Six years (1989--1994) of weekly composite satellite data are analyzed with the objective of retrieving the aerosol optical depth over boreal forests. The average surface bidirectional reflectance distribution (BRD) was estimated from a sample of the original data, yielding similar results in different regions around the world. Four analytical models are compared in their ability to reproduce the surface BRD. In channel 1 (0.58--0.68 μm) the surface reflectance averages 0.05 and exceeds 0.10 in the backscattering region. Thus the non-Lambertian ground contribution could not be neglected in the aerosol retrievals. A relevant radiative transfer code was run in iterative mode to retrieve the aerosol optical depth which best accounts for the difference between modeled surface BRD and measured satellite reflectance in all conditions. From comparisons with Sun photometer data we conclude that a large-scale, persistent aerosol enhancement such as that resulting from Mount Pinatubo's eruption in 1991 is clearly discernible from weekly composite AVHRR data over boreal forests. ¿ 1998 American Geophysical Union