Simultaneous spectroradiometry. A study of solar UV irradiance at two altitudes

Spectrally resolved measurements of solar ultraviolet (UV) radiation were made at adjacent mountain top and valley sites, vertically separated 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 cover. As there was no snow cover the local albedo was similar at both sites. Irradiances at the mountain site were 9 per cent higher than the valley at 370 nm, increasing gradually to 11 per cent at 320 nm. Across the UVB waveband the altitude effect increased dramatically to 24 per cent at 300 nm. The magnitude of the altitude effect was independent of zenith angle, within the measurement accuracy, for the range z = 30 degree-70 degree. These measured differences between the mountain and valley station can be interpreted by model calculations using the corresponding differences in air pressure, column ozone and aerosol content.