Twelfth ARM Science Team Meeting Proceedings, St. Petersburg, Florida, April 8-12, 2002 Use of AERONET Aerosol Retrievals to Calculate Clear-Sky Irradiance at the Surface G. L. Schuster National Aeronautics and Space Administration Langley Research Center Hampton, Virginia O. Dubovik National Aeronautics and Space Administration Goddard Space Flight Center Laboratory for Terrestrial Physics Greenbelt, Maryland Motivation The worldwide aerosol robotic network (AERONET) of ground-based radiometers was developed (in part) as a satellite validation tool (Holben et al. 1998). These sites utilize spectral sky-scanning radiometers, providing more information for aerosol retrievals than conventional sunphotometer measurements. The use of the almucantar sky radiance scans in conjunction with the aerosol optical thicknesses are the basis of the AERONET Dubovik retrievals, which provide the aerosol size distribution and the refractive index at four wavelengths (Dubovik and King 2000). The accuracy of the Dubovik retrieval has been assessed with the retrieval of synthetic data (Dubovik et al. 2000), but an assessment utilizing independent measurements is desirable. Hence, the AERONET retrievals are used here as input to a discrete-ordinates radiative transfer model (DISORT, Stamnes et al. 1988) utilizing 12 computational streams to calculate the solar radiation at the surface, and the results are compared to co-located surface measurements. Included in the comparison are principle plane radiance measurements from the Cimel sunphotometer, irradiance measurements from the Multi-filter Rotating Shadowband Radiometer (MFRSR, Michalsky et al. 2001), and irradiance measurements from the Rotating Shadowband Spectroradiometer (RSS, Harrison et al. 1999). Method The aerosol size distributions and refractive index at four wavelengths (0.440, 0.670, 0.870, and 1.020 µm) are provided by the AERONET Dubovik retrievals. The retrievals may be used at these wavelengths to calculate the phase functions, optical depths, and single-scatter albedos required for radiative transfer calculations, but a refractive index extrapolation is necessary for comparisons to radiation measurements at non-AERONET wavelengths. A 3-component χ 2 -tuning technique was developed, whereby the composition of retrieved aerosol size distributions are “tuned” by varying the amount of internally-mixed water, soot, and ammonium sulfate until a minimum χ 2 -fit of the available refractive index are achieved. (Typically, the volume fraction of soot calculated with this technique is 1