The USDA Ultraviolet Radiation Monitoring Program

Abstract

The U.S. Department of Agriculture's Ultraviolet (UV) Radiation Monitoring Program has been measuring UV radiation since 1994. The initial network of 12 stations employed broadband meters to measure UVB irradiance and included ancillary measurements of temperature, humidity, and irradiance at seven wavelengths in the visible produced by a Multi-Filter Rotating Shadowband Radiometer (MFRSR). Since that beginning the network has expanded to more than 20 stations and the broadband meters have been supplemented with a seven-wavelength Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR). The network has been designed to include 30 stations, each with a full complement of instrumentation. Annual characterizations of the network's filter radiometers indicate that gradual shifts in instrument response are manageable but must be accounted for to achieve accurate and precise measurements of UV irradiance. The characterization and calibration of the filter instruments is discussed along with filter stability and instrument precision. Broadband instruments are shown to be quite stable and collocated instruments are shown to agree to within 2.3% for zenith angles less than 80° under all sky conditions. Preliminary investigations into the accuracy of the UV-MFRSR calibrated with the Langley method are presented and successful column ozone retrievals are demonstrated with the UV-MFRSR under clear skies.