Single Scattering Albedo parameter retrieval requires accurate information about the aerosol refractive index (or chemical composition). In AERONET, the volume size distribution parameters (mode radii and width) are retrieved along with the refractive index (wavelength dependent) information. You can use this information in a Mie Scattering code to derive the single scattering albedo and scattering phase function (or asymmetry parameter) along with aerosol extinction or optical depth at a particular wavelength. This may not be the best solution but you can give a try!