Investigations on the effect of high surface albedo on erythemally effective UV irradiance: Results of a campaign at the Salar de Uyuni, Bolivia

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Abstract

Measurements and model calculations have been performed to study the effect of high surface albedo on erythemally effective UV irradiance. A central part of the investigation has been a one week measurement campaign at Salar de Uyuni in the Southern part of the Bolivian Altiplano. The Salar de Uyuni, the largest salt lake of the world, is characterized by largely homogeneous surface conditions during most of the year. Albedo measurements performed by an UV radiometer result in a reflectivity for erythemally effective radiation of 0.69 ± 0.02. The measurements show hardly any dependency on solar elevation, indicating the homogeneity of the surface and nearly isotropic reflection properties of the Salar.

The effects of the high albedo surface on the erythemally effective irradiance, i.e. the UV index (UVI), has been experimentally determined by simultaneous measurements of several UV radiometers located at different sites around and on the Salar. In this context a method for the minimization of systematic deviations between the individual detectors used for the investigation is presented. It ensures the intercomparability of the performed UV measurements within ±2% which is a distinct improvement compared to the typical absolute accuracy of UV irradiance measurements in the order of ±5%. For solar elevations around 50° the UVI measured close to the center of the Salar is typically enhanced by 20% compared to the values determined outside. Towards lower solar elevations this increase becomes slightly weaker. The measurements agree well with both, own corresponding 1D and previously published 3D radiative transfer calculations from literature.

Introduction

There is wide evidence about the effects of excessive exposure to UV radiation on the human health [1], [2]. Most important in this context are acute (e.g. sun burn, snow blindness) and chronic (e.g. skin cancer, eye cataracts) irritations or damages of human skin and eyes [3], [4], [5] as well as a potential suppression of the immune system [6], [7]. People living in the tropics are especially at risk to these effects due to the naturally low levels of total ozone and the high solar elevations reached at those latitudes. Moreover the average UV exposure increases markedly with altitude (e.g. [8], [9]). In addition, high surface albedo may reinforce the irradiance for both horizontal and inclined surfaces due to multiple scattering processes of reflected radiation. Bolivia, where this present study has been carried out, has approximately three million people living above 3000 m a.s.l., making it one of the most densely populated regions in the world at such high altitudes [10]. Hence, it is worthwhile to investigate the radiative properties of tropical regions located at high altitude that have extreme surface albedo values. Due to its high elevation, its high albedo over a large area and its homogeneous surface reflection properties, the Salar de Uyuni is an ideal natural laboratory to carry out experiments on albedo effects on UV radiation.

Model simulations on the basis of three-dimensional radiative transfer calculations for the determination of the albedo effect on UV irradiance [11] indicate that the regional reflection properties of the surface within several tenth of kilometers around the measurement site should affect the UV irradiance level significantly. These investigations based on two different surface albedo values of 0.03 for vegetation and 0.85 for snow have been performed for various horizontal distribution patterns of high and low surface albedo areas in a model test region of 200 km × 200 km. The results for the UV wavelength indicate that the maximum increase of UV irradiance due to the albedo change from 0.03 to 0.85 will be close to 50% for 330 nm and around 35% for 300 nm. The magnitude and spectral course of the effect is in excellent agreement with published theoretical investigations [12]. The diameter of the high albedo area around a detector to produce 80% of this maximum effect ranges from 45 km at a wavelength of 330 nm to 12 km for 300 nm. Until now the effects of surface albedo inhomogeneities on UV radiation have been studied experimentally by measurements at the ice edge of Antarctica [13], [14] and in glaciated areas in the Alps [15]. However these investigations encountered difficulties with respect to inhomogeneities in the terrain and access to the boundary between low and high surface albedo due to the polar environment. Changes in UV irradiance in comparable magnitude and of similar distance dependence can be expected in the surroundings of the Salar de Uyuni. In contrast to the model simulations mentioned above that have been performed at sea level, the Salar de Uyuni is located around 3700 m a.s.l. To compare measurements and 3D model simulations, additional sensitivity experiments have been conducted by using a 1D radiation transfer model.

The basic data for the present study on albedo effects on erythemally effective UV irradiance have been obtained during a measurement campaign in May 2005, using several UV broadband instruments. The results from the present study might contribute to enrich our current knowledge in albedo and albedo effects and expected to be for the public benefit with respect to the welfare of the local population and the relevant tourist industry of the region. The measured UV irradiances and albedos will provide important data for the validation of radiative transfer model simulations and for the interpretation and evaluation of satellite-borne data.

Section snippets

Campaign and instrumentation

The Salar de Uyuni is located in the Southern region of the Bolivian Altiplano between 19.7° and 20.7°S and 66.9° and 68.3°W at a mean altitude of 3700 m a.s.l. It covers an area of around 12,000 km2, with a maximum meridional and zonal extension of 120 km by 150 km. The actual size of the Salar may vary slightly due to seasonal changes at its outer borderline which is characterized by a variable mixture of mud, salt and water. Except for the rainy season (December–March), when a thin layer of

UV measurements

UV irradiances have been measured using broadband radiometers described in Section 2. These instruments are designed to coincide in their spectral response with the erythema action spectrum defined by CIE [20] to measure the erythemally effective UV irradiance, i.e. the UV index (UVI) [21], [26]. Due to inevitable deviations of the detectors spectral response from the CIE definition, elaborate calibration processes and data processing are required to provide reliable UVI measurements. All

Model simulations

The model simulations of albedo effects on UVI have been performed by the radiation transfer model STAR [29], [30]. Fig. 8 presents a sensitivity study on the UVI as function of solar elevation and surface albedo for average conditions of the Bolivian Altiplano. Calculations have been performed for an altitude of 3700 m a.s.l. and a corresponding pressure of 650 hPa. A total ozone content of 250 DU, representing average Altiplano conditions, has been used. Furthermore the aerosol type ‘clean

Summary and outlook

The effect of a high albedo surface on erythemally effective UV irradiance, i.e. the UVI, has been investigated by a combination of measurements and model calculations. A central part of the investigation has been a one week measurement campaign at the Salar de Uyuni. The erythemally effective UV irradiance has been measured by three broadband UV radiometers simultaneously at various sites on and outside the Salar at different distances to its edge. For an improved intercomparability of the

Acknowledgements

The authors are very grateful to Mr. Gonzalo Gutiérrez for experimental help. We acknowledge the Volkswagen Foundation grant support which enabled this collaborative project.

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