Articles | Volume 16, issue 19
Research article
04 Oct 2016
Research article |  | 04 Oct 2016

Mineralogical properties and internal structures of individual fine particles of Saharan dust

Gi Young Jeong, Mi Yeon Park, Konrad Kandler, Timo Nousiainen, and Osku Kemppinen

Abstract. Mineral dust interacts with incoming/outgoing radiation, gases, other aerosols, and clouds. The assessment of its optical and chemical impacts requires knowledge of the physical and chemical properties of bulk dust and single particles. Despite the existence of a large body of data from field measurements and laboratory analyses, the internal properties of single dust particles have not been defined precisely. Here, we report on the mineralogical organization and internal structures of individual fine ( <  5 µm) Saharan dust particles sampled at Tenerife, Canary Islands. The bulk of Tenerife dust was composed of clay minerals (81 %), followed by quartz (10 %), plagioclase (3 %), and K-feldspar (2 %). Cross-sectional slices of Saharan dust particles prepared by the focused ion beam technique were analyzed by transmission electron microscopy (TEM) to probe the particle interiors. TEM analysis showed that the most common particle type was clay-rich agglomerate, dominated by illite–smectite series clay minerals with subordinate kaolinite. Submicron grains of iron (hydr)oxides (goethite and hematite) were commonly dispersed through the clay-rich particles. The median total volume of the iron (hydr)oxide grains included in the dust particles was estimated to be about 1.5 % vol. The average iron content of clay minerals, assuming 14 wt % H2O, was determined to be 5.0 wt %. Coarse mineral cores, several micrometers in size, were coated with thin layers of clay-rich agglomerate. Overall, the dust particles were roughly ellipsoidal, with an average axial ratio of 1.4 : 1.0 : 0.5. The mineralogical and structural properties of single Saharan dust particles provide a basis for the modeling of dust radiative properties. Major iron-bearing minerals, such as illite–smectite series clay minerals and iron (hydr)oxides, were commonly submicron- to nano-sized, possibly enhancing their biogeochemical availability to remote marine ecosystems lacking micronutrients.

Short summary
Individual Saharan dust particles were investigated by transmission electron microscopy of cross-sectional slices. We classified internal structures, and determined the volume of iron oxide included in the dust particles, the iron content of clay minerals, and the shape of the dust particles. The mineralogical and structural properties of single dust particles provide a basis for the modeling of dust optical properties and the supply of iron as a micronutrient to remote ocean ecosystem.
Final-revised paper