This study presents the validation of atmospheric turbidity databases. It is divided in two parts. The first part presents the validation of three Aerosol Optical Depth (AOD) databases with a worldwide coverage. AOD at 550nm estimates from MISR, MODIS-L3 satellites and MACC model and water vapor from NCEP are compared against measurements from 865 AERONET stations as ground-truth data for a period of more than one year in each location. The second part addresses the limitation that ground measurements of AOD are not always available. An alternative methodology is presented here to evaluate the AOD databases. It consists of comparing the daily Linke Turbidity index (TL) obtained from the databases to the corresponding TL derived from DNI measurements. All databases show a significant overestimation of AOD and errors in terms of rRMSD are between 85 and 118% in total for the AERONET stations validated.