Inference of aerosol optical depth over land through the retrieval of surface BRDF parameters from the AVHRR pathfinder atmosphere data set

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, e.g. the 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) binned into approximately (110km)² equal area grid cells. This extraordinarily long period of observation allows explicit characterization of the Earth's surface reflectance properties through retrieval of the bi-directional reflectance distribution functions (BRDF). Besides molecular extinction - which is well understood - both aerosols and the surface contribute to the magnitude and angular variability of clear-sky visible reflectances over land. The aerosol contribution depends on the aerosol amount (i.e. 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 make it seasonally variable. 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 and surface measurements of AOD, these two contributions can be separated. PATMOS data is used to estimate surface BRDF parameters. These parameters are then used to calculate the top-of-the-atmosphere reflectance for an aerosol-free atmosphere from a radiative transfer model. Deviations in the PATMOS clear-sky reflectances from model estimates are correlated to ground-based measurements of AOD. Further research will provide BRDF retrievals for different land surface types, and the residual aerosol signal in the PATMOS data will be used to estimate AOD over surface types which show signs of an aerosol signal.