CALIBRATING BROAD-BAND UV INSTRUMENTS - OZONE AND SOLAR ZENITH ANGLE DEPENDENCE

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), H awaii, in July 1995. This instrument has been employed to characterize several broadband UV instruments of a type commonly used to estimate erythemal irradiance at many sites around the globe. One year of clear -sky data from MLO has been analyzed for solar zenith angles (SZAs) of 5 degrees-85 degrees, in steps of 5 degrees, and for total ozone valu es in the range 220-310 DU measured with a Dobson spectrophotometer. B ecause the spectral responses of various broadband instruments can be quite different, and particularly because the erythemal response defin ed for human skin is significantly different than that of many broadba nd instruments, the calibration of a broadband instrument reporting in erythemal units is strongly dependent on total ozone and SZA. When a broadband instrument is placed in the field it is necessary to know th e calibration as a function of ozone and SZA to determine accurate ery themal irradiance. However, the manufacturers of broadband instruments do not generally provide information on the ozone dependence of the c alibration. A procedure is described here for determining the calibrat ion of a broadband UV instrument by comparison with a calibrated spect roradiometer. This procedure does not require precise knowledge of the spectral response of the broadband instrument. This analysis shows th at if, for example, total ozone concentration decreased from 300 to 20 0 DU, the calibration constant of a broadband instrument should be inc reased by almost 20%. Therefore, the broadband instrument would signif icantly underestimate the increase of erythema.