Monitoring of erythemal irradiance in the Argentine ultraviolet network

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D13, 4165, /2001JD001206, 2002 Monitoring of erythemal irradiance in the Argentine ultraviolet network Alexander Cede, 1 Eduardo Luccini, 2,3 Liliana Nuñez, 4 Rubén D. Piacentini, 2,3 and Mario Blumthaler 1 Received 14 August 2001; revised 25 October 2001; accepted 5 November 2001; published 3 July [1] The Ultraviolet (UV) Monitoring Network of the Argentine Servicio Meteorológico Nacional (National Weather Service) consists at present of nine stations from 22 to 64 latitude south equipped with biometers, broadband instruments that measure the erythemal irradiance. After a complete calibration of the instruments and reprocessing of the database a preliminary climatology of erythemal irradiance and erythemal exposure was built by analyzing the data of the first 2 4 years for each station. The influence of the measurement interval on the UV Index is quantified. A tropical high-altitude station in the Andean Altiplano reaches top UV Index values of 20 and daily doses of 10.6 kj/m 2, among the highest UV levels worldwide. Characteristic UV lndex and erythemal exposure in the central, most populated areas of Argentina are similar to those in other Southern Hemisphere stations and are higher than in comparable latitude regions of the Northern Hemisphere. The southern stations in the UV network are often affected by the Antarctic ozone hole. By comparing clear-sky irradiance measurements with aerosol-free radiative transfer calculations using the Total Ozone Mapping Spectrometer surface albedo climatology, typical attenuations from 2 to 15% due to aerosols were determined, reaching maxima of 30% at urban locations. On the other hand, increases of 3 6% due to higher surface albedo were obtained for snow-free conditions, and increases up to 15% were obtained for snow-covered terrain. INDEX TERMS: 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques 1. Introduction [2] The discovery of the Antarctic ozone hole [Farman et al., 1985] and the global decrease in the concentration of the stratospheric ozone [World Meteorological Organization (WMO), 1995, 1999] have led to concerns about increasing levels of ultraviolet (UV) radiation on the Earth s surface [Kerr and McElroy, 1993; Seckmeyer et al., 1997; Zerefos et al., 1998], possibly causing enhanced damage to the whole biosphere [Caldwell et al., 1998; Andrady et al., 1998; United Nations Environment Programme, 1998]. This motivated more detailed investigations in measuring and modeling of UV radiation. [3] The continuously recorded measurements by a calibrated and controlled surface instrument are indispensable 1 Institute for Medical Physics, University of Innsbruck, Innsbruck, Austria. 2 Instituto de Física Rosario, Consejo Nacional de Investigaciones Científicas y Técnicas Universidad Nacional de Rosario, Rosario, Argentina. 3 Facultad de Ciencias Exactas, Ingeniería y Agrimensura, Universidad Nacional de Rosario, Rosario, Argentina. 4 Servicio Meteorológico Nacional de Argentina, Buenos Aires, Argentina. Copyright 2002 by the American Geophysical Union /02/2001JD in order to determine an irradiance climatology for a place and use it as a reference for other methods, e.g., satellitederived data. The complete climatology for a given region can be studied if sufficient data are available from the instruments covering its different zones, and the climatology provides the possibility of evaluating the typical expected UV levels during the year, UV variability, and trends in order to prevent the risk of damage to life and materials. Particularly, to prevent hazards in the human population, the UV Index has been defined as a practical code to measure the risk of solar overexposure. [4] A large quantity of spectral and broadband UV instruments is distributed around the world (World Meteorological Organization, UV monitoring around the world, available at ). Several countries arranged a so-called UV monitoring network (UV net), in which the detectors are strategically distributed to cover the extent of the different regions. They are calibrated and maintained in the same way, and their data are collected and analyzed by a central authority [see, e.g., Silbernagl and Blumthaler, 1998; Bigelow et al., 1998; National Institute of Water and Atmospheric Research (NIWA), Ozone and UV information, available at katipo.niwa.cri.nz/lauder/uvinfo.htm, 2001; Bureau of Meteorology Research Center (BMRC), Ultraviolet index analysis and forecast, available at bmrc/daas/uvi.html, 2001]. AAC 1-1

2 AAC 1-2 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA Figure 1. Table 1. Geographical location of the Argentine UV monitoring network stations, numbered as in [5] The majority of the ground UV measurements are done in the Northern Hemisphere; therefore the less frequent measurements in the Southern Hemisphere are especially important to describe and understand UV levels on the whole globe [McKenzie et al., 1999]. With a latitudinal extension from 21 Sto56 S and furthermore a presence in the Antarctic continent, Argentina has very complete coverage from tropical to polar regions for the study of the ozone depletion and its consequences. [6] Starting in 1994, with the support of the World Meteorological Organization the Argentine Servicio Meteorológico Nacional provided nine locations with instrumentation to form an Argentine UV net. The database consists of at least 2 years for most of the stations, permitting a preliminary characterization of the erythemal irradiance at each place. A. Cede et al. (Calibration and uncertainty estimation of erythemal radiometers in the Argentine ultraviolet network, submitted to Applied Optics, 2002) (hereinafter referred to as Cede et al., submitted manuscript, 2002) describe the procedure to calibrate the instruments in the UV net and the reprocessing of the data and analyzed the uncertainty of the measurements. The present paper presents a detailed analysis of the erythemal irradiance (the UV Index) at each station. 2. Instruments and Locations [7] The locations of the UV net are distributed from the northernmost to the southernmost location on the continental area of Argentina and also in the Antarctic continent (see Figure 1 and Table 1). All the sites are equipped with biometers, either the model 501 UV radiometer of the Solar Light Co. or the model UVB-1 of Yankee Environmental Systems, Inc. These broadband UV instruments weight the incoming radiation with a function similar to the Commission Internationale de l Eclairage (CIE) action spectrum for erythema [McKinlay and Diffey, 1987]. The instruments were calibrated by means of a transfer biometer against the spectrometer of the University of Innsbruck, Austria, and a complete analysis was made of the measurement uncertainty (Cede et al., submitted manuscript, 2002). [8] Together with the measurement of classical meteorological data, several stations are additionally provided with instruments to measure total ozone column (see Table 1) and surface ozone, pyranometers with and without shadow

3 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA AAC 1-3 Table 1. Stations and Details of the Argentine UV Monitoring Network Station Number and Name (Province) Latitude South Longitude West Height, a m Start of Database Measurement Interval, min Ozone Instrument b 1, La Quiaca (Jujuy) July , Pilar (Cordoba) March AFO 3, Mendoza (Mendoza) March , Rosario (Santa Fe) May , Buenos Aires (Capital Federal) October Dobson 6, Comodoro Rivadavia (Chubut) January Dobson 7, San Julián (Santa Cruz) September AFO 8, Ushuaia (Tierra del Fuego) June Dobson 9, Base Marambio (Antarctica) October Dobson a Height is measured in meters above sea level. b AFO stands for a Russian ozonemeter; Dobson stands for a Dobson spectrometer. band to measure total ( nm) irradiance, and instruments to determine total net irradiance. The data acquisition and storage and also the maintenance of the instruments is done by specialized meteorologists of the Argentinean Servicio Meteorológico Nacional, who make an observation of the actual weather situation every hour or every 3 hours, including a detailed description of quantity and type of clouds. Table 2 shows characteristic values of different parameters for each station; the climatological albedo data are from Herman and Celarier [1997], and the total ozone values are Total Ozone Mapping Spectrometer (TOMS) level 2 data for the analyzed measurement period from the start of the database (Table 1) until 31 December 1999 or 30 June 2000 (for stations 6 and 7 only). The detector s signals, recorded at intervals of 1, 2, or 15 min (see Table 1), were converted to erythemal irradiance as described by Cede et al. (submitted manuscript, 2002). 3. Measurements [9] Erythemal irradiance is defined as the wavelengthintegrated spectral UV irradiance weighted with the CIE action spectrum for erythema [McKinlay and Diffey, 1987], normalized to 1 at 298 nm. It may be expressed as weighted or effective mw/m 2. However, this is no longer a purely physical unit as the CIE action spectrum is arbitrarily normalized at a certain wavelength; it is more a relative indicator for the risk of damage from exposure to the Sun. The consequences for each person, for example, to get a sunburn, are still strongly dependent on his personal characteristics, especially skin type [Fitzpatrick et al., 1974; Longstreth et al., 1998]. [10] In order to put the information in a simple and standardized form, the World Meteorological Organization and the World Health Organization define the UV Index (UVI) as 1 UVI = 25 mw/m 2 of erythemal irradiance in an open-ended scale ranging typically between 0 and 16 [WMO, 1994], which is very appropriate for regions at high to middle latitudes and low altitude but is sometimes exceeded at tropical and high-altitude sites. [11] When the UV Index is presented as one single number, then it is usually the highest UV Index measurement of the day. This value is, of course, dependent on the measurement interval; for example, an hourly integrated value will never reach a maximum compared to a 1 min integrated value. [12] To estimate the influence of the integration time, the UV Index was determined for the stations of the Argentine UV net in five different ways: (1) 1 min integration time; (2) moving 15 min integration time, starting every minute; (3) fixed 15 min integration time, starting every quarter hour; (4) moving 30 min integration time, starting every minute; and (5) fixed 30 min integration time, starting every half hour. Obviously, the moving indices must be larger than the Table 2. Characteristic Meteorological Data of the UV Net Stations Parameter Mean total ozone (TOMS), a DU Standard deviation total ozone (TOMS), DU Different ozone ground to TOMS, % Mean air temperature, C Median relative humidity, % Median wind velocity, m/s Median cloud coverage, oktas Portion 0 oktas moments, % Portion 8 oktas moments, % Mean air pressure, mbar Median visibility, b km 60 (70) 20 (25) 15 (20) 10 (20) 30 (30) 30 (30) 30 (30) 15 (20) Albedo climatology, % a TOMS stands for Total Ozone Mapping Spectrometer. b This is an observation of the most distant point in the horizon that can clearly be observed and is therefore dependent on the horizon. The value in parentheses is the most distant point for each station; the observed visibility can never exceed this number. Station

4 AAC 1-4 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA Figure 2. Medians of the differences of the various UV Index measurements relative to the fixed 30 min UV Index, as a function of the observed cloud coverage. fixed ones, and the shorter integration time indices must be larger than the longer ones. [13] As the distribution of the UV Index is not symmetric, its analysis will be based on median and percentiles. Figure 2 shows the medians of the differences M of the various UV Index measurements relative to the fixed 30 min UV Index as a function of the observed cloud coverage. They were calculated for the data from all the UV net stations together as M turned out to be quite independent of the station. For clear-sky conditions the difference is only given by the curvature of the daily course of the erythemal irradiance and is very small. With increasing cloud coverage and therefore a much more structured daily course, the differences in the UV Indices increase, reaching maxima at 8 oktas cloud coverage of 132%, 73%, 51%, and 38% for the 1 min index, the moving 15 min index, the fixed 15 min index, and the moving 30 min index, respectively. [14] Weighting the values of Figure 2 with the cloud coverage distribution of a selected station yields the differences of the UV Indices relative to the fixed 30 min UV Index. The medians of the differences for stations with little cloudiness (such as stations 1 5) are 5.0%, 2.3%, 0.6%, and 0.5% for the 1 min Index, the moving 15 min Index, the fixed 15 min Index, and the moving 30 min Index, respectively; more cloudy stations (like stations 8 and 9) show differences of 7.0%, 4.1%, 2.5%, and 1.1%. [15] These considerations should be taken into account when measurements of the UV Index with different integration times are compared. Since the WMO recommends an integration time of 30 min for the UV Index [WMO, 1998], the next sections use the fixed 30 min UV Index to discuss the irradiance levels in Argentina. The differences between the moving and the fixed 30 min UV Index are quite small for most days; under special conditions, however, the differences may be significant. [16] To describe the UV levels in the Argentine UV net, the whole database of stations is analyzed for dependence on the time in the year. Station 3 at Mendoza is not included, as its database is still too short (see Table 1). Figure 3 shows the moving median (solid curves), 16 84% range (dark shaded areas), and 5 95% range (light shaded areas) of the UV Index for each station. The moving values are calculated for a 30 day interval centered at every day in the year from 1 January to 31 December. For station 4 at Rosario, no median and percentile ranges were calculated in March because <20 measured days were available. [17] The UV levels in the Argentine stations are generally higher than at corresponding latitudes and altitudes in the Northern Hemisphere [Austrian Federal Ministry of Environment, Youth and Family Affairs, 1999; Frederick et al., 2000; U.S. Department of Agriculture (USDA) UVB Radiation Monitoring Program, Climatological site locations and data, available at uvb/uvb_personnel.html, 2001] and are similar to those of other Southern Hemisphere stations (NIWA, 2001). At the high-altitude station La Quiaca all conditions favoring extreme UV levels are present (except for high surface albedo). There the UV levels with extreme values of UV

5 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA AAC 1-5 Figure 3. Moving median (solid curves), 16 84% range (dark shaded areas), and 5 95% range (light shaded areas) of the UV indices along the year for each station, numbered as in Table 1. Index 20 and daily doses above 10 kj/m 2 are among the highest worldwide, even higher than the previously reported at the similar latitude and altitude Mauna Loa Observatory (Hawaii) (USDA, 2001). [18] Table 3 lists the days of the highest daily erythemal doses measured at each station. The maximum daily dose at Ushuaia and Comodoro Rivadavia was registered on 8 December 1998, when the ozone hole touched the

6 AAC 1-6 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA Table 3. Days of Extreme Erythemal Dose Registered at Each Station Station Date Daily Dose, kj/m 2 UV Index Cloud Coverage, oktas Ozone TOMS, DU Ozone Ground, DU 1, La Quiaca 28 December , Pilar 30 December , Rosario 19 December , Buenos Aires 10 December , Comodoro Rivadavia 8 December , San Julián 2 January , Ushuaia 8 December , Marambio 4 December South American continent, which usually occurs in September or October. On that day the ozone column was also very low at San Julián, but the cloud coverage prevented the daily dose from reaching a high level. In Marambio the long daytime near the summer solstice and the Antarctic ozone hole may result in daily doses above 7 kj/m 2, comparable to the maximum values at the Pampas stations. For example, the highest measured daily dose on 4 December 1998 equals even the highest measured at Buenos Aires (Table 3). It must also be considered that as this is the dose of the erythemal irradiance on a horizontal plane, the dose on inclined surfaces following the Sun or the actinic dose may be even higher than on a clear-sky summer day at the latitude of the Pampas stations. [19] A still commonly used unit for the erythemal dose is the minimum erythemal dose (MED). It corresponds to the dose, which causes a minimum erythema for a typical Caucasian skin type. When 1 MED is defined as 210 J/m 2, then the daily dose on 28 December 1997 at La Quiaca (see Table 3) corresponds to more than 50 MED! [20] Figure 4 presents the medians (squares) and percentile ranges (shaded areas) of the measured UV indices at 50 solar zenith angle, corrected for the Sun-Earth distance. The solid squares represent all atmospheric conditions; the open squares represent only clear-sky data, i.e., with <1 okta cloud coverage observation from the ground and no clouds in the Sun s direction. As the period in the year where 50 solar zenith angle is not reached at noon grows with the station s latitude, this data selection does not comprise all months at all stations. 4. Discussion 4.1. Effects of Clouds, Ozone, and Altitude [21] The lower fraction of totally overcast situations at La Quiaca (station 1) and the Patagonian stations 6 and 7 causes narrower percentile ranges of the UV Index than at Figure 4. Medians (squares) and percentile ranges (shaded areas) of the measured UV indices at 50 solar zenith angle, corrected for the Sun-Earth distance for all data (solid squares) and clear-sky data only (open squares) as a function of the station number.

7 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA AAC 1-7 Figure 5. Ratios of measured over aerosol-free modeled erythemal irradiances (RMC) along the year for each station, numbered as in Table 1. Squares are daily averages of the RMC for the solar zenith angle (SZA) (noon) SZA SZA (noon) +5. The solid curve is the smoothed median, the dark shaded area is the 16 84% range, and the light shaded area is the 5 95% range. the remaining stations (see Table 2 and Figure 3). The 5% percentile of the UV Index at 50 solar zenith angle given in Figure 4 is smaller than 1 UVI only for the Pampas stations 2, 4, and 5 and for the Antarctic station 9 as a consequence of very thick cloud layers at these locations, which are very rare at the other sites.

8 AAC 1-8 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA Table 4. UV Radiative Transfer Model a Characteristics Description/Value Model type DISORT Model [Stamnes et al., 1988], Pseudospherical Version [Mayer et al., 1997] Number of streams 16 Wavelength grid 121 wavelengths from 280 to 400 nm, 1 nm step Extraterrestrial flux SUSIM-Atlas 3 spectrum (M. E. Van Hoosier, personal communication, 1996) Layers 58 layers of 0.5 km width from 0 to 2 km, of 1 km width from 2 to 50 km, and of 5 km width from 50 to 80 km (0 km, height of the station above sea level) Rayleigh scattering formula from Bodhaine et al. [1999] Ozone absorption Cross sections by Bass and Paur [1985] Ozone profile TOMS Version 7 climatology of ozone profiles Total ozone column TOMS/Earth Probe/level 2 data (Overpass site list Earth Probe TOMS, Atmospheric Chemistry and Dynamics Branch, Goddard Space Flight Center, NASA, gov/eptoms/ep_ovplist_l.html, 2001) Temperature profile COSPAR (International Reference Atmosphere of 1986 (0 km to 120 km), nasa.gov/space/model/atmos/cospar1.html, 1986) atmosphere for corresponding latitude and time, corrected with the measured surface temperature Pressure ground surface pressure measurement Surface albedo climatology from Herman and Celarier [1997] a Definitions are as follows: DISORT, discrete ordinates radiative transfer model; SUSIM, Solar Ultraviolet Spectral Irradiance Monitor; and COSPAR, Committee on Space Research. [22] The so-called Bolivian winter is a regular meteorological event of the Puna de Atacama region where the cloud cover in the nominal summer months of January and February is higher than in the rest of the year. Therefore the percentile ranges for the UV Index are larger in these months than those of the corresponding spring months of October and November with the same solar zenith angles (see station 1 in Figure 3). Out of this Bolivian winter period the little cloudiness causes a very narrow 5 95% range. So 95% of the days in December at La Quiaca give UV indices above 16 and daily doses above 7 kj/m 2. [23] The annual total ozone variation at midlatitudes with typical highest values in spring [Herman et al., 1991, 1993] causes more erythemal irradiance in summer than in spring for stations 2 7 (see Figure 3). However, this is inverted at higher latitudes, where the influence of the Antarctic ozone hole causes broader percentile ranges and higher maximum UV indices during spring compared to summer (see station 8 and especially station 9 in Figure 3). [24] The median of the UV Index at 50 solar zenith angle for all conditions is 65% higher at the high-altitude station 1 than at the remaining stations near sea level (see Figure 4). This corresponds to an average increase of erythemal irradiance of 20% per kilometer and is also influenced by the average ozone difference of >20 Dobson units (DU) (1 DU = atm cm) and the little cloud cover at station 1 (see Table 2). At clear-sky conditions this altitude effect reduces to 15% per kilometer Effects of Aerosols and Albedo [25] A way to eliminate the influence of the solar zenith angle, the cloud cover, the total ozone amount, the station s altitude, and partly the surface albedo is to compare the measured clear-sky data with model calculations, leaving the aerosols as the principle remaining parameter. Figure 5 presents the ratios of the measured over the aerosol-free calculated erythemal irradiances (RMC) for all the stations, using the radiative transfer model described in Table 4. The squares are the daily averages of the RMC for the solar zenith angle (SZA) which obey SZA (noon) SZA SZA (noon) +5, e.g., for Buenos Aires a SZA interval from 11 to 16 in the summer solstice and 58 to 63 in the winter solstice. An additional limitation to solar zenith angles <70 is included for the stations with higher latitude. The limit was chosen as the uncertainty of the biometer data becomes larger than ±10% for higher solar zenith angles (Cede et al., submitted manuscript, 2002). The moving median (solid curve), 16 84% range (dark shaded area), and 5 95% range (light shaded area) are calculated for 30 day intervals centered at every day in the year from 1 January to 31 December and then smoothed. The discontinuous zones result from too few clear-sky data (stations 1 and 4) or from the solar zenith angle limitation (stations 7 9). [26] The uncertainty of the RMC depends on the absolute level of the measured and modeled erythemal irradiances. For the biometer data a total uncertainty (2 standard deviations) of ±10% at 70 solar zenith angle and ±6% for solar zenith angles below 50 was estimated (Cede et al., submitted manuscript, 2002). The total uncertainty of the modeled irradiances includes that of the extraterrestrial spectrum, ±3% in this wavelength range (M. E. Van Hoosier, The ATLAS-3 solar spectrum, personal communication, 1996), and the even larger uncertainties of the other input parameters (e.g., vertical profiles, temperature, pressure, etc.) and the calculation of the atmospheric transmission. Because of the uncertainty of the absolute levels of measurements and model calculations the curves in Figure 5 may collectively shift up or down by several percent. Particularly, if ground-based total ozone measurements, which are always lower than the TOMS/Earth Probe level 2 data (see Table 2), were used as input in the model, the RMC would decrease by a percentage slightly higher than that listed in Table 2 as the radiation amplification factor for erythemal irradiance is 1.1 [Madronich et al., 1998]. [27] Most of the stations show an annual course of the RMC with a minimum in winter. One reason is that the SZA is smaller in summer than in winter. As the attenuation of the erythemal irradiance at a constant aerosol optical depth increases with SZA, a seasonal variation is generated. However, when the same analysis is made with the daily means of just the SZA interval from 65 to 70 for the whole year, the winter minima do not totally disappear, indicating a seasonality in the aerosol optical depth too. [28] The RMC at the high-altitude station La Quiaca is always near 100% because of the low noon SZA and small aerosol optical depth during the whole year. No industry, poor vegetation, and little wind provide no big local aerosol sources, and the few aerosols do not enlarge by condensed water vapor in this region of very low humidity (see Table 2). [29] Significant attenuation of the RMC to nearly 70%, compared to the occasional clean days with >95%, is sometimes observed at the Pampas stations 2, 4, and 5. For this reduction of the global erythemal irradiance, aerosol optical depths of 1.0 at 340 nm with single-scatter-

9 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA AAC 1-9 ing albedo 0.9 are needed. For the big cities Rosario and Buenos Aires such conditions with urban aerosols are possible [Kylling et al., 1998]. However, for Pilar, situated in a rural-urban environment, these occasionally low RMC are surprising at first glance. Therefore another contribution to take into account is the possible arrival of northern Argentine and Amazonian biomass burning aerosols to this region especially in late winter and spring, transported by winds from low (tropical) to middle latitudes, as described in the results of the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment [Gleason et al., 1998; Kaufman et al., 1998; Prins et al., 1998; Trosnikov and Nobre, 1998; Eck et al., 1998; Dubovik et al., 1998]. [30] For the southernmost stations an important fact must be pointed out: The surface albedo climatology by Herman and Celarier [1997] was based on the lowest reflectivity measurements from TOMS/Nimbus 7 in the whole period , representing therefore a lower limit of the surface albedo at each place all over the world. So the surface albedo climatology input in the model will not be representative each time the ground presents a different condition. [31] The Patagonian stations 6 and 7, Comodoro Rivadavia and San Julián, are in rural-urban and rural environments, respectively, near the Atlantic coast. The median and percentiles of RMC at station 6 are, as expected, near 100% in summer. For station 7, however, the annual variation of the RMC is much more pronounced, and some days in summer reach almost 110%. A possible explanation for an underestimation of the aerosol-free erythemal irradiance by the model calculations is the surface albedo. The terrain in this region consists of a sandy base with scattered tufts of dry, yellow grass. On the occasional rainy days the surface albedo is probably similar to the climatology, between 1 and 4%, more or less corresponding to wet beach conditions, whereas during the much more frequent dry periods the surface albedo is possibly more like that of the Saharan desert or dry beach conditions, i.e., around 8% [Herman and Celarier, 1997] or up to even 15% [Feister and Grewe, 1995]. This albedo difference could explain a reduction of 3 6% of the high summer RMC of 110% at San Julián. [32] The median RMC at Ushuaia, station 8, slightly exceeds 100%; several winter days reach even >115%. Together with the small aerosol content during the year, the explanation may also be the surface albedo: Each time, when the terrain is snow-covered in a season where it was sometimes not, the model will underestimate the irradiance. A surface albedo of 60% in August (because of snow cover) instead of the climatological value of 26% can explain the RMC on these days. Station 9, Marambio, is not continuously snow- and ice-covered (in summer the station s ground consists frequently of muddy earth); therefore the same albedo problem as at Ushuaia could be expected. However, the annual curves rarely exceed 100%. Eventually, the real albedo was near the climatological value for the period of this data set. 5. Conclusion [33] On the basis of the database of recently calibrated biometers the erythemal irradiance in the Argentine UV Monitoring Network stations, covering different characteristic zones from tropical to Antarctic regions, is analyzed. The central, most populated areas of Argentina at sea level present typical summer UV Indices of around 12, which implies a high risk of damage especially for people that spend many hours outside and for the more sensitive skin types. Southern Patagonia shows UV Indices of 8, and the Antarctic Peninsula has a typical UV Index between 4 and 5 in summer, with an increasing variability in springtime for the higher-latitude stations because of the ozone hole event. Probably in no constantly populated region of the world are the UV levels as extreme as they are in the Puna de Atacama. In this high-altitude Andean plane, where several million people are living, extreme values of UV Index 20 and daily doses above 10 kj/m 2 can be measured. There the UV exposure may probably cause health problems to darker skin types too. [34] The present work can be considered the basis for future studies. The periodic control of the stability of the instruments will allow the confident extension of the Argentine UV monitoring network database, including the possibility of analyzing long-term tendencies in the UV irradiance. [35] Acknowledgments. This work was supported by the Argentine Skin Cancer Foundation, the World Meteorological Organization, the Argentine Servicio Meteorológico Nacional, the Argentine Agency for the Promotion of Scientific and Technological Research and the University of Innsbruck, Austria. The authors would like to thank the president of the Argentine Skin Cancer Foundation, Fernando Stengel, for his great personal dedication to advance the project; Rumen Bojkov for his support in the installation of the UV net; and the scientists of the Atmospheric Chemistry and Dynamics Branch of the Goddard Space Flight Center, NASA, for their support and the incorporation of the Argentine locations in the overpass data. A special thanks to the people of the Argentine Servicio Meteorológico Nacional for the very friendly reception and collaboration and to Pablo García for his computational assistance. References Andrady, A. L., S. H. Hamid, X. Hu, and A. Torikai, Effects of increased solar ultraviolet radiation on materials, J. Photochem. Photobiol. B, 46, , Austrian Federal Ministry of Environment, Youth and Family Affairs, Monitoring of Total Ozone and UV-B Radiation in Austria, Berger, Horn, Austria, Bass, A. M., and R. J. Paur, The ultraviolet cross-section of ozone, I, The measurements, in Atmospheric Ozone: Proceedings of the Quadrennial Ozone Symposium, edited by C. Zerefos and A. Ghazi, pp , D. Reidel, Norwell, Mass., Bigelow, D. S., J. R. Slusser, A. F. Beaubien, and J. H. Gibson, The USDA Ultraviolet Radiation Monitoring Program, Bull. Am. Meteorol. Soc., 79(4), , Bodhaine, B. A., N. B. Wood, E. G. Dutton, and J. R. Slusser, On Rayleigh optical depth calculations, J. Atmos. Ocean. Technol., 16, , Caldwell, M. M., L. O. Bjorn, J. F. Bornman, S. D. Flint, G. Kulandaivelu, A. H. Teramura, and M. Tevini, Effects of increased solar ultraviolet radiation on terrestrial ecosystems, J. Photochem. Photobiol. B, 46, 40 52, Dubovik, O., B. N. Holben, Y. J. Kaufmann, M. Yamasoe, A. Smirnov, D. Tanré, and I. Slutsker, Single-scattering albedo of smoke retrieved from the sky radiance and solar transmittance measured from ground, J. Geophys. Res., 103(D24), 31,903 31,923, Eck, T. F., B. N. Holben, I. Slutsker, and A. Setzer, Measurements of irradiance attenuation and estimating of aerosol single-scattering albedo for biomass burning aerosols in Amazonia, J. Geophys. Res., 103(D24), 31,865 31,878, Farman, J. C., B. G. Gardiner, and J. D. Shanklin, Large losses of total ozone in Antarctica reveal seasonal ClOX/NOX interaction, Nature, 315, , Feister, U., and R. Grewe, Spectral albedo measurements in the UV and visible region over different types of surfaces, Photochem. Photobiol., 62, , 1995.

10 AAC 1-10 CEDE ET AL.: ERYTHEMAL UV IN ARGENTINA Fitzpatrick, T. B., M. A. Pathak, L. C. Harber, M. Seiji, and A. Kukita, Sunlight and Man, Univ. of Tokyo Press, Tokyo, Frederick, J. E., J. R. Slusser, and D. S. Bigelow, Annual and interannual behavior of solar ultraviolet irradiance revealed by broadband measurements, Photochem. Photobiol., 72, , Gleason, J. F., N. C. Hsu, and O. Torres, Biomass burning smoke measured using backscattered ultraviolet radiation: SCAR-B and Brazilian smoke interannual variability, J. Geophys. Res., 103(D24), 31,969 31,978, Herman, J. R., and E. Celarier, Earth surface reflectivity climatology at 340 nm to 380 nm from TOMS data, J. Geophys. Res., 102(D23), 28,003 28,011, Herman, J. R., R. McPeters, R. Stolarski, D. Larko, and R. Hudson, Global average ozone change from November 1978 to May 1990, J. Geophys. Res., 96(D9), 17,297 17,305, Herman, J. R., R. McPeters, and D. Larko, Ozone depletion at northern and southern latitudes derived from January 1979 to November 1991 Total Ozone Mapping Spectrometer, J. Geophys. Res., 98(D7), 12,783 12,793, Kaufman, Y. J., et al., Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment, J. Geophys. Res., 103(D24), 31,783 31,808, Kerr, J. B., and C. T. McElroy, Evidence for large upward trends of ultraviolet-b radiation linked to ozone depletion, Science, 262, , Kylling, A., A. F. Bais, M. Blumthaler, J. Schreder, C. S. Zerefos, and E. Kosmidis, Effect of aerosols on solar UV irradiances during the Photochemical Activity and Solar Ultraviolet Radiation campaign, J. Geophys. Res., 103(D20), 26,051 26,060, Longstreth, J., F. R. de Gruijl, M. L. Kripke, S. Abseck, F. Arnold, H. I. Slaper, G. Velders, Y. Takizawa, and J. C. van der Leun, Health risks, environmental effects of ozone depletion, 1998 assessment, J. Photochem. Photobiol. B, 46, 20 39, Madronich, S., R. L. McKenzie, L. O. Björn, and M. M. Caldwell, Changes in biologically active ultraviolet radiation reaching the Earth s surface, J. Photochem. Photobiol. B, 46, 5 19, Mayer, B., G. Seckmeyer, and A. Kylling, Systematic long-term comparison of spectral UV measurements and UVSPEC modeling results, J. Geophys. Res., 102(D7), , McKenzie, R. L., B. J. Connor, and G. E. Bodeker, Increased summertime UV observed in New Zealand in response to ozone loss, Science, 285, , McKinlay, A. F., and B. L. Diffey, A reference action spectra for ultraviolet introduced erythema in human skin, in Human Exposure to Ultraviolet Radiation: Risks and Regulations, edited by W. R. Passchier and B. M. F. Bosnajakovich, pp , Elsevier Sci., New York, Prins, E. M., J. M. Feltz, W. P. Menzel, and D. E. Ward, An overview of GOES-8 diurnal fire and smoke results for SCAR-B and 1995 fire season in South America, J. Geophys. Res., 103(D24), 31,821 31,835, Seckmeyer, G., B. Mayer, G. Bernhard, A. Albold, R. Erb, H. Jaeger, and W. R. Stockwell, New maximum UV irradiance levels observed in central Europe, Atmos. Environ., 31(18), , Silbernagl, R., and M. Blumthaler, First experiences with the Austrian UV monitoring network, paper presented at European Conference on Atmospheric UV Radiation, Eur. Comm., Helsinki, Stamnes, K., S. C. Tsay, W. Wiscombe, and K. Jayaweera, A numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media, App. Opt., 27, , Trosnikov, I. V., and C. A. Nobre, Estimation of aerosol transport from biomass burning areas during the SCAR-B experiment, J. Geophys. Res., 103(D24), 32,129 32,137, United Nations Environment Programme, Environmental effects of ozone depletion: 1998 assessment, J. Photochem. Photobiol B Biol., 46, 1 108, World Meteorological Organization (WMO), Report of the WMO meeting of experts on UV-B measurements, data quality and standardization of UV indices (Les Diablerets, July, 1994), WMO Rep. 95, Geneva, Switzerland, World Meteorological Organization (WMO), Scientific assessment of ozone depletion: 1994, Global Ozone Research and Monitoring Project, WMO Rep. 37, Geneva, World Meteorological Organization (WMO), Report of the WMO-WHO meeting of experts on standardization of UV indices and their dissemination in the public (Les Diablerets, July, 1997), WMO Rep. 127, Geneva, Switzerland, World Meteorological Organization (WMO), Scientific assessment of ozone depletion: 1998, Global Ozone Research and Monitoring Project, WMO/Rep. 44, Geneva, Switzerland, Zerefos, C. S., C. Meleti, D. S. Balis, K. Tourpali, and A. F. Bais, Quasibiennial and longer-term changes in clear-sky UV-B solar irradiance, Geophys. Res. Lett., 25(23), , A. Cede and M. Blumthaler, Institute for Medical Physics, University of Innsbruck, Mullerstrasse 44, A-6020 Innsbruck, Austria. (alexander.cede@ uibk.ac.at; mario.blumthaler@uibk.ac.at) Eduardo Luccini and Rubén D. Piacentini, Instituto de Física Rosario, CONICET-Universidad Nacional de Rosario, 2000 Rosario, Argentina. (luccini@ifir.edu.ar; ruben@ifir.edu.ar) Liliana Nuñez, Servicio Meteorológico Nacional de Argentina, C1002 ABN Capital Federal, Buenos Aires, Argentina. (lnunez@meteofa.mil.ar)