• Introduction
• Results
• Conclusions
• Acknowledgements
• References

FIGURES

• Figures 1a, 1b
• Figure 2
• Figure 3
• Figure 4

In the present work we made detailed radiative transfer model calculations based on the pseudo-spherical DISORT algorithm, of solar ultraviolet irradiances incident over the Atacama desert region around 18º S within a 6º latitudinal interval as function of altitude, from 0 to 7 km, in order to interpolate all possible situations in the region. The resulting irradiances weighted with the erythemal action spectra of McKinlay and Diffey, transformed in UV index through the internationally accepted factor of 0.04 if the first quantity is expressed in mW/m2, are compared with ground based measurements done with a biometer of the SCO3P (Southern Hemisphere Ozone Project)/WMO network at La Quiaca, Argentina, (3459 m a.s.l.), and at Arica (23 m a.s.l. at the Pacific coast) and Copaquilla (3166 m a.s.l.) Chili. The comparison between these measurements and model calculations shows rather good agreement.

UV index for different months of the year are presented, considering typical as well as extreme values of the most significant geophysical variables (ozone, aerosol and albedo for ground surface with and without snow). A mean curve derived from results for all months normalized to the maximum value, gives a slope of 5.6 %/km for no snow and normal ozone conditions and 4.1 %/km for snow (high albedo) and low ozone. In this last case, the absolute value of the erythemal irradiance at the highest mountains picks are within the highest of the Earth. These mean values can be improved significantly if the altitude range is divided in three parts: 0-1 km, 1-3 km and 3-7 km. The corresponding slopes are: 9, 5.9 and 3.9 %/km for the first (normal) case and 6.4, 4.4 and 2.8 %/km for the second (extreme) case.

Also, the yearly variation of this UV index is presented for different altitudes.

Introduction

The Puna of Atacama is a high altitude intertropical very dry desert region placed on the Andes mountains in the latitude-longitude range of about (12º - 24º S, 65º - 71º W). Since in the summer Southern Hemisphere the Earth-Sun actual distance is the lowest one (it pass at the minimum distance, in the beginning of January), the corresponding extraterrestrial solar irradiance, proportional to the inverse square of the ratio of this distance to the mean one, is 6.4 % higher than the Northern Hemisphere one (see for example, Iqbal, 1983). Due to all these circumstances, which determine naturally low intertropical ozone total column and low aerosol atmospheric content, the global as well as UV solar irradiances are within the highest in the world (Piacentini and Herman, 1996; Piacentini and Cede, 2000).

The detailed knowledge of the solar UV irradiance is of great importance in relation to its absolute value in each region of the Earth as well as in order to determine its variation due to ozone and aerosol changes by natural or antropogenic causes (see for example, Herman et al, 2000). Also it is related to different biological and material degradation actions (UNEP, 1998; Zerefos and Bais, 1997).

Table 1. Solar risk qualification for the different ranges of the UV Index.

 

UV Index

Solar risk qualification
0-2.9	Very low
3-4.9	Low
5-6.9	Moderate
7-9.9	High
10-13.9	Very high
14 or more	Extreme

The solar risk measured though the UV index, is directly related to the erythemal irradiance (in SI units) through the factor 40 (or 0.04 when the common mW/m2 unit of irradiance is used). This irradiance is obtained multiplying the spectral solar UV irradiance by the well established erythemal action spectra of McKinlay and Diffey and integrating the product in the UV (280 nm - 400 nm) range. This action spectra measures the effectiveness of the incident photons in reddening the normal reference skin and it is mainly concentrated in the UVB (280 nm -320 nm) range. So, it is an alarm signal for skin damage (Jagger, 1985). Table 1 gives the relation between the numbers related to the solar UV Index and the qualification adopted for describing the correspondent erythemal intensity.

Results

In this work we present results of UVB and erythemal irradiances incident over the Atacama desert region around 18º S within a 6º latitudinal interval as function of altitude obtained employing the radiative transfer pseudo-spherical DISORT algorithm. The ozone total column has a small dependence with latitude in this region and the global trends produced in the last decades by antropogenic causes has also very small contribution from this intertropical region (Bojkov et al, 1999). So an annual mean constant value has been considered. The other geophysical variables that mainly influence the UV irradiance is the aerosol total content of the atmosphere and the albedo (surface reflectivity). The program automatically adapt the ozone, aerosol and pressure to the corresponding altitude.

The erythemal irradiance, calculated for each month of the year as function of altitude from 0 to 7 km, a very low ozone layer (the mean minus 2 standard deviations) and for snow and no-snow conditions is displayed in figure 1. It can be seen that the UV index with ground not covered with snow, is systematically lower for each month and altitude than the UV index for snow conditions. Normalizing all curves to the absolute maximum and obtaining the mean value, it is possible to evaluate the mean slope for all altitude or for different altitudinal ranges (see also Tables 2 and 3).

Figure 1. Top: UV Index determined through radiative transfer model calculations for all months of the year and altitude values present in the (18º±6º) S geographical area of the Atacama region at the Andes Mountains for snow (left) and now snow (right) covered surface. This index is internationally defined as UVI=0.04*Erythemal Irradiance in mW/m². Bottom: yearly mean UV Index increase for different altitudinal ranges, in percent per km.

Table 2. Solar risk UV index slope (in percent per km) for different altitudes, for the Atacama Region (18º±6º) S and different months of the year corresponding to equinox and solstice. More common situations are emphasized.

 

Altitude range

 

Month

(0 - 0.5) km				(6 - 7) km
Decem	March	June	Sept	Dec	March	June	Sept
Without snow	9.07	9.07	10.22	9.34	3.00	3.05	3.56	3.13
With snow	6.46	6.69	7.55	6.78	1.91	1.99	2.43	2.05

Table 3. Yearly mean of the solar risk UV index slope for different altitudes, for the Atacama Region (18.5° S). More common situations are emphasized.

 

Altitude range

(0-1) km	(1-3) km	(3-7) km
Without snow	9	5.9	3.9
With snow	6.4	4.4	2.8

A verification of the calculated values can be made comparing these values with those measured at La Quiaca (3459 m asl), Argentina with a YES biometer (figure 2) and at Arica (23 m asl at the Pacific coast) and Copaquilla (3166 m asl), Chili using an IL 1400 A biometer (see Tables 4 and 5). Kondratyev and Varotsos (2000) presented results of erythemal irradiance measured with a UV-501 Solar Light biometer over Athens, Greece on June 1997. Even if they were obtained in a region of the Earth with different atmosphere and surface albedo, the data show the same qualitative "saturation to high altitude" behavior as the present calculations.

Table 4. Percentage variation of erythemal irradiance (directly related with the solar risk) measured with an IL1400A instrument at horizontal plane around solar noon in Copaquilla (18.40º S, 69.65º W, 3166 m a.s.l.) and in Arica (18.47º S, 70.31º W, 23 m. a.s.l.), Chili and comparison with the results of calculations with radiative transfer model in "no snow" case for December month (see figure 2). Both results are within error bands.

 

RESULT OF

RELATIVE DIFFERENCE (%)
MEASUREMENT	16.4
MODEL	15.5

Table 5. UV Index (solar risk) determined at La Quiaca (22.11º S, 65.57º W, 3459 m a.s.l.), Argentina through the measurement of the erythemal irradiance done with a YES UVB-1 biometer of the SCO3P (Southern Cone Ozone Project)/WMO at solar noon on February 20, 1999 compared with model result (see figure 2). Both results are within error bands and correspond to Extreme qualification

 

RESULT OF

UV Index
MEASUREMENT	18.6
MODEL	17.9

Results for very high altitude conditions like the top of the Parinacota volcano (Atacama desert, Andes mountains, 6342 m asl) for all months in snow ground conditions are described in Table 6. During all months the values are in the Very High or Extreme ranges.

Erythemal irradiances for consecutive days are given in figure 3 for normal mean ozone and low and high albedo.

Table 6. UV Index (solar risk) at the pick of Parinacota volcano (Atacama desert, Andes mountains, 6342 m a.s.l.) for all months in snow ground conditions. Note that during all months the values are in the Very High or Extreme ranges.

 

MONTH

 

January

 

February

 

March

 

April

 

May

 

June

 

July

 

August

 

September

 

November

 

December

UV INDEX
28.7
27.9
24.8
19.5
15.1
13.3
14.8
19.0
24.1
October	27.5
28.4
28.4

Figure 2. UV index determined from erythemal irradiance measurements done with the biometer of the SCO3P (Southern Hemisphere Ozone Project)/WMO network at La Quiaca, Argentina (3459 m asl). Mean of the last 3 years clear sky maxima plus and minus one standard deviation.

Figure 4 shows the UV index for the Southern Hemisphere solstice and equinoxes and the day of the maximum observed irradiance at all altitudes (January 29).

Figure 3. Erythemal irradiance calculated for the Atacama desert region for consecutive days, considering normal mean ozone (260 DU) and low (0.1) no snow and high (0.6) snow albedo, the last one at high altitudes (between 5 and 7 km).

Figure 4. Solar UV index for the Atacama desert region as function of altitude, for Southern Hemisphere solar solstices and equinoxes and the day of maximum absolute UV irradiance at all altitudes (January 29). Values for mean normal ozone (260 DU) and albedo (0.1) are given as black curves and high albedo (0.6) corresponding to snow at high altitudes (5 to 7 km) are given as red curves.

Conclusions

In spring and summer, extreme values of the UV index, larger by a factor of two with respect to those registered at mid-latitudes near sea level, were determined for this very dry and desert high altitude region of Atacama, and verified at particular locations and periods of the year. Even in autumn and winter, rather high values of this index are common. Persons that live permanently there or that spend some periods of the year, must implement measures for protecting their skin and eyes.

Acknowledgements.

This work was partially financed by ANPCyT (Agencia Nacional de Promoción Científica y Tecnológica) of Argentina, FOMEC (Fondo para el Mejoramiento de la Enseñanza Universitaria, Ministerio de Educación de la Nación), Universidad Nacional de Rosario, Universidad de Tarapacá, Chile, Servicio Meteorológico Nacional de Argentina and CONICET. The technical computational assistance of Pablo García at the Astronomical Observatory of Rosario, Argentina, is kindly appreciated.

References

Bojkov R. "Ozone variability and trends" in "Scientific Assessment of Ozone Depletion: 1998". WMO Report 44, chapter 4, 1999.

Herman J, Piacentini R D, Ziemke J, Celarier E and Larko D. "Interannual variability of UVB radiations". Journal of Geophysical Research, in press (2000).

Iqbal M. "Introduction to Solar Radiation", Academic Press, 1983.

Jagger C. "Solar UV Action on Living Cells". Praeger, New York, 1985

Kondratyev K Ya. and Varotsos C. "Atmospheric Ozone Variability. Implications for Climate Change, Human Health and Ecosystems". Springer, Berlin, 2000.

Piacentini R D and Cede A. "Very high solar irradiance at Puna of Atacama desert", submitted for publication, 2000.

Piacentini R D and Herman J. Energías Renovables y Medio Ambiente (in spanish). 1, 6, 1996.

UNEP Report "Environmental Effects of Ozone Depletion". Published in the special issue of Journal of Photochemistry and Photobiology B: Biology. 46, Numbers 1-3, 1998.