Nearly all anthropogenically-influenced aerosol sources are located over land masses. However, most satellite observations of aerosols are limited to ocean areas. Not until recently were instruments in space which could perform instantaneous retrievals of aerosol optical depth (AOD) over land such as Moderate Resolution Imaging Spectro-radiometer (MODIS). However, the Advanced Very High Resolution Radiometer (AVHRR) Pathfinder Atmosphere (PATMOS) data set provides daily observations for nearly twenty years (July 1981 through present). This long period of observation allows explicit characterization of the Earth's surface reflectance properties through retrieval of the bi-directional reflectance distribution functions (BRDF). The aerosol contribution to the atmospheric reflectance depends on the aerosol amount (optical depth) and optical properties (single scatter albedo and scattering phase function). The surface contribution depends on its topography and vegetation, which determine the BRDF and it seasonal variability. While both contributions can be accurately simulated with a radiative transfer model, the dependent parameters of neither are available on a global basis. However, using the extensive PATMOS data together with radiative transfer models, these two contributions can be separated. PATMOS clear-sky visible reflectances are used to estimate surface BRDF parameters which are then used to retrieve AOD. When and where this retrieval is accurate, as compared to ground-based AOD observations, the AOD climatology is compiled.