SUSPEN intercomparison of ultraviolet spectroradiometers

Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece July, 1-13 1997, are presented . Nineteen instrument systems from 15 different countries took part and pro vided spectra of global solar UV irradiance for two consecutive days from s unrise to sunset every half hour. No data exchange was allowed between part icipants in order to achieve absolutely independent results among the instr uments. The data analysis procedure included the determination of wavelengt h shifts and the application of suitable corrections to the measured spectr a, their standardization to common spectral resolution of 1 nm full width a t half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instrume nts which were objectively selected and used as comparison norms for the as sessment of the relative agreement among the various instruments. With rega rd to the absolute irradiance measurements, the range of the deviations fro m the reference for all spectra was within +/- 20%. About half of the instr uments agreed to within +/-5%, while only three fell outside the +/- 10% ag reement limit. As for the accuracy of the wavelength registration of the re corded spectra, for most of the spectroradiometers (14) the calculated wave length shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majori ty of the instruments was good and comparable to the commonly accepted unce rtainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercom parison campaigns, held in 1995 in Ispra, Italy and in 1993 in Garmisch-Par tenkirchen, Germany. As a result of this series of intercomparison campaign s, several of the currently operating spectroradiometers operating may be r egarded as a core group Of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.