Abstract. A dust production model (DPM) is obtained by combining preexisting models of saltation and sandblasting, the two processes that lead to mineral aerosol release in arid areas. From a description of the soil characteristics and wind conditions, the DPM allows computation of the amounts of aerosol released and of their size distributions. Semiquantitative comparisons of the model outputs with the few field data available in the litterature validate its main implications. The first one is that the aptitude of a soil to release particles smaller than 20 lam depends on (15 the dry size distribution of aggregates constituting its loose fraction, (2) its roughness length, and (35 the wind velocity. The second implication is that the size distribution of aerosols released in source areas also strongly depends on these parameters.

[1] The column-integrated optical properties of aerosol in the central eastern region of Asia and midtropical Pacific were investigated based on Sun/sky radiometer measurements made at Aerosol Robotic Network (AERONET) sites in these regions. Characterization of aerosol properties in the Asian region is important due to the rapid growth of both population and economic activity, with associated increases in fossil fuel combustion, and the possible regional and global climatic impacts of related aerosol emissions. Multiyear monitoring over the complete annual cycle at sites in China, Mongolia, South Korea, and Japan suggest spring and/or summer maximum in aerosol optical depth (t a ) and a winter minimum; however, more monitoring is needed to establish accurate climatologies. The annual cycle of Angstrom wavelength exponent (a) showed a springtime minimum associated with dust storm activity; however, the monthly mean a 440 -870 was >0.8 even for the peak dust season at eastern Asian sites suggesting that fine mode pollution aerosol emitted from population centers in eastern Asia dominates the monthly aerosol optical influence even in spring as pollution aerosol mixes with coarse mode dust originating in western source regions. Aerosol optical depth peaks in spring in the tropical mid-Pacific Ocean associated with seasonal shifts in atmospheric transport from Asia, and $35% of the springtime t a500 enhancement occurs at altitudes above 3.4 km. For predominately fine mode aerosol pollution cases, the average midvisible ($550 nm) single scattering albedo (w 0 ) at two continental urban sites in China averaged $0.89, while it was significantly higher, $0.93, at two relatively rural coastal sites in South Korea and Japan. Differences in fine mode absorption between these regions may result from a combination of factors including aerosol aging during transport, relative humidity differences, sea salt at coastal sites, and fuel type and combustion differences in the two regions. For cases where t a was predominately coarse mode dust aerosol in the spring of 2001, the absorption was greater in eastern Asia compared to the source regions, with w 0 at Dunhuang, China (near to the major Taklamakan dust source), $0.04 higher than at Beijing at all wavelengths, and Anmyon, South Korea, showing an intermediate level of absorption. Possible reasons for differences in dust absorption magnitude include interactions between dust and fine mode pollution aerosol and also variability of dust optical properties from different source regions in China and Mongolia.

Abstract. In order to develop a model providing the mass size distribution of the dust raised from the ground by the sandblasting process, mechanical characteristics of 240 [tm saltating sand grains meant to be used as projectiles in wind tunnel sandblasting experiments were carefully determined. It was found that for values of the measured friction velocity less than about 55 cm/s, the constraint of the relatively small dimensions of the wind tunnel test section did not prevent saltation from developing freely. The kinetic energy of the sand grains was also determined. Aerosols were then produced in the wind tunnel by bombarding a clay target with the saltating 240 [tm quartz grains. The size distributions of these aerosols were measured for three wind speeds with an optical particle analyzer. For the lowest wind speed the size distribution of the aerosol was similar to that of the 8.6 [tm aggregates originally constituting the agglomerates of clay, but disaggregation into smaller particles became more important when wind speed increased. A theo• of sandblasting was then developed that gave theoretical results agreeing with the experimental ones. A consequence of this theo• was that submicron particles could be released from aggregates for high wind speeds. Experiments meant to check this implication were carried out and confirmed it. Cohesion energies of kaolin particles of three different sizes, 8.6, 2.8, and approximately 0.5 gm, were calculated and found to be a decreasing exponential function of the particle size. This explains (1) why the soil-derived aerosol size distributions present a common mode in the 1-10 [tm size range, these particles being readily released even in not particularly energetic conditions, and (2) why observation of a submicron mode in natural aerosols requires a higher friction veloci .ty than for the common 1-10 [tm mode.

Mineral dust concentrations have been measured from a monitoring aerosol station set up at Sal, Cape Verde Island between December 1991 and December 1994 in order to assess the transport process of African dust over the North‐eastern Tropical Atlantic. These measurements indicate a pronounced seasonal pattern, with maximum dust concentrations observed during winter. A meteorological analysis shows that the transport of dust occurs at low altitudes, in the trade winds layer, during this season. Large quantities of dust are carried out of North‐western Africa, in particular from the Sahel at these altitudes. Such process could constitute the major supply of atmospheric mineral matter to the surface sea water of the Eastern Atlantic.

Abstract. The aim of this study is to find a tracer allowing retrieval of the regional origin of mineral dust for Saharan aerosols transported over the North Atlantic Ocean. Because of physical and chemical ffactionation processes occurring at the soil-atmosphere interface mid during the atmospheric transport of dust, clay mineral species seein to be the best candidate. This study shows that the ratio between relative abundance of illite and kaolinire (I/K ratio) is the parameter that is the most sensitive to the regional origin of Saharan dust collected on Sal Island (Cape Verde). By comparing the I/K ratio measured in dust enfitted from the same Saharan source and collected along its transport both on Sal Island and Barbados (Caribbean Sea), we show that this ratio seems to remain unchanged after long-range transport.

Abstract. Size distributions of mineral aerosols released by wind-aerosol particle populations that can be released by sandblasting erosion from arid areas are of primary importance to model their from a given soil. transport patterns as well as their effect on climate. Wind tunnel experiments meant to test the influence of wind strength and soil Experimental characteristics are earned out with two natural soils differing both 1. Set-up in texture and in mineral composition. In all cases, the aerosols can be considered as mixtures, in proportions depending on wind speed, of the same three lognormally distributed particle populations. By using appropriate binding energies for these populations, a sandblasting model previously developed allows retrieval of the size distributions of the aerosols produced by the two soils at any wind speed. A major implication of this study is that the size distributions of mineral aerosols greatly depend on the wind conditions prevailing during their generation. Aerosols optical properties being size dependent, this should clearly be taken into account in climate modeling.

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During June, July and August 2006 five aircraft took part in a campaign over West Africa to observe the aerosol content and chemical composition of the troposphere and lower stratosphere as part of the African Monsoon Multidisciplinary Analysis (AMMA) project. These are the first such measurements in this region during the monsoon period. In addition to providing an overview of the tropospheric composition, this paper provides a description of the measurement strategy (flights performed, instrumental payloads, wing-tip to wing-tip comparisons) and points to some of the important findings discussed in more detail in other papers in this special issue