Atmospheric aerosols absorb and scatter solar radiation, thereby altering the flux of solar energy reaching Earth. Quantifying the aerosol-radiation interaction is difficult and the effect of aerosols on the propagation of sunlight through the atmosphere is poorly characterized. Accurate, precise observations of aerosol absorption and scattering are necessary to improve radiative transfer models. We have developed a broadband cavity-enhanced spectrometer, dubbed BBCES-III, to measure the extinction (extinction = absorption + scattering) by atmospheric aerosols throughout the UV-Vis spectrum. In this talk, I will discuss the design and validation of the BBCES-III. Then, I will show how the BBCES-III was used to determine the refractive index and size distribution of polystyrene latex spheres, a common calibration standard, using non-linear least squares optimization.