Path radiance technique for retrieving aerosol optical thickness over land

The key issue in retrieving aerosol optical thickness over land from shortw ave satellite radiances is to identify and separate the signal due to scatt ering by a largely transparent aerosol layer from the noise due to reflecti on by the background surface, where the signal is relatively uniform compar ed to the highly inhomogeneous surface contribution. Sensitivity studies in aerosol optical thickness retrievals reveal that the apparent reflectance at the top of the atmosphere is very susceptible to the surface reflectance , especially when aerosol optical thickness is small. Uncertainties associa ted with surface reflectance estimation can greatly amplify the error of th e aerosol optical thickness retrieval. To reduce these uncertainties, we ha ve developed a "path radiance" method to retrieve aerosol optical thickness over land by extending the traditional technique that uses the "dark objec t" approach to extract the aerosol signal. This method uses the signature o f the correlation of visible and middle-IR reflectance at the surface and c ouples the correlation with the atmospheric effect. We have applied this me thod to a Landsat TM (Thematic Mapper) image acquired over the Oklahoma sou thern Great Plains site of the Department of Energy Atmospheric Radiation M easurement (ARM) program on September 27, 1997, a very clear day (aerosol o ptical thickness of 0.07 at 0.5 mu m) during the first Landsat Intensive Ob servation Period. The retrieved mean aerosol optical thickness for TM band 1 at 0.49 mu m and band 3 at 0.66 mu m agree very well with the ground-base d Sun photometer measurements at the ARM site. The ability to retrieve smal l aerosol optical thickness makes this path radiance technique promising. M ore importantly, the path radiance is relatively insensitive to surface inh omogeneity. The retrieved mean path radiances in reflectance units have ver y small standard deviations for both TM blue and red bands. This small vari ability of path radiance further supports the current aerosol retrieval met hod.