Methodology for deriving clear-sky erythemal calibration factors for UV broadband radiometers of the US Central UV Calibration Facility

In the United States, there are several federal agencies interested in the effects of UV radiation, which has resulted in the establishment of UV moni toring programs each with their own instrumentation and sites designed to a ddress their specific needs. In 1993, participating agencies of the U.S. Gl obal Change Research Program organized a UV Panel for coordinating the diff erent agencies' programs in order to ensure that UV data are intercalibrate d, have common quality assurance and control procedures, and that the effor ts among agencies are not duplicated. In order to achieve these goals, in 1994 the UV Panel recommended formation of the U.S. Central UV Calibration Facility (CUCF), which is operated by t he Surface Radiation and Research Branch of the Air Resources Laboratory of National and Oceanic Atmospheric Administration. The CUCF is responsible f or characterizing and calibrating UV measuring instruments from several U.S . federal agencies. Part of this effort is to calibrate UVB broadband radio meters from these agencies. The CUCF has three Yankee Environmental Systems (YES UVB-I) and three Solar Light (SL 501A) broadband radiometers as refer ence standards that are routinely calibrated. For the past three years, cle ar-sky erythema calibration factors were determined for these standard UVB broadband radiometers by using simultaneously measured erythema-weighted ir radiance determined during the annual North American Intercomparison. Compa risons between erythemally weighted irradiance calculated spectra supplied by spectroradiometers typically agreed better than +/-2% for solar zenith a ngles less than 60 degrees. The spectroradiometers were participating in an intercomparison event organized by the National Institute of Standards and Technology and the CUCF. In this article, the calibration methodology is described for transferring the calibration from the spectroradiometers to the CUCF's standard broadban d radiometers. The CUCF standard broadband radiometers are used to calibrat e UVB broadband radiometers from several U.S. UV monitoring networks. Eryth emal calibration factors for the CUCF's YES UVB-1 standard broadband radiom eter triad are reported for 1994, 1995, and 1996. Erythemal calibration fac tors for CUCF's SL 501A standard broadband radiometer triad are reported fo r 1996.