Physico-chemical and optical properties of Sahelian and Saharan mineral dust : in situ measurements during the GERBILS campaign

2011

Article référencé dans le Web of Science WOS:000293968900008

Klaver A., Formenti P., Caquineau Sandrine, Chevaillier S., Ausset P., Calzolai G., Osborne S., Johnson B., Harrisone M., Dubovik O.

Quarterly Journal of the Royal Meteorological Society, 2011, 137 (658), p. 1193-1210 ISSN 0035-9009

This paper presents new results on the composition, size and shape of mineral dust particles from African sources which were obtained by analysis of bulk filter samples collected in June 2007 onboard the BAe-146 research aircraft of the Facility for Airborne Atmospheric Measurements (FAAM). The aircraft was operated over Mauritania, Mali and Niger during the Geostationary Earth Radiation Budget Intercomparisons of Longwave and Shortwave radiation (GERBILS) campaign. Dust sampled during the campaign originated from various sources, including locally in the Sahel as a result of large-scale convective activity. Regardless of origin, clays (illite, kaolinite) dominated the total volume (79-90%); the remainder was composed of quartz, calcium-rich minerals (calcite, dolomite, gypsum) and alkali feldspars. Iron oxides, measured using a selective chemical extraction method, accounted for 1-3% of the total dust mass. The dependence of particle number size and shape distribution on the origin of dust seems minor too, although our results might be slightly misleading due to the fact that those kinds of data have been gathered on flights when dust had comparable origins and residence time. Mineral dust is only weakly absorbing in the mid-visible wavelengths (single scattering albedo omega(0) > 0.95 at 550 nm), and omega(0) measured values can be reproduced by measuring the bulk fractions of the major minerals, i.e. clays, quartz, calcite and iron oxides. At this wavelength, knowledge of the nature of clays and iron oxides, or the state of mixing of the minerals, does not induce significant differences in the results. A more precise description of the nature of clays and iron oxides is necessary at lower wavelengths owing to larger differences in their spectral optical properties. In particular, knowledge of the nature of the dominant clay is important for determining light scattering in the backward hemisphere.

Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Sciences du milieu [021]

Fonds IRD [F B010053768]

fdi:010053768