Solar UV-B and UV-A irradiance in arid high-mountain regions: Measurements on the island of Tenerife as compared to previous tropical Andes data

Spectroradiometric measurements of global and diffuse solar UV‐B and UV‐A irradiance were performed as a function of solar elevation under clear‐sky conditions at altitudes between sea level and the top of the volcano El Teide (3555 m above sea level (asl)) at the arid part of the island of Tenerife (Spain) and compared with irradiance data previously measured in the tropical Chilean Andes. Total optical depths and their components were calculated by using data of direct solar irradiance determined in dependence of the solar elevation angle and of altitude. The Ratio total/Rayleigh optical depth increased with altitude in the short‐wavelength UV range and decreased with altitude for long‐wavelength UV radiation. Rayleigh scattering contributed dominantly to the attenuation of irradiance for both UV‐A and for wavelengths between 308 nm and 319 nm in the total range of altitudes investigated as compared with the contributions of ozone absorption and of aerosol absorption and scattering. The latter effects were more effective for the attenuation of solar irradiance in the spectral range of UV‐B for altitudes above about 500–960 m asl as compared with the effect of Rayleigh scattering. Increases of global irradiance with increasing altitude were mainly caused by an increase of the direct component in both spectral ranges. Because of the attenuation of direct solar irradiance during its transmittance through vertically stratified near‐surface layers characterized by increased values of the aerosol optical depth, vertical gradients of global irradiance and of its direct component observed at Tenerife in the lower part of the investigated atmospheric layer exceeded in both spectral ranges the gradients determined in the upper part in the case of small solar elevation angles. Similarity with the vertical gradients of global irradiance in the Andes was observed in the case of solar elevation angles above about 40° for both UV‐A and UV‐B within the upper layer and above about 60° for both layers and for both spectral ranges whereas the gradients observed in both layers at Tenerife covered the gradients derived in the Andes for smaller solar elevation angles.