Mass size distributions of the major elements of the mineral aerosol of northem Sahara were obtained from measurements carried out with an inertial cascade impactor. A fitting procedure by data inversion was applied to these data. This technique considers the major mechanisms of nonideal behavior such as particle bounce-off, wall losses and cross sensitivity which affect the accuracy of impactor measurements of aerosol size distribution. Accurate size distributions of Saharan aerosol (range 0.1-20 gm) were so obtained for different conditions of soil mobilization. These measurements, coupled with wind speed information, show the shape of the size distribution for desert aerosols in the considered range to be characterized by a similar pattern with a common mode of particles having diameter between 1 and 20 [am whatever the weather conditions. A second mass peak of submicron particles appears when the wind speed increases above the threshold value for erosion. Scanning electron microscope photographs indicate that the whole granulometric spectrum of the sampled aerosol between 0.1 and 20 [am mainly consists of clay easily disaggregated by sandblasting. The common origin of submicron particles with those between 1 and 20 [am is established by showing the similar composition of dusts in the both modes. The results suggest that the presence of submicron mineral particles during dusty and sandstorm conditions is consistent with a sandblasting process.13,927 the confr&erie of E1 Abiod Sidi Cheikh (Algeria) and to Y. Callot of University of Oran (Algeria) for their kind assistance. We also would like to mention the friendly cooperation of P. Blanc for SEM and EDXA examinations and B. Chatenet for her technical support, especially for illustration. We are particularly grateful to hypothesis that this population of very fine particles W.F. Fitzgerald and F. Dulac for reviewing our manuscript. This originates from windblown dust, and is derived from the work was supported by the Centre National de la Recherche soils by a sandblasting process. Scientifique (France).

No abstract

Bulk filtration samples of dust were collected on Fuerteventura (Canary Islands) in July 1985 in order to investigate African dust transport over this region during summer. Air mass trajectories and elemental and mineralogical analyses allowed us to distinguish between oceanic and continental influences: three African dust transport cases occured during this period. Geochemical and mineralogical tracers, combined with air mass trajectories, indicate that two different source regions of dust (Sahelian and Moroccan ones) are involved. Each of these exhibits specific Si/Al and Fe/Al ratios that can be considered as tracers of dust origin. A meteorological study, based on satellite imagery, meteorological maps, and air mass trajectories, confirms the previous identification of the source region. It is also possible to describe the meteorological situation controlling the atmospheric pathway of the dusts from the source regions up to the Canary Islands. This study suggests that some source regions (especially the Sahelian one) of dust affecting the Canary Islands during summer are rather different from the Moroccan ones observed in a spring situation by Coudé‐Gaussen et al. (1987).

From April 12 to 19, 1984, dust was [Windom, 1975; Chester et al., 1979; Sarnthein et collected by cascade impactor on Fuerteventura al., 1982] and, probably, to the mineral feeding Island, 100 km from the Saharan coast. The sam-of marine plankton. The dust outbreaks from ples were analyzed by X ray fluorescence spectro-northwestern Africa to the middle Atlantic and metry (elemental composition), by X ray diffrac-Western Europe have been much less studied. The tion (mineral constituents), and by scanning only samplings were carried out during oceanic electron microscope (SEM) and energy dispersive X cruises [Chester and Johnson, 1971; Parkin et ray analyzer (EDXA) examination (shape, surface al., 1972] and the results were compared to the features, and mineralogy of particles), with the marine sedimentation [Tetzlaff and Wolter, 1980; aim of determining the possible source areas of Sarnthein et al., 1982]. On the other hand, the the dust. On the scale of dust clouds, another fallout on the islands was not considered, except method allowed us to define the displacements of in a short note about the sampling of dust on the dust: using remote sensing (METEOSAT II, Tenerife island [Fernandez-Navarro, 1921] and National Oceanic and Atmospheric Administration another about the dust effects upon the under-(NOAA)) and computing trajectories for any star-ground water chemistry in the Gran Canaria ting or ending point at any time. These various [Logan, 1974]. methods gave complementary results and pointed A research project, supported by the French out two successive episodes of dust with two Centre National de la Recherche Scientifique different continental origins. The first ori-(CNRS), started with the purpose of characteriginated from a nearby source in the Souss plain zing Saharan dust [Coud•-Gaussen, 1982, 1984; (southern Morocco) and was characterized by very Coud•-Gaussen and Rognon, 1983]. Fuerteventura fine quartz particles. The second drifted above Island (eastern Canaries) is a particularly intemore remote calcareous regions. This study allows resting area for this study because aeolian parus to show how different methods can be used to ticles have been encountered in various silty characterize dust and to find the source areas. deposits of surface formations: desert quartz, The results suggest some general questions about allochthonous minerals, feltlike grains of fithe occurrence of coarse particles in the atmos-brous clays, etc. [Chamley et al., 1987]. Relaphere and their role in the atmospheric depositing to this project, dust samples were collected tion flux budget. from April 12 to 19, 1984, on Fuerteventura Island. This island is 100 km from the Saharan Recherche Scientifique, Universit• Pierre et study on the year 1979 from the imagery given by Marie Curie, Paris, France. the GOES East and METEOSAT satellites [01iva et 2 Laboratoire de Physico-Chimie de l'Atmosph•-al., 1983].

Holocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north‐eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin‐suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.