A method of obtaining the spectral aerosol optical thickness that uses a multispectral instrument and a crude instrument with one spectral band only, here the blue at 450 nm, is presented. The instrument in the blue range is calibrated from a stratospheric balloon. The method is shown to be efficient by using experimental data collected in West Africa, where the daily variations of the aerosol optical thicknesses are large because of frequent dust storms and where the classic Langley plot method could not be used.