THE 1996 NORTH-AMERICAN INTERAGENCY INTERCOMPARISON OF ULTRAVIOLET MONITORING SPECTRORADIOMETERS

Concern over stratospheric ozone depletion has prompted several govern ment agencies in North America to establish networks of spectroradiome ters for monitoring solar ultraviolet irradiance at the surface of the Earth. To assess the ability of spectroradiometers to accurately meas ure solar ultraviolet irradiance, and to compare the results between i nstruments of different monitoring networks, the third North American Interagency Intercomparison of Ultraviolet Monitoring Spectroradiomete rs was held June 17-25, 1996 at Table Mountain outside Boulder, Colora do, USA. This Intercomparison was coordinated by the National Institut e of Standards and Technology (NIST) and the National Oceanic and Atmo spheric Administration (NOAA). Participating agencies were the Environ mental Protection Agency; the National Science Foundation; the Smithso nian Environmental Research Center; the Department of Agriculture; and the Atmospheric Environment Service, Canada. The spectral irradiances of participants' calibrated standard lamps were measured at NIST prio r to the Intercomparison. The spectral irradiance scales used by the p articipants agreed with the NIST scale within the combined uncertainti es, and for all lamps the spectral irradiance in the horizontal positi on was lower than that in the vertical position. Instruments were char acterized for wavelength uncertainty, bandwidth, stray-light rejection , and spectral irradiance responsivity, the latter with NIST standard lamps operating in specially designed field calibration units. The spe ctral irradiance responsivity demonstrated instabilities for some inst ruments. Synchronized spectral scans of the solar irradiance were perf ormed over several days. Using the spectral irradiance responsivities determined with the NIST standard lamps, the measured solar irradiance s had some unexplained systematic differences between instruments.