Meteorologisches Institut Theresienstr. 37 D –80333 Munich moni@meteo.physik.uni-muenchen.de

1. Dependence of UV radiation on altitude and aerosol optical depth: example Bolivia Monika Pfeifer, Joachim Reuder, Peter Koepke, Frank Wagner Background Generally, solar UV radiation increases with increasing altitude due to smaller optical air masses. This increase in solar radiation is called the altitude effect (AE) which is given as an increase in irradiance relative to the valley side in percent per 1 km. . Compared with low altitudes, radiation at high altitudes traverses a shorter path length through the atmosphere and thus undergoes less scattering and absorption. The constituents of the atmosphere in the layer between the mountain and the valley sites and their interactions with UV radiation are responsible for the different irradiances at the two sites. These interactions are Rayleigh scattering by air molecules, absorption by tropospheric ozone and other gases, scattering and absorption by aerosols. All these effects depend on the solar zenith angle (SZA). Since aerosols are mainly concentrated in the boundary layer, the altitude effect will strongly increase if the mountain station happens to come out of the aerosol layer. Because of the decrease of temperature with altitude, the probability of snow cover at the mountain station increases. Moreover, there is the possibility of clouds between the two levels. Both enhance the albedo of the mountain station and UV radiation is increased through multiple scattering. Figure 1 shows an overview of all the parameters contributing to the altitude effect. Figure 2 : Map of Bolivia with the stations used in this study. Altitude SZA Albedo Measurements Erythemally weighted UV radiation was measured with broadband radiometers in Bolivia in 2000/2001 at four altitudes. In 2002 UV radiation and aerosol optical depth were measured in Caranavi, La Paz and Chacaltaya. Caranavi is located in the tropical part of Bolivia at an altitude of 605 m. Because of daily precipitation and wash out of the aerosols, the mean aerosol optical depth was exceptionally low. La Paz (3420 m) and El Alto (4000 m), the stations of the plateau, are important sources of aerosol in altitudes above 3000 m. Mount Chacaltaya (5240 m) is an example of an isolated mountain peak out of the boundary layer. See figure 2 for locations. Clouds Boundary Layer (O3, Aerosol) Pressure Figure 1: Overview of all the parameters contributing to the altitude effect. Methods Results Figure4: The altitude of the station against the averaged UVI for elevations 20 – 70 °. For clarity, the data has been grouped into boxes corresponding to the fixed elevations examined. This elevation is displayed above each box. Figure 3: Calibrated UV measurements in Caranavi. In the upper figure UV radiation varies with clouds, changes in the total ozone content, aerosol properties and albedo. In the lower figure only cloud free cases are considered and the data is normalized to a fixed total ozone content. Figure 4 shows the altitude of the station against the averaged UVI for an elevation of 20 - 70 °. The difference in UVI for an elevation divided by the difference in altitude of the two stations gives the altitude effect. The mean altitude effect measured between Caranavi and La Paz was 5.5 – 10.1 %/ km dependent on the solar zenith angle. These values are in good agreement with the values reported by Cabrera (4 –10 %/ km) and Piazena (8 -10 %/ km) from the Chilean Andes. An altitude effect of 9.2-12.7%/ km between La Paz and Chacaltaya and 8.1-12.5 %/ km between Caranavi and Chacaltaya was obtained. Because the Chacaltaya is already located outside the boundary layer, the differences in UV radiation between the plateau and the mountain are very high. Cabrera presented an altitude effect of only 2 %/ km for altitudes from 3000 – 5000 m. Our effect is more pronounced because of the important aerosol sources in altitudes of more than 3500 m. This effect is even stronger at El Alto. The altitude effect between Caranavi and El Alto shows relative small values of 4.1 - 6.8 %/ km. The altitude effect between El Alto and Chacaltaya reaches values from 14.3 - 23.0 %/ km. In El Alto less UV radiation was measured than in La Paz, although it lies 580 m higher than La Paz. A comparision of La Paz and El Alto would result in negative altitiude effects, because of the dominating aerosol effect. The maximal altitude effect for erythemally weighted UV radiation was measured for an elevation of 20 °. Figure 3 shows the measured UV irradiance at Caranavi after calibration. In the upper figure, UV radiation is affected by changes in cloudiness, total ozone content, aerosol properties and albedo. In the lower figure, the irradiance was normalized to a fixed total ozone content and only cloud free measurements were taken into account, because variations in cloud cover between the mountain and valley site would dominate the altitude effect. At this point the UV radiation varies only with solar elevation, atmospheric turbidity and changes in the albedo of the terrain. The albedo in La Paz, Caranavi and El Alto can be assumed to be constant during the measurements, so the remaining variability of the UVI represents natural changes of atmospheric turbidity during the measurements. The mean UVI and standard deviation were calculated for elevations from 20 – 70 ° in steps of 10 ° and this data was used to calculate the altitude effect. Conclusion Due to the great natural variability in the altitude effect, even under cloudless conditions, it is impossible to describe this effect by a single number. Moreover, it is necessary to identify the actual atmospheric conditions and albedo of the terrain at the measurement sites in order to interprete the effect correctly and understand it properly. We propose to seperate the effects of the alittude, the albedo and the aerosol optical depth on the altitude effect and to describe it in the future as the combination of three different effects. . Meteorologisches Institut Theresienstr. 37 D –80333 Munich moni@meteo.physik.uni-muenchen.de References: Pfeifer, M (2003): Höhenabhängigkeit der UV Strahlung: Untersuchung am Beispiel Boliviens (Diploma Thesis) Cabrera et al. (1995): Variations in UV radiation in UV radiation in Chile. Journal of Photochemistry and Photobiology, Biology 28:137-142 Piazena, H (1996): The effect of altitude upon the solar UV-B and UV-A irradiance in the tropical chilean andes. Solar Energy, 57: 133-140 Meteorological Institute of the University of Munich Theresienstr. 37; 80333 Munich; Germany moni@meteo.physik.uni-muenchen.de