Relationship between erythemal irradiance and total solar irradiance in South-Western Spain

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007jd009627, 2008 Relationship between erythemal irradiance and total solar irradiance in South-Western Spain M. Antón, 1 A. Serrano, 1 M. L. Cancillo, 1 and J. A. García 1 Received 20 November 2007; revised 10 March 2008; accepted 19 March 2008; published 22 July [1] In the last years, many studies have focused on the study of the ultraviolet radiation or the total solar radiation in Southwestern Europe. This area presents a particular interest due to the intense solar radiation which reaches the Earth s surface at these latitudes. Despite of this high interest, the influence of the atmospheric factors on the relationship between both variables is not well documented for that region. In this work, the ultraviolet erythemal irradiance (UVER) and the total solar irradiance ( nm) at three locations in Southwestern Spain were measured between January 2001 and December The main goal of this paper is to analyze the relationship between ultraviolet and total solar irradiance as described by their ratio. This ratio represents the percentage of UV erythemal irradiance over the total solar irradiance at Earth s surface. The results show that this ratio varies in time. It shows a day-to-day and seasonal variability mainly due to changes in sky conditions (regarding cloudiness and aerosols) and in the slant column of ozone, respectively. Differences in ozone may cause a relative increase in the ratio higher than 100% between winter and summer. This fact stresses the importance of reporting information about the UV radiation at ground level to the people mainly during summer. Clouds play also an important role, since highly different conditions between consecutive days are responsible for up to 60% of the ratio variability. The results show that cloudiness attenuates more the total solar radiation than the UV solar radiation. Additionally, high changes in aerosol load can produce a short-term decrease in the ratio of 13 15%, attenuating differently UVER than total solar irradiance. Citation: Antón, M., A. Serrano, M. L. Cancillo, and J. A. García (2008), Relationship between erythemal irradiance and total solar irradiance in South-Western Spain, J. Geophys. Res., 113,, doi: /2007jd Introduction [2] Solar radiation received at ground level is the most important term for the energy balance of the earth-atmosphere system. Thus a detailed knowledge of its variability, both temporal and spatial, is of great importance for many topics; such as soil-vegetation-atmosphere energy balance models, validation of climate models and studies focused on the use of solar radiation as source of energy. [3] Although ultraviolet radiation ( nm) represents only the 8.03% of the incoming solar radiation at the top of the atmosphere [Iqbal, 1983], the analysis of this radiation has received considerable attention during last decades mainly because of its notable increase at the Earth s surface due to the ozone depletion. In this sense, Fioletov et al. [2001] reported an increase of 20% in UV spectral irradiance at 300 nm at Toronto (Canada) for the period [4] The continuous monitoring of the UV solar radiation at ground level is very important since it affects many 1 Departamento de Física, Universidad de Extremadura, Badajoz, Spain. Copyright 2008 by the American Geophysical Union /08/2007JD biological, ecological and photochemical processes, being often harmful for living organisms. In particular, an increase in UV radiation increase can cause substantial adverse effects on plant growth, photosynthesis and aquatic ecosystems (the major contributor to the Earth s biomass) [Diffey, 1991]. In addition, a long-term over-exposure of humans to UV radiation can have detrimental consequences. According to the study by Lopez-Abente et al. [1996], the number of cases of melanoma (the most dangerous type of skin cancer) annually increased about 5% in Spain during the period The World Health Organization (WHO) has estimated that up to 20% of the cataracts developed annually could be related to exposure to ultraviolet radiation [WHO, 1995]. Therefore nowadays, regional, national and international organizations promote the installation of UV measuring networks in order to monitor the UV solar radiation that reaches the Earth s surface. [5] According to the biological effects on human skin, UV-B ( nm) is the most detrimental band. In order to account for this wavelength dependence, an erythemal action spectrum is defined, having its maximum weight at 297 nm. Thus the erythemal effect of UV-B radiation on the human skin is commonly calculated weighting the solar UV radiation with the erythemal action spectrum, resulting in 1of7

2 the so-called ultraviolet erythemal radiation (UVER). The curve adopted in 1987 by the Commission Internationale de l Eclairage (CIE) [McKinlay and Diffey, 1987] is currently recommended as the standard erythemal action spectrum. [6] Many studies about the relationship between the UV irradiance and the total solar irradiance have been performed during the last years for different areas such as Spain [Foyo-Moreno et al., 1998; Martínez-Lozano et al., 1999; Cañada et al., 2003; Murillo et al., 2003], Germany [Feister and Grasnick, 1992], Egypt [Robaa, 2004], Kuwait [Al-Aruri, 1990], Saudi Arabia [Elhadidy et al., 1990; Khogali and Al-bar, 1992] and Malaysia [Ilyas et al., 1999]. The main goal of these studies was to propose empirical expressions for estimating UV values from measurements of total solar irradiance (variable usually available at many meteorological stations). These studies are of high interest in Iberian Peninsula, and particularly for its South- Western area due to the notably high UV irradiance recorded at these [Serrano et al., 2006]. In spite of its great interest, only few studies have analyzed the influence of the different attenuation factors on the relationship between UV irradiance and total solar irradiance. Lorente et al. [1994] analyzed the effect of aerosols on the ratio of the UV-A ( nm) and UV-B components with respect to the total solar irradiance under cloud-free conditions at Barcelona. Cañada et al. [2000] analyzed the empirical relationship between UV irradiance (UV-A + UV-B) and the total solar irradiance for four intervals of clearness index in Valencia and Córdoba. Using the same clearness index, Foyo-Moreno et al. [1999] studied the influence of clouds over the ratio between UV irradiance (UV-A + UV-B) and the total solar irradiance at Granada and Almería. Instead of the clearness index, Foyo-Moreno et al. [2003] analyzed the same ratio with respect to the cloud cover amount (in octas) at Granada. Den Outer et al. [2005] and Alados-Arboledas et al. [2003] studied the influence of clouds on UVER and total solar radiation at Bilthoven (Netherlands) and three different locations in Spain, respectively. Moreover, few results were found in literature about the combined effect of attenuation factors on the ratio between UV radiation and total solar radiation [Koronakis et al., 2002; Ogunjobi and Kim, 2004; Jacovides et al., 2006]. [7] In this framework, this paper aims to analyze in detail the contribution of the main attenuation factors (clouds, ozone and aerosols) to the variability of the ratio (UVER/ Total solar irradiance) at three locations in Extremadura region (southwestern Spain). [8] This article is structured as follows: section two presents the main characteristics of instruments and data used in this work. This section also describes the method followed for establishing the relationship between UVER and total solar irradiance. Section three presents and discusses the results obtained in this work and, finally, section four summarizes the main conclusions. 2. Instruments, Data and Method [9] Total solar irradiance and UVER have been measured at three stations located in Badajoz (38.99 N, 7.01 W, altitude = 199 m a.s.l.), Cáceres (39.48 N, 6.34 W, 397 m a.s.l.) and Plasencia (40.06 N, 6.04 W, 372 m a.s.l.) (Extremadura, Spain). These radiometric stations have been installed and maintained by the AIRE (Atmósfera, Clima y Radiación en Extremadura) research group of the University of Extremadura. Each station is equipped with two instruments: a UV-S-ET broadband pyranometer for measuring UVER and a CM-6B pyranometer for measuring total solar irradiance values ( nm). Both instruments are manufactured by Kipp & Zonen (Delft, Netherlands). Total and UV radiation were sampled every 10 s and recorded as 1-min mean voltages on Campbell CR10X data acquisition systems. The CM-6B pyranometer complies with the specifications for the first-class WMO classification of this instrument (resolution better than ±5 W/m 2 ). On the other hand, the output voltages provided by UV-S-ET instrument were converted to UVER values by applying the calibration factors. These factors were obtained in several outdoor intercalibration campaigns with a well-characterized Brewer MK-III spectrophoradiometer as the reference instrument. During the period , three biennial calibration campaigns were carried out at the Atmospheric Sounding Station (ESAt), which belongs to the National Institute of Aerospace Technology (INTA), located at El Arenosillo in Huelva, Spain (37.1 N, 6.7 W, 20 m a.s.l.). These three campaigns showed that the calibration factors of the broadband UV instruments present a negative trend attributable to the aging effect of the instruments. Thus a linear interpolation as function of time was calculated in order to account for this trend in the calibration factors [Antón, 2007]. In addition, relative uncertainties of UVER are lower than 5% for all the values considered in this study. More details about the calibration of the UV broadband radiometers can be found in the papers by Cancillo et al. [2005] and Cancillo et al. [2007]. [10] For this work, one daily value of UVER (E UV ) and total solar irradiance (E G ) data were calculated as the average of all available measurements for each variable between 11:00 and 13:00 (true solar time). A daily average was calculated since the Total Ozone Mapping Spectrometer (TOMS) instrument provides only a single estimation of total ozone column (TOC) around solar local noon. [11] This paper focuses on the ratio defined by the following expression: R ð%þ ¼100 E UV E G : [12] This ratio indicates the percentage of the UVER with respect to the total solar irradiance. Its value can be considered constant at the top of the atmosphere. Therefore the variability of the ratio measured at the Earth s surface can be exclusively attributed to atmospheric attenuation factors. [13] TOC values estimated by TOMS satellite instrument were used in order to describe the influence of ozone on the relationship between UVER and total solar irradiance. This remote sensor has worked operatively on board of two satellites: Nimbus-7 ( ) and Earth Probe (EP) ( ). The TOMS instrument measures the backward scattered Earth solar radiance in six 1-nm bands in the UV-A and UV-B spectral regions and the extraterrestrial solar radiance in the same six wavelength channels. This information is used to estimate spatial distributed daily 2of7

3 Figure 1. Daily mean evolution of variable R for the period at Cáceres station. values of total ozone column expressed in Dobson Units (DU) at the satellite overpass time. TOMS ozone data are available via anonymous ftp service (ftp://toms.gsfc.nasa. gov). The horizontal spatial resolution of TOMS ozone data is 1 latitude 1.25 longitude. In general, there is a very good agreement between the TOMS ozone estimations and ground based measurements. Thus McPeters et al. [1998] reported that the TOMS total ozone is about only 1% higher than the ground measurements in 30 midnorthern latitude stations. Other studies confirm the good agreement between satellite and ground based ozone data [Fioletov et al., 2002; Masserot et al., 2002]. [14] The actual ozone amount crossed through by the solar radiation in the atmosphere is the slant ozone column defined as: Z ¼ TOC m ; where m is the cosine of the solar zenith angle averaged between 11:00 and 13:00 h. This expression is only valid for the direct solar irradiance. However, it can be used as a good approximation also for the global solar irradiance (direct + diffuse), since the largest part of ozone absorption occurs at high altitudes above the scattering processes by aerosols and clouds. [15] The daily clearness index was used in order to characterize the cloudiness. This variable has been broadly used for this purpose [Foyo-Moreno et al., 1999; Cañada et al., 2000; Ogunjobi and Kim, 2004] and it is defined as: K ¼ E G EG TOA where E G TOA is the extraterrestrial total solar irradiance on a horizontal surface averaged between 11:00 and 13:00 h. In order to select cloud-free cases a threshold of K > 0.75 was ; preferred instead the value of 0.65 used by Kudish et al. [1983] and Udo [2000]. This higher threshold guarantees the cloud-free conditions of the selected days. Additionally, in order to analyze the influence of aerosol load over the relationship between UVER and total solar irradiance, measurements from an automatic Cimel Sun photometer was used. This instrument works operatively since June 2005 at Cáceres station, being part of NASA AERONET network ( [Holben et al., 1998]. This photometer measures direct sun and sky radiation at seven wavelengths between 340 nm and 1020 nm (10 nm FWHM). In this study, the aerosol optical depth (AOD) at 380 nm from the AERONET direct Sun data (version 1) was used to characterize the aerosol load. In addition to calibration performed by AERONET, the aerosol optical depth was also corrected against diurnal cycle [Cachorro et al., 2004, 2008]. The AERONET uncertainty in AOD retrievals is approximately (wavelength dependent) due to the calibration uncertainty for the field instruments [Eck et al., 1999]. [16] In this work, all daily data were obtained around solar noon. This restriction large solar zenith angles at mean latitudes removes. Thus the variables used in this paper: UVER, total solar irradiance, AOD, clearness index and slant ozone column were calculated for solar zenith angles lower than 65. [17] The period of study ranges between January 2001 and December 2005, guaranteeing that a wide range of TOC, sky conditions and solar elevation angles are included in the study. 3. Results and Discussion [18] Considering the spectral solar values given by Frohlich and Wherli [Iqbal, 1983], the solar constant for the UV band ( nm) weighted by the standard CIE spectral response [McKinlay and Diffey, 1987] result in 10.3 Wm 2 (erythemal solar constant). In addition, the solar constant corresponding to the UV bands A + B ( nm) results in 78.3 Wm 2 (UV solar constant). The last result agrees with the works of Cañada et al. [2003] and Foyo-Moreno et al. [2003]. Therefore the erythemal solar constant is about 87% lower than UV solar constant which shows the magnitude of the CIE spectral response reduction. Regarding the solar constant for the total solar spectrum (1367 Wm 2 ), the variable R presents a fixed value of 0.75% (R TOA )atthe top of the atmosphere. [19] At the Earth s surface, the mean R value (±one standard deviation) is (0.017 ± 0.005) % for Badajoz, (0.017 ± 0.005) % for Cáceres and (0.017 ± 0.006) % for Plasencia. It can be seen that mean R values at ground level are quite lower than R TOA (relative decrease higher than 90%). This fact is mainly due to the absorption by atmospheric gases (mainly ozone) which are strongly wavelengthdependent, increasing as wavelength decreases [Iqbal, 1983]. [20] The daily mean R values for the whole period of study are shown in Figure 1 for the Cáceres station. A clear seasonal behavior of variable R can be seen with maximum values during summer and minimum during winter. This temporal evolution shows a low day-to-day variability. The ratio between UVER and total solar radiation values is not constant because it depends on the main attenuation atmo- 3of7

4 Figure 2. Monthly mean evolution of slant ozone column and variable R (only cloud-free conditions) at Cáceres station. spheric factors. In the following subsections, the influence of slant ozone column, clouds and aerosols over the variable R is analyzed in detail Influence of Slant Ozone Column [21] Only the solar radiation corresponding to a certain small interval of wavelengths (from 280 to 315 nm) is significantly affected by changes in ozone amount. Thus the fluctuations of this variable affects mainly to UVER and consequently to the variable R. [22] The TOC presents a notable seasonal variability mainly due to dynamical factors (i.e., Dobson-Brewer circulation) [Nikulin and Karpechko, 2005]. In middle latitudes the maximum TOC is recorded during spring and the minimum during autumn. The TOC values measured by TOMS instrument from 2001 to 2005, in Badajoz ranged from its minimum value (mn) of 200 DU to the maximum value (mx) of 469 DU. For Cáceres, these parameters had the following values: mn = 190 DU and mx = 476 DU, and for Plasencia: mn = 189 DU and mx = 478 DU. [23] The slant ozone column shows also a marked annual cycle with maximum values during summer and minimum during winter. This fact is shown in Figure 2, which presents the monthly averaged evolution of slant ozone column and R values (cloud-free conditions) at Cáceres station. The inverse correlation between these two variables is quite striking. A similar behavior is found for the other two stations: the monthly mean value of R ranges from ( ± ) % (December) to ( ± ) % (July) for Badajoz and from ( ± ) % (December) to ( ± ) % (June) for Plasencia. Thus the high R values reached during summer months (see Figures 1 and 2) may be mainly attributed to the low slant ozone values recorded during that season. Therefore besides the fact that the UVER evolution follows a sinusoidal curve driven mainly by astronomical factors with the highest UVER values during summer [Serrano et al., 2006], the percentage of UVER respect to the total solar irradiance increases significantly during that season. [24] It is interesting to note that the influence of slant ozone in the ratio UVER/Total is mainly caused by changes in the length of the optical path crossed by the solar radiation within the atmosphere. Therefore the R variable presents a clear seasonal behavior (Figures 1 and 2) with a marked dependence with the annual solar cycle. The actual ozone amount crossed by the solar radiation in summer (low optical depth) is lower than in winter (high optical depth). Thus due to the high absorption in the ultraviolet region by ozone, the UVER values decreases (increases) respect to the total solar radiation in winter (summer), causing a notable drop (increase) of the R variable. [25] All these results stress the importance of reporting UV index during summer in order to advise people to protect themselves against UV radiation over-exposure at the latitudes corresponding to Extremadura Influence of Clouds [26] It is well documented that the lowest hourly and daily UVER and total solar irradiance values correspond to days with a notable attenuation by clouds [Cañada et al., 2000; Cede et al., 2002; Serrano et al., 2006]. Therefore beside the variation in solar zenith angle, cloudiness is the main attenuation factor responsible for most part of the variability presented by both UVER and total solar irradiance. In this subsection the influence of this factor over the variable R is analyzed. [27] Figure 3 shows the relationship between the variable R and the clearness index for two representative months corresponding to winter (January) and late summer (September) at Cáceres station. September was selected since June, July and August lack of enough cloudy days for the study. The study cases were 124 in January and 104 in September. The regression lines for the two months have been also plotted in the Figure 3. Although the coefficients Figure 3. Variable R versus the clearness index for two representative months corresponding to winter (January) and late summer (September) at Cáceres station. 4of7

5 Figure 4. Relationship between the relative difference of clearness index and the relative difference of variable R between consecutive days at Cáceres station. of correlation are not high (0.47 for September and 0.75 for January), a strong negative-correlation between the clearness index and the variable R can be observed. Thus when K is low (high cloudiness) the variable R increases notably its value. For example, for cloud-free days (K > 0.75), the variable R has a value of ( ± ) % (January) and ( ± 0.010) % (September). In contrast, for cases with thick clouds (K < 0.45), the variable R presents higher values than for cloud-free days, with ( ± ) % for January and ( ± ) % for September. These results indicate that cloud cover reduces total solar irradiance more strongly than the UVER, leading to an increase in the ratio R. This fact may be attributed to the cloud absorption by liquid water content, much stronger in visible and near-infrared region than in the ultraviolet region [Lenoble, 1993]. [28] Cloudiness is the main factor responsible for the dayto-day variability of solar radiation, and, therefore, also the main cause for the R day-to-day fluctuations. This fact is confirmed by Figure 4, which presents the relationship between the relative difference of clearness index and the relative difference of variable R between consecutive days. It can be observed that when K decreases (a cloud-free day is followed by cloudy day), the variable R shows a notable relative increase (up to 60%, depending on the K change). Conversely, when K increases, the variable R suffers a lower relative variation (decrease up to 30%) Influence of Aerosols [29] An analysis of the correlation between mean daily R values and the corresponding AOD data at 380 nm has been performed under cloud-free conditions for the Cáceres station. In order to remove the strong dependence of the variable R on the slant ozone column, the set of measurements was split into three subsets corresponding to certain intervals of slant ozone column (subset 1: 320 DU 340 DU, subset 2: 420 DU 440 DU and subset 3: 620 DU 640 DU). Figure 5 shows the variable R versus AOD at 380 nm and the regression lines for each subset. It can be seen that as AOD increases, R decreases. This fact is confirmed when the variable R is averaged for cases with low aerosol load (AOD < 0.1) and high aerosol load (AOD > 0.4). The following values are obtained for low aerosol load cases: ( ± ) % for subset 1, ( ± ) % for subset 2 and ( ± ) % for subset 3. When this average was performed for cases with high aerosol load, the variable R notably decreases: ( ± ) % for subset 1, ( ± ) % for subset 2, and ( ± ) % for subset 3. Therefore the relative decrease in variable R due to the aerosol attenuation is between 13% (subset 3) and 16% (subset 1). This result indicates that aerosols do not attenuate total solar irradiance as highly as UVER. Although aerosols affects UVER and total solar radiation similarly by scattering processes, the higher attenuation of UVER could be partly attributed to an amplification of its absorption by the selfsame aerosols, tropospheric ozone and other anthropogenic gases, due to the enhancement in the optical path because of the scattering processes [Bernard et al., 2003; Kaskaoutis et al., 2006; Badarinath et al., 2007]. 4. Conclusions [30] This work focuses on the study of the ratio between UVER and total solar irradiance at three locations in South- Western Spain. The atmospheric attenuation reduces notably (about 90%) this ratio at the Earth s surface when compared to its value at the top of the atmosphere. This decrease is mainly due to the atmospheric components which attenuate more effectively UVER than total solar radiation. [31] The influence of slant ozone, cloudiness and aerosol load over the variability ratio measured at surface has been analyzed in detail. While slant ozone produces a strong Figure 5. Variable R versus AOD at 380 nm for three subsets corresponding to selected slant ozone intervals at Cáceres station. 5of7

6 seasonal variability, cloudiness and aerosols cause notable day-to-day fluctuations. The seasonal cycle of the slant ozone column with minimum values in summer, causes the annual cycle of the ratio UVER/Total, which presents maximum values in that season. This behavior is mainly due to the shorter optical path crossed by the solar radiation in the atmosphere in summer. In addition, aerosol load produces an effect more accused on the UVER than on the total solar irradiance. Thus the ratio decreases more than 13% when the atmospheric conditions change from low to high aerosol load. In contrast, clouds cause the opposite effect, since they attenuate more the total solar radiation than the UVER. Thus the ratio increases up to 60% when a cloudy day is followed by a cloud-free day. [32] Finally, it is interesting to note that the highest ratio at the Earth s surface occurs during summer with low aerosol load conditions. [33] Acknowledgments. This work was partially supported by MEC under project CGL C03-03/CLI, by the Junta de Extremadura under Convenio para la creación y mantenimiento de una red de medida de la radiación solar ultravioleta en Extremadura, and under the Project GRU References Alados-Arboledas, L., I. Alados, I. Foyo-Moreno, F. J. Olmo, and A. Alcántara (2003), The influence of clouds on surface UV erythemal irradiance, Atmos. Res., 66, Al-Aruri, S. D. (1990), The empirical relationship between global radiation and global ultraviolet ( ) nm solar radiation components, Sol. Energy, 45, Antón, M. (2007), Modelos empíricos para la estimación de la irradiancia solar ultravioleta, Ph.D. thesis, University of Extremadura, Badajoz, Spain. Badarinath, K. V. S., S. K. Kharol, D. G. Kaskaoutis, and H. D. Kambezidis (2007), Influence of atmospheric aerosols on solar spectral irradiance in an urban area, J. Atmos. Sol. Terr. Phys., 69, Bernard, W. F., V. K. Saxena, B. N. Wenny, and J. J. DeLuisi (2003), Daily surface UV exposure and its relationship to surface pollutant measurements, J. Air Waste Manage. Assoc., 53, Cachorro, V. E., P. M. Romero, C. Toledano, E. Cuevas, and A. M. de Frutos (2004), The fictitious diurnal cycle of aerosol optical depth: A new approach for in situ calibration and correction of AOD data series, Geophys. Res. Lett., 31, L12106, doi: /2004gl Cachorro, V. E., C. Toledano, M. Sorribas, A. Berjón, A. M. de Frutos, and N. Laulainen (2008), An in situ calibration-correction procedure (KCICLO) based on AOD diurnal cycle: Comparative results between AERONET and reprocessed (KCICLO method) AOD-alpha data series at El Arenosillo, Spain, J. Geophys. Res., 113, D02207, doi: / 2007JD Cancillo, M. L., A. Serrano, M. Antón, J. A. García, J. M. Vilaplana, and B. de la Morena (2005), An improved outdoor calibration procedure for broadband ultraviolet radiometers, Photochem. Photobiol., 81, Cancillo, M. L., A. Serrano, M. Antón, J. A. García, and J. M. Vilaplana (2007), Improving the solar zenith angle dependence of broadband UV radiometers calibration, Atmos. Chem. Phys. Disc., 7, Cañada, J., G. Pedrós, A. López, and J. V. Boscá (2000), Influences of the clearness index for the whole spectrum and the relative optical air mass on UV solar irradiance for two locations in the Mediterranean area, Valencia and Cordoba, J. Geophys. Res., 105, Cañada, J., G. Pedrós, and J. V. Boscá (2003), Relationships between UV ( nm) and broadband solar radiation hourly values in Valencia and Córdoba, Spain, Energy, 28, Cede, A., E. Luccini, L. Nuñez, R. D. Placentini, and M. Blunthaler (2002), Effects of clouds on erythemal and total irradiance as derived from data of the Argentine Network, Geophys. Res. Lett., 24(29), 2223, doi: /2002gl Den Outer, P. N., H. Slaper, and R. B. Tax (2005), UV radiation in the Netherlands: Assessing long-term variability and trends in relation to ozone and clouds, J. Geophys. Res., 110, D02203, doi: / 2004JD Diffey, B. L. (1991), Solar ultraviolet radiation effects on biological systems, Phys. Med. Biol., 36, Eck, T. F., B. N. Holben, J. S. Reid, O. Dubovik, A. Smirnov, N. T. O Neill, I. Slutsker, and S. Kinne (1999), Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosol, J. Geophys. Res., 104, 31,333 31,349. Elhadidy, M. A., D. Y. Abdel-Nabi, and P. D. Kruss (1990), Ultraviolet solar radiation at Dhahran, Saudi Arabia, Sol. Energy, 44, Feister, U., and K. H. Grasnick (1992), Solar UV radiation measurements at Postdam ( N, E), Sol. Energy, 49, Fioletov, V. E., L. McArthur, J. E. Kerr, and D. I. Wardle (2001), Long-term variations of UV-B irradiance over Canada estimated from Brewer observations and derived from ozone and pyranometer measurements, J. Geophys. Res., 106(D19), 23,009 23,027. Fioletov, V. E., J. B. Kerr, D. I. Wardle, N. Krotkov, and J. R. Herman (2002), Comparison of Brewer ultraviolet irradiance measurements with total ozone mapping spectrometer satellite retrievals, Opt. Eng., 41, Foyo-Moreno, I., J. Vida, and L. Alados-Arboledas (1998), Ground based ultraviolet ( nm) and broadband solar radiation measurements in south-eastern Spain, Int. J. Climatol., 18, Foyo-Moreno, I., J. Vida, and L. Alados-Arboledas (1999), A simple all weather model to estimate Ultraviolet solar radiation ( ), Int. J. Climatol., 18, Foyo-Moreno, I., I. Alados, F. J. Olmo, and L. Alados-Arboledas (2003), The influence of cloudiness on UV global irradiance ( nm), Agric. For. Meteorol., 120, Holben, B. N., et al. (1998), AERONET A federated instrument network and data archive for aerosol characterization, Remote Sens. Environ., 66, Ilyas, M., A. Pandy, and S. I. S. Hassan (1999), UV-B radiation at Penang, Atmos. Res., 51, Iqbal, M. (1983), An Introduction to Solar Radiation, Academic Press, Canada. Jacovides, C. P., V. D. Assimakopoulos, F. S. Tymviosa, K. Theophilouc, and D. N. Asimakopoulosa (2006), Solar global UV ( nm) radiation and its relationship with solar global radiation measured on the island of Cyprus, Energy, 31, Kaskaoutis, D. G., H. D. Kambezidis, C. P. Jacovides, and M. D. Steven (2006), Modification of solar radiation components under different atmospheric conditions in the Greater Athens Area, Greece, J. Atmos. Sol. Terr. Phys., 68, Khogali, A., and O. F. Al-Bar (1992), A study of solar ultraviolet radiation at Makkah solar station, Sol. Energy, 48, Koronakis, P. S., G. K. Sfantos, A. G. Paliatsos, J. K. Kaldellis, J. E. Garofalakis, and I. P. Koronaki (2002), Interrelations of UV-global/global/diffuse solar irradiance components and UV-global attenuation on air pollution episode days in Athens, Greece, Atmos. Environ., 36, Kudish, A., D. Wolf, and Y. Machalv (1983), Solar radiation data for Beer Sheva, Israel, Sol. Energy, 30, Lenoble, J. (1993), Atmospheric Radiative Transfer, pp. 532, Deepak Publishing, Hamptom, VA. Lopez-Abente, G., M. Pollán, A. Escolar, M. Errezola, andv. Abraira (1996), Atlas of cancer mortality and other causes of death in Spain Fundación Científica de la Asociación Española Contra el Cáncer Madrid, Avilable at: Atlas92.html (accessed June 2, 2007). Lorente, J., A. Redaño, and X. de Cabo (1994), Influence of urban aerosol on spectral solar irradiance, J. Appl. Meteorol., 33, Martínez-Lozano, J. A., F. Tena, and M. P. Utrillas (1999), Ratio of UV to Global broad band irradiation in Valencia, Spain, Int. J. Climatol., 19, 903. Masserot, D., J. Lenoble, C. Brogniez, M. Houet, N. Krotkov, and R. McPeters (2002), Retrieval of ozone column from global irradiance measurements and comparison with TOMS data. A year of data in the Alps, Geophys. Res. Lett., 29(9), 1309, doi: /2002gl McKinlay, A. F., and B. L. Diffey (1987), A reference spectrum for ultraviolet induced erythema in human skin, CIE J., 6, McPeters, R. D., et al. (1998), Earth probe Total Ozone Mapping Spectrometer (TOMS) data products user guide, NASA Tech. Rep. TP , NASA, Greenbelt, Md. Murillo, W., J. Cañada, and G. Pedrós (2003), Correlation between global ultraviolet ( nm) and global irradiation in Valencia and Córdoba, Spain, Renew. Energy, 28, Nikulin, G., and A. Karpechko (2005), The mean meridional circulation and midlatitude ozone buildup, Atmos. Chem. Phys., 5, Ogunjobi, K. O., and Y. J. Kim (2004), Ultraviolet ( mm) and broadband solar hourly radiation at Kwangju, South Korea: Analysis of their correlation with aerosol optical depth and clearness index, Atmos. Res., 71, of7

7 Robaa, S. M. (2004), A study of ultraviolet solar radiation at the Cairo urban area, Egypt, Sol. Energy, 77, Serrano, A., M. Antón, M. L. Cancillo, and V. L. Mateos (2006), Daily and annual variations of erythemal ultraviolet radiation in Southwestern Spain, Ann. Geophys., 24, Udo, S. (2000), Sky conditions at Ilorin as characterized by clearness index and relative sunshine, Sol. Energy, 69, World Health Organization (WHO) (1995), Protection against exposure to ultraviolet radiation, Technical Report WHO/EHG/95.17, Geneva. M. Antón, M. L. Cancillo, J. A. García, and A. Serrano, Departamento de Física, Universidad de Extremadura, Avd. de Elvas, s/n, Badajoz, Spain. (mananton@unex.es; mcf@unex.es; agustin@unex.es; asp@unex.es) 7of7