Comparison of column ozone retrievals by use of an UV multifilter rotatingshadow-band radiometer with those from Brewer and Dobson spectrophotometers

The U.S. Department of Agriculture UV-B Monitoring Program measures ultravi olet light at seven wavelengths from 300 to 368 nm with an ultraviolet mult ifilter rotating shadow-band radiometer (UV-MFRSR) at 25 sites across the U nited States, including Mauna Loa, Hawaii. Column ozone has been retrieved under all-sky conditions near Boulder, Colorado (40.177 degrees N, 105.276 degrees W), from global irradiances of the W-MFRSR 332- and 305-nm channels (2 nm FWHM) using lookup tables generated from a multiple-scattering radia tive transfer code suitable for solar zenith angles (SZA's) up to 90 degree s. The most significant sources of error for UV-MFRSR column ozone retrieva ls at SZA's less than 75 degrees are the spectral characterizations of the filters and the absolute calibration uncertainty, which together yield an e stimated uncertainty in ozone retrievals of +/-4.0%. Using model sensitivit y studies, we determined that the retrieved column ozone is relatively inse nsitive (<+/-2%) to typical variations in aerosol optical depth, cloud cove r, surface pressure, stratospheric temperature, and surface albedo. For 5 m onths in 1996-1997 the mean ratio of column ozone retrieved by the W-MFRSR divided by that retrieved by the collocated Brewer was 1.024 and for the W- MFRSR divided by those from a nearby Dobson was 1.025. The accuracy of the retrieval becomes unreliable at large SZA's of more than 75 degrees as the detection limit of the 305-nm channel is reached and because of overall ang ular response errors. The W-MFRSR advantages of relatively low cost, unatte nded operation, automated calibration stability checks using Langley plots, and minimal maintenance make it a unique instrument for column ozone measu rement. (C) 1999 Optical Society of America.