REMOTE-SENSING OF AEROSOLS OVER BOREAL FOREST AND LAKE WATER FROM AVHRR DATA

A complete set of advanced very high resolution radiometer (AVHRR) dat a and ground-based measurements of aerosol and water-vapor content are used to test an algorithm for the retrieval of aerosol properties ove r dense vegetation in the red and over lake water in both the red and the near-infrared AVHRR channels. With the assumptions of a weak and r easonably constant surface reflectance and an appropriate aerosol mode l in the radiative transfer code, the remaining variance in the satell ite signal is interpreted in terms of aerosol optical thickness. From theoretical computations, it appears that the algorithm is particularl y sensitive to the surface albedo and that an uncertainty of 0.01 in r eflectance leads to an error of +/-0.1 in the retrieved optical thickn ess. This theoretical estimate is confirmed by data acquired over a bo real forest region in Canada and over one of the Great Lakes (Ontario) . In particular, channel 1 observations over vegetation in the forward scattering direction are well suited for retrievals because vegetatio n pixels appear darker owing to shadowing effects. Conversely, the for ward scattering geometry over lakes introduces large errors in both ch annels owing to specular reflections (glint effects). Even for observa tions well removed from the forward scattering principal plane, lake s urface reflections due to sky radiance glint have to be taken into acc ount. Because the accuracy of the retrieval algorithm is affected by w ater-vapor absorption in channel 2 and by variations in lake-water tur bidity in channel 1, the optimal retrieval configuration is to employ vegetation observations in channel 1. Bidirectional effects have to be considered, however, for observations in the backscatter directions. (C) Elsevier Science Inc., 1997.