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Here the goal is to study the behaviour of the ratio between aerosol mass loading M and extinction tau(a)(lambda) at a given wavelength, generally at 0.55 mu m. The simple ratio M/tau(a) is currently applied in aerosol studies to estimate total vertical mass aerosol content, but this ratio shows a strong variability. Obviously the M/tau(a) ratio is dependent on the aerosol size distribution model so we have carried out a theoretical analysis to observe this dependence. Two functions were selected, the Junge and the lognormal function. By means of their cumulative values the M/tau(a) ratio is evaluated as a function of aerosol parameters. The results clearly show the dependence and variability of the M/tau(a) on the aerosol size distribution model and parameters. An additional problem is analysed in the case of the Junge function due to the dependence of this function on the limits r(1) and r(2) Of the size distribution. The study is applied to current desert aerosol models together with a real experiment, carried out in Senegal, Africa. (C) 1997 Elsevier Science Ltd.