Altitude effects on UV spectral irradiance deduced from measurements at Lauder, New Zealand, and at Mauna Loa Observatory, Hawaii

Measurements from Lauder, New Zealand, and from the high-altitude Mauna Loa Observatory, Hawaii, are used to determine the altitude effects on spectra l UV irradiance and to relate these altitude differences to other factors t hat influence UV radiation. The measured ratios UVMauna (Loa) / UVLauder ar e complex functions of both wavelength and solar zenith angle (SZA). Spectr ally, the ratios tend to increase toward shorter wavelengths through most o f the UV-A region. For small SZA (SZA < similar to 40 degrees) the ratios c ontinue to increase as wavelength decreases throughout the UV-B region. For SZA = 60 degrees a local maximum occurs in the UV-B region. As the SZA inc reases, this turning point moves to longer wavelengths and its peak value d ecreases. For SZA > -80 degrees, local minima in the ratios are seen at sho rter wavelengths in the UV-B region. For biologically weighted irradiances, the peak ratios occur near SZA = 70 degrees, where UV-A, erythemally weigh ted UV, UV-B, and DNA-weighted UV irradiances at Mauna Loa Observatory exce eded those at Lauder by similar to 17 %, 26%, 27%, and 29% respectively. Th e ratios of irradiances at the two altitudes, as functions of SZA and wavel ength, were related to differences expected from radiative transfer calcula tions. For small SZA, modeled and measured ratios agreed within the limits of experimental uncertainty without taking differences in altitude distribu tions of ozone and temperature into account. However, for larger SZA and sh orter wavelengths these profile shapes had a significant effect. In the mod el calculations, satisfactory agreement with the measurements was achieved only when the contribution from radiation scattered from air or cloud tops below the observation height at Mauna Loa Observatory was included. To mode l this accurately, a three-dimensional radiative transfer code should be us ed in conjunction with a topographical model of the surrounding terrain.