Red-ox speciation and mixing state of iron in individual African dust particles

The Fe distribution in African dust particles collected in Senegal (North-Western Africa) during the African Monsoon Multidisciplinary Analysis Special Observation Period 0 (AMMA-"SOP 0,"February 2006) was assessed using individual particle analysis ( Scanning and Transmission Electron Microscopy respectively equipped with X-ray Spectrometry (SEM-EDX) and Electron Energy Loss Spectrometry (TEM-EELS)). Senegal is not a dust source area; the chemical composition of collected dusts indicates that they originate primarily in the North-Western Sahara, which is consistent with previous studies of the area. Fe can be present inside dust particles as a substitution element in the crystalline lattice of aluminosilicate, but a high proportion (62%) of aluminosilicate Fe-containing particles are also found as an internal mixture of aluminosilicate with Fe oxide grains ( including both oxide and hydroxide species). The 3D structure of such particles obtained by tomography reveals that these Fe-rich inclusions are often found at the surface of aluminosilicate particles but that some are also included inside particles. These Fe oxide grains can result from crustal earth or atmospheric processes during long-range transport. Fe-III is dominant in both the aluminosilicate matrix and the Fe oxide grains (Fe-III/Sigma Fe ratio = 76.8% and 90.0%, respectively, on average), with notable heterogeneities of Fe valence inside grains at a nanometer scale.