AEROSOL RETRIEVAL OVER LAND FROM AVHRR DATA - APPLICATION FOR ATMOSPHERIC CORRECTION

Correction of AVHRR imagery for the aerosol effect requires retrieval of the aerosol loading from the images. In this paper two retrieval algorithms that were previously developed for Landsat are modified for the AVHRR. The methods which determine the aerosol optical thickness over land surfaces from AVHRR band 1 data independent of ancillary information are described. The first method retrieves aerosols based on the atmospheric effect on the path radiance. This method requires the surface reflectance to be 0.02 +/- 0.01, which is found over forests in the red channel. Two techniques are used to screen an AVHRR scene for pixels that have this low reflectance. One is based on selecting the lowest NDVI or by selecting the minimum 3.75-mu-m radiance. The qualifying requirements for these techniques are discussed and the method demonstrated to retrieve aerosol optical thicknesses to approximately +/- 0.1. The second method uses the change in contrast for several scenes to determine the change in the optical thickness between the scenes. A reference scene allows absolute determination. The method has an rms error of approximately 0.1. The two methods are discussed with respect to other methods as well as their implications for global scale aerosol monitoring and atmospheric correction.