The correlation between particle mass loading and extinction: Application to desert dust aerosol content estimation

We are interested in aerosol mass loading estimation from optical thickness measurements. Therefore, the goal is to determine the correlation between the mass loading M and the aerosol extinction tau(a)(lambda) at a given wavelength and the improvement resulting from the use of the spectral dependence. Methods using the simple ratio M/tau(a) = cte are obviously dependent on the aerosol size distribution model and show a strong dependence on the aerosol size distribution parameters. Ground-based sun photometer measurements under variable atmospheric conditions were taken during two campaigns in April and May in both 1986 and 1967 at M'bour (80 km south of Dakar, Senegal) and are used to derive the desert aerosol columnar size distribution by a nonlinear least squares fitting ''inversion'' technique, using the lognormal function (the Junge function was used in his study only as an academic reference), the mass loading, and hence the M-tau(a) relation. Our results are also compared with those given by current desert aerosol models. Several M-tau(a) relations are proposed to estimate desert aerosol mass content with an accuracy greater than the simple ratio approach. The differences of these estimates of M are between +/-8% and +/-15%, as deduced from our experimental data, when the simple ratio method reaches uncertainty tip to +/-40%. (C) Elsevier Science Inc.