Deriving aerosol parameters from absolute UV sky radiance measurements using a Brewer double spectrometer

A Brewer MKIII double spectrophotometer has been modified to measure direct sun and sky radiance from 303nm to 363mn for the purpose of measuring aerosol optical depth, Angstrom parameter, and single scattering albedo. Results from a detailed instrument calibration showed that there is a temperature dependence of -0.3% per degree Celsius, the field of view was 2.6degrees full width half maximum, and the wavelength calibration was accurately determined using a dye-LASER. Using both integrating sphere and lamp-diffuser plate combinations, absolute diffuse radiometric calibration was performed and converted into direct calibration using the measured field of view. Aerosol optical depth and Angstrom parameter were measured on 4 clear sky days in June 2003 at Greenbelt, Maryland and compared to AERONET-data at the same location. The average difference in the aerosol optical depth at 340nm was smaller than 0.02. A depolarizing element was inserted in the Brewer's optical path to reduce the very pronounced polarization sensitivity, and additional polarized filters were added to explore the possibility to obtain additional aerosol information. Because of a defect in the depolarizer, the current residual polarization is 5%, which has to be reduced to less than 1% to derive additional aerosol parameters from sky radiance measurements.