Aerosol particles affect the global energy budget and Earth's climate through the absorption and scattering of both solar and terrestrial radiation. Here we present a study of column-averaged aerosol optical properties derived from clear-sky measurements of the direct and diffuse components of the solar downwelling spectral and broadband irradiance during a 10-day observation period at the Atmospheric Radiation Measurement (ARM) facility in central Oklahoma in the fall of 2001. Using a radiative transfer model and Mie calculations of the optical properties of standard continental aerosol components and assuming a constant aerosol composition with altitude, we determine the phenomenological aerosol model that best fits both the direct and diffuse spectral irradiances. Internal and external homogeneous aerosol mixtures were considered, along with single-aerosol and coated sphere models. We find that the irradiance data is well fit statistically by at least one of the models for every clear-sky time considered during the observations period, with no excess gaseous absorption required to fit the data. By far the best fitting model consisted of ultrafine (~5--30 nm radius) coated spheres with absorbing black carbon cores and nonabsorbing shells of water or a similar substance. The mean value of the 500-nm aerosol single-scattering albedos for the entire 10-day period was 0.76. These results indicate that absorbing carbonaceous aerosols can have a significant radiative impact even in rural settings.