maximum radiation values for both the UV-B and global radiation are received in March and September. The daily total global radiation shows a bigger seasonal variation than UV-B radiation. On clear days, the erythemal UV radiation flux is in the high or extreme range for about 5 h beginning at around 1030 hours throughout the year. Penang Ž5.38N. and Natal Ž68S. are found to exhibit similar daily maximum erythemal UV irradiance values. A simple empirical relationship for the noontime UV-B flux and global irradiance in the equatorialrtropical regions has been established. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Radiation; Ultraviolet; Global; Erythemal; Equator
1. Introduction Atmospheric ozone is the primary absorber of solar UV-B radiation. Recent changes to the stratospheric ozone levels have attracted strong interest from the scientific and environmental communities as well as from the policy makers. Ozone column amount in the tropics is lower than in the mid and higher latitudes. As a result, the tropical zone is richer in the shorter wavelength of the UV spectrum. Enhanced UV-B radiation at the ground level has the potential to cause adverse biological and environmental effects ŽUNEP, 1989.. Long-term exposure of human skin to UV radiation could induce skin cancer Žbasal and squamous cell carcinoma., cause ocular effects Žcataract, photokeratitis, lens capsule deformation and ocular melanoma. )
Corresponding author
0169-8095r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 Ž 9 9 . 0 0 0 0 5 - 8
142
M. Ilyas et al.r Atmospheric Research 51 (1999) 141–152
and decrease body immune functions ŽHenriksen et al., 1990; Kripke, 1992.. Tremendous variability exists among plants in their sensitivity to UV-B radiation. Various responses have been reported including changes in leaf secondary chemistry Žflavonoid accumulation., alterations in leaf anatomy and morphology, reductions in net carbon assimilation capacity Žphotosynthesis. and changes in biomass allocation and growth ŽSullivan, 1992.. On aquatic ecosystems, experiments have demonstrated that UV-B radiation could cause damage to fish larvae and juveniles, shrimp larvae, crab larvae and plants essential to the marine food web ŽWorrest, 1992.. The amount of UV-B radiation received at the ground level depends on various temporal, spatial and meteorological factors such as time of day, season, latitude, altitude, clouds, surface albedo, ozone, air particulates and aerosols ŽFrederick et al., 1989.. Any small increases in UV-B flux could be offset by other competing factors. In fact, some studies ŽBerger and Urbach, 1982; Scotto et al., 1988. have shown lower levels of UV-B radiation reaching the ground. These decreases have been attributed to scattering and absorption by pollutant gases and dust particles ŽGrant, 1988., and by tropospheric ozone. In urban areas, pollutants like NOx and hydrocarbons, through photodissociation by the absorption of UV radiation, produce increasing amounts of atomic oxygen, thus increasing the tropospheric ozone levels. Tropospheric ozone is generally less than 15% of the total ozone ŽWang and Lenoble, 1994.. According to Bruhl and Crutzen Ž1989., tropospheric ozone absorbs scattered radiation more effectively than the direct beam. However, recent studies have shown increases in UV-B radiation at the surface level ŽBlumthaler and Ambach, 1990; Kerr and McElroy, 1993; Zheng and Basher, 1993; Feister and Grewe, 1995.. During the passage of the Antarctic ozone hole over Punta Arenas ŽChile. in October 1992, the erythemal UV radiation increased from about 70 to 180 mW my2 ŽKirchhoff et al., 1997b.. Although solar UV-B irradiance data collection for the higher latitudes have been carried out intensively at many locations for many years now, it is not the same for the equatorialrtropical regions. At the tropical latitudes, the erythemally-weighted irradiance far exceeds that incident at higher latitudes ŽFrederick and Erlick, 1994.. At the Antarctic latitudes, even during the occurance of the ozone hole, direct transmission of UV radiation of wavelength higher than 290 nm remains lower than at the tropics ŽDavies, 1993.. At Penang, measurements of surface level solar ultraviolet radiation were started in 1978 ŽIlyas and Barton, 1983; Ilyas et al., 1988; Ilyas, 1993.. In this paper, we present the diurnal and seasonal variations of solar UV-B radiation as measured at Penang Ž5.348N, 100.308E. during a new series of measurements initiated in 1994. The UV data during the first 2-year period ŽSeptember 1994 to August 1996. have been used in this study.
2. Experimental The site area is situated in an urban environment close to the sea at an elevation of about 50 m above sea level. Penang has an equatorial climate and its humidity is high
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throughout the year, about 85% ŽIlyas et al., 1981.. The solar radiation measuring instruments were mounted on a raised platform on the tower block at the Astronomy and Atmospheric Science Research Unit ŽAASRU. building at the University of Science Main Campus, Penang Ž5.348N, 100.308E.. The solar UV-B radiation was measured using a UVB-1 ultraviolet pyranometer ŽYankee Environmental Systems, 1991.. The instrument measures global Ždirect and scattered. broadband ultraviolet irradiance of wavelengths between 280 and 330 nm from the entire hemisphere of the sky. Coloured glass filters and a UV-B sensitive phosphor are used to block all of the sun’s visible light and convert the UV-B light into green light. The intensity of the green light is then measured by a solid state photodiode and the instrument output is a 0–5 V dc signal. The instrument output signal Žin volts. can be converted to the effective UV-B irradiance Ž280–330 nm. using a calibration factor of 0.5076 W my2 Vy1 Žor 1.97 V s 1 W my2 .. As shown in Fig. 1, the spectral response of the instrument is very similar to the International Commission of Illumination ŽCIE. human erythemal action spectra ŽDi-
Fig. 1. Relative spectral response of the UVB-1 instrument compared with the CIE ŽDiffey. and Parrish erythemal action spectra.
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chter et al., 1993.. It must be noted that the spectra of any broadband pyranometer varies strongly with ozone levels, aerosols and solar zenith angle. In the UV-B range, the spectral response of the UVB-1 instrument drops at the same rate as the CIE action curve. However, due to the shift of the UVB-1 instrument spectrum toward the longer wavelength by 10 nm, it is less sensitive to changes in ozone. The spectral error of the instrument will also change with the solar zenith angle. This is due to the change of the UV portion of the solar spectrum with zenith angle at the earth’s surface. However, this error is not expected to be more than 2% for effective UV-B irradiance and 4% for erythemal irradiance ŽDichter et al., 1993.. The UVB-1 instrument employs thermally regulated optics that eliminates temperature dependent wavelength problems. Calibration of the instrument is done at half-yearly intervals on clear days using a reference unit. At the time of installation, the difference between the instrument and reference unit was 1.8%. Stability of the instrument for the first year was very good, as the difference from the reference unit came up to only 2.1% at the end of its first year in operation. At the end of the second year in operation this difference increased slightly to 3.8%. These differences are not included in the analysis of the UV-B values. The global Ždirect and diffuse. solar radiation was measured using a CM-11 Pyranometer ŽKipp & Zonen Delft. which has a spectral response with 95% points over 335–2200 nm, and 50% points over the wavelength range 305–2800 nm. The UV-B and global irradiances are sampled every 5 s and averaged at the end of 30-min intervals. The irradiances were measured from about 1 h before sunrise Ž0600 Malaysian Standard Time. to about 1 h after sunset Ž2000.. All measurements are of radiation incident on a plane horizontal surface. The sensors were connected to a Datalogger and the data is retrieved regularly using a Personal Computer. The same sensors were used throughout the measurement period. The sensor domes were cleaned on a regular basis to prevent soiling which could seriously affect the radiation data.
3. Results The seasonal and diurnal variations for the UV-B and global radiation are shown in Fig. 2. The maximum flux is received between 1230 and 1300 Malaysian Standard Time. For each type of radiation, in the month of September Penang receives the lowest maximum radiation level, the highest radiation level being in March; this represents an increase from September to March of about 27% for the UV-B radiation and 50% for the global radiation. The lower maximum flux in September and October were influenced by the occurrence of haze and increased cloud cover. The bigger seasonal variation shown by the total global radiation means that it is more strongly attenuated by clouds. The absorptivity of the global radiation increases in the presence of clouds due to multiple scattering which increases the photon pathlength. According to Forster Ž1995., UV radiation is not absorbed much by water or ice clouds. Whenever cloud and pollution particles are present, UV-B radiation undergoes strong Mie scattering in the forward direction. The effect of clouds on the UV irradiance
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Fig. 2. Ža. Seasonal diurnal variation for measured global UV-B radiation at Penang Ž5.348N, 100.38E. for the period from September 1994 to August 1996. The curve is symmetrical about solar noon. Žb. Seasonal diurnal variation for measured global radiation at Penang. The curve broadens after solar noon due to attenuation by clouds.
depends on its amount. Feister and Grewe Ž1995. have shown that at Potsdam ŽGermany., the presence of lower than normal cloud amount further enhanced the biologically effective solar UV irradiance at the surface.
4. Discussion Long term cloud cover data at Penang ŽIlyas et al., 1981. shows lower cloud cover in February and March, and higher cloud cover in September and October. The maximum average daily total UV-B radiation received at Penang is in April Ž1.74 = 10 4 J. with the minimum in November Ž1.36 = 10 4 J.. For the total global radiation, the maximum average daily total is in March Ž2.19 = 10 7 J. and the minimum in September Ž1.57 = 10 7 J.. The UV-B radiation data appears to be very symmetrical around the local solar noon. Cloud variability could cause large temporal changes in the instantaneous irradiance. The total global radiation curve shows a broadening after the solar noon. In
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tropicalrequatorial regions, clouds normally build up in the afternoon. This implies that clouds attenuate the UV-B lesser than global irradiance. Even though we have measured the basic UV-B irradiance as presented earlier, the practical consideration requires us to relate this data to erythemal UV-B dosage for the McKinlay–Diffey spectrum. This would allow us to evaluate the degree of danger to human health due to the UV-B radiation as well as comparisons with measurements made at other locations which are generally presented in erythemal dosages. In order to reduce our data to Erythemal Dosage, we can establish the direct inter-conversion between UV-B irradiance and Erythemal Dosage with the help of calibration information given by the manufacturer as follows. Effective UV-B irradiances 0.5076 W my2 = signal in volts Erythemal Dosages 0.141 W my2 = signal in volts
Ž i. Ž ii .
Hence, for the same signal we can write, Erythemal dosage
0.141
s 0.27778 Effective UV-B irradiance 0.5076 For zenith angles smaller than 658, the accuracy of the calibration factor is within 4% ŽYankee Environmental Systems, 1991.. Bodhaine et al. Ž1998. have shown that the calibration of the instrument depends strongly on total ozone for smaller solar zenith angles ŽSZA. Ž- 658. and also a strong SZA dependence at larger SZA’s. According to the manufacturers, the Model UVB-1 pyranometer was subjected to three types of absolute calibration, details of which are available in the instruction manual. The calibration was done for total ozone values of about 322 Dobson units ŽDU.. The mean daily total ozone for 1994 as measured by the Total Ozone Mapping Spectrometer ŽTOMS. on the satellite Meteor-3 is 256 DU for Petaling Jaya, Malaysia, Ž3.108N, 101.658E. which is close enough to Penang. Measurements by Bodhaine et al. Ž1997. at Mauna Loa ŽHawaii. with a Yankee UVB-1 broadband pyranometer suggest that the calibration as provided by the manufacturer was within about 2% of spectroradiometer measurements for effective UV-B irradiance. Errors as high as 10% can occur if the effects of total ozone are not taken into account in the calibration of the instrument ŽBodhaine et al., 1998.. However, this error is expected to be very small for low latitudes due to the small seasonal variability in total ozone. Using the Canadian UV Index model ŽWilson, 1993., where a UV Index value of 1 represents approximately 25 mW my2 of erythemal UV flux reaching a horizontal plane on the ground, we found that on clear days the erythemal flux at Penang is in the high or extreme for 5 h a day, between 1030 and 1530 Malaysian Standard Time ŽFig. 3.. During this time of the day, many outdoor activities are carried out. Distribution of the predicted UV Index will enable individuals to take precautionary measures Žusing hats, proper clothing, sunglasses, sunscreens.. Comparison of the Penang erythemal UV radiation data with another equatorial site which has similar climatic and weather conditions, Natal ŽBrazil. Ž68S, 358W., shows that there is not much difference between them in the daily maximum readings with both sites showing an average of 0.30 W my2 ŽKirchhoff et al., 1997a. ŽTable 1.. The Natal s
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Fig. 3. Variation of the erythemal UV-B flux with time of day on a typical clear day in March. The UV Index is based on the Canadian AES scale.
data used is the CIE weighted UV-B radiation integrated for each day. The daily maximum erythemal UV irradiance at Stockholm Ž59.48N. in July is 0.14 W my2 ŽWester, 1992. and at Leba Ž54845X N, 17832X E., a Baltic coastal city in Poland, the maximum during the summer months is 0.12 W my2 ŽLitynska, 1994.. This means that tourists from the high latitudes will be exposed to more than twice the erythemal UV irradiance they receive at home. It is therefore of utmost importance that sun seekers from the high latitudes be made aware of overexposure to solar UV radiation.
Table 1 Daily maximum erythemal UV irradiance at different equatorial and high latitudes Location
Latitude
Daily maximum erythemal UV flux ŽW my2 .
Penang ŽMalaysia. Natal ŽBrazil. Leba ŽPoland. Stockholm ŽSweden. Punta Arenas ŽChile.
5.38N 6.08S 54.88N 59.48N 53.08S
0.30 0.30 0.12 0.14 0.07
At Punta Arenas ŽChile. the value was 0.18 W my2 during the occurance of the ozone hole in October 1992. This value is only half the flux received at the equatorial region.
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We also present the daily total UV-B and global solar radiation for the experimental period under discussion ŽFig. 4.. The daily total radiation is calculated by multiplying the daily summation of the half hourly irradiance by 1800 s. Large variations are observed for the daily totals, due mainly to cloud, tropospheric pollutants and seasonal ŽEarth–Sun distance. factors. The upper boundary represents the maximum radiation that would be received on a clear sky day. The total ozone levels shown in Fig. 4a are measurements from TOMS on Meteor-3. It can be seen that the total ozone levels has two peaks. A lower peak in April and a higher one in late September and early October. The higher maximum in September is due to the equinox effect ŽTeh, 1998.. The mean daily total UV-B radiation during the measurement period is 1.43 = 10 4 J and for global radiation is 1.77 = 10 7 J. The daily total radiation reaches maximum
Fig. 4. Ža. Seasonal variation of the total column ozone for the period 1991–1994 as measured by TOMS. Žb. Daily total global solar UV-B radiation as measured at Penang. The maximum radiation is received in March and September. Žc. Measured daily total global radiation at Penang. The large variations are mainly due to clouds.
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values in March and September with a value of 2.3 = 10 4 J and 2.8 = 10 7 J for the daily total UV-B and global radiation, respectively. This corresponds to the time of the year when the path length of the solar radiation through the atmosphere is at its shortest. The mean daily total UV-B and global radiation during September and October would have been higher if not for the heavier cloud cover, increased ozone levels and perturbation due to the local hazy period Žfrom mid-September to mid-October 1994. believed to be caused by widespread forest fires in Sumatra and Kalimantan ŽIndonesia. ŽSTAR, 1994.. During the haze period, both types of radiation were reduced appreciably. We found that the daily total UV-B radiation was reduced by 12% and the global radiation by 9% compared to the 1995 values during the same period. The enhanced absorption of the UV-B radiation was probably due to increased levels of tropospheric NO x and ozone, through photodissociation and oxidation processes ŽIlyas et al., 1996.. The relationship between 1r2 hourly UV-B and global radiation about local noon time Žbetween 1300 and 1330. for all sky conditions are shown in Fig. 5. At Penang, we find that close to the solar noon, usually there are almost no clouds or only some broken
Fig. 5. Relationship between the measured noontime irradiance of UV-B and global radiation. A strong linear correlation can be observed with a mean noontime UV-B to global irradiance ratio of 1.05=10y3 .
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and scattered clouds. There appears to be a strong linear relationship between the two types of radiation with most of the data being within the 95% confidence interval. The stray points are caused by the occasional clouds and perturbations on ozone levels. The mean noontime UV-B to global irradiance ratio is 1.05 = 10y3 . Regression of the UV-B irradiance on the corresponding global irradiance gives a linear relation of the form: I v s aIg q b where Iv and Ig are the UV-B and global noontime irradiance, and, a s 0.00105, b s 0.05. Although the regression coefficients may vary slightly for measurements at different tropicalrequatorial locations, it could form a good basis for estimation. In places where facilities for UV-B radiation measurements are not available, this kind of empirical relationship could be used for general risk-assessments, e.g., in determining the noontime UV index using global radiation measurements.
5. Conclusions The maximum daily total UV-B and global radiation values are highest in March and September with mean values of 1.43 = 10 4 J and 1.77 = 10 7 J, respectively. Seasonally, the total global radiation shows a larger variation than UV-B radiation due to its stronger attenuation by clouds. Based on the Canadian UV Index scale, the surface level solar erythemal radiation is high or extreme for about 5 h, beginning at 1030 Malaysian Standard Time. A good linear relationship exists between the UV-B and global irradiance which enables estimation of UV-B flux in tropicalrequatorial areas where facilities for UV-B measurements are not available but global radiation flux data are available. The erythemal UV radiations are of the same magnitude at Penang and Natal ŽBrazil., both equatorial cities but in different hemispheres.
Acknowledgements The authors wish to thank the staff of the Astronomy and Atmospheric Science Research Unit who were involved in the maintenance of the solar radiation measuring system. Part of this work is supported through an IRPA ŽRM 6. funded project under the Ministry of Science, Technology and Environment, Malaysia.
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