Chemical characterization of the Saharan dust end-member:: Some biogeochemical implications for the western Mediterranean Sea -: art. no. 4258

[1] Two types of samples were used to chemically characterize the Saharan end-member: fine fractions of surface soil samples collected in Northern Africa and particulate phases of typical Saharan rains. Since the concentrations measured in the particulate phase of the Saharan rains were corrected from the dissolution losses in rainwater, these particles were considered to be representative of the transported Saharan dust before being blended into rainwater. Al, Fe, P, and Pb were analyzed: except for lead, the chemical composition of the transported Saharan dust was more homogeneous than the composition of individual soils. As confirmed by the air mass back trajectories, the higher level of homogeneity of the aerosol is partly due to the fact that a dust event affects a large area of the Saharan desert, and the composition of the particles reflects the average composition of the eroded areas. Pb concentration in the transported dust reflected an anthropogenic fraction. By using Pb/Al measurements from the soils it was shown that a typical Saharan rain event with no mixing with air masses from Europe appears to be very rare in the Mediterranean environment. The following values are proposed to characterize the Saharan dust end-member: Al (%) = 7.09 +/- 0.79; Fe (%) = 4.45 +/- 0.49; P (%) = 0.082 +/- 0.011; Pb (ppm) = 24 +/- 9. This study suggests that the [element/Al or Fe)] ratio is also useful to characterize the Saharan end-member as they are very homogeneous for the two sample types. Saharan dust represents a potential source of nutrients (P, Fe) for the Mediterranean water. Indeed, it accounts for similar to30-40% of the total atmospheric flux of phosphorus in the western Mediterranean, and it governs the biogeochemical cycle of iron being the main source of dissolved iron in the western Mediterranean waters.