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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-211
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
01 Mar 2018
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event
Carlos Rodriguez-Navarro, Fulvio di Lorenzo, and Kerstin Elert Dept. Mineralogy and Petrology, University of Granada, Fuentenueva s/n, 18002 Granada, Spain
Abstract. The mineralogy and physicochemical features of Saharan dust particles help to identify source areas and determine their biogeochemical, radiative and health effects, but their characterization is challenging. Using a multianalytical approach, here we characterized with unprecedented level of detail the mineralogy and physicochemical properties of Saharan dust particles massively wet deposited (~ 18 g m−2) following an extreme red rain event triggered by a north African cyclone that affected the southern Iberian Peninsula during February 21–23, 2017. Abundant palygorskite and illite, and relatively high carbonate contents, well-known northern and north-western Saharan dust indicators, along with low chlorite content and significant amounts of smectites and kaolinite, whose abundance increases southwards in the western Sahara, complemented by satellite imagery and back/forward trajectories, show that the most probable dust source areas were (i) south/central Algeria, north Mali and northwest Niger, and (ii) north Algeria, south Tunisia and north-west Libia. Scanning and transmission electron microscopy analyses, including Z-contrast high angle annular dark field (HAADF) imaging and analytical electron microscopy (AEM) show that clay minerals include abundant structural Fe (57 % of the total Fe) and typically form nanogranular aggregates covered or interspersed with amorphous/poorly crystalline iron oxyhydroxide nanoparticles (ferrihydrite), which account for 28 % of the free Fe, the rest being goethite and hematite. These nanogranular aggregates tend to form rims lining large silicate and carbonate particles. Such internally mixed iron-containing phases are main contributors to the observed absorption of solar and thermal radiation, and along with the abundant coarse/giant particles (> 10 μm), strongly affect the dust direct radiative forcing. The lack of secondary sulfates in aggregates of unaltered calcite internally mixed with clays/iron-rich nanoparticles shows that iron-rich nanoparticles did not form via atmospheric (acid) processing but were already present in the dust source soils. Such iron-rich nanoparticles, in addition to iron-containing clay (nano)particles, are an important source for bioavailable (soluble) iron. The dust particles are a potential health hazard, specially the abundant and potentially carcinogenic iron-containing palygorskite fibers. Ultimately, we show that different source areas are activated over large desert extensions, and large quantities of complex dust mixtures are transported thousands of kilometers and wet-deposited during such extreme events, which thwart any other Saharan dust event affecting south-western Europe. The past, present, and future trends, as well as impacts, of such extreme events must be taken into account when evaluating and modeling the manifold effects of the desert dust cycle.
Citation: Rodriguez-Navarro, C., di Lorenzo, F., and Elert, K.: Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-211, in review, 2018.
Carlos Rodriguez-Navarro et al.
Carlos Rodriguez-Navarro et al.

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Short summary
Saharan dust wet deposited in Granada (Spain) in February 21–23, 2017 during the most extreme red rain event of the last decades led to the deposition of ~ 140,000 T of dust just in the city of Granada, thwarting any other standard Saharan dust events taking place in the area. The multianalytical study of Saharan dust disclosed potentials source areas and the mineralogy and composition of the size-fractions of desert dust as well as its potential biogeochemical, radiative and health effects.
Saharan dust wet deposited in Granada (Spain) in February 21–23, 2017 during the most extreme...
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