SIMULTANEOUS SPECTRORADIOMETRY - A STUDY OF SOLAR UV IRRADIANCE AT 2 ALTITUDES

Spectrally resolved measurements of solar ultraviolet (UV) radiation w ere made at adjacent mountain top and valley sites, vertically separat ed by 1 km. Data was collected over a period of 4 days in August 1993 providing simultaneously measured spectra for a range of zenith angles with both sites in direct sunlight and less than 2 octas of cloud cov er. As there was no snow cover the local albedo was similar at both si tes. Irradiances at the mountain site were 9% higher than the valley a t 370 nm, increasing gradually to 11% at 320 nm. Across the UVB waveba nd the altitude effect increased dramatically to 24% at 300 nm. The ma gnitude of the altitude effect was independent of zenith angle, within the measurement accuracy, for the range z = 30 degrees-70 degrees. Th ese measured differences between the mountain and valley station can b e interpreted by model calculations using the corresponding difference s in air pressure, column ozone and aerosol content.