Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event

• Published in: Atmospheric chemistry and physics 2018-07, Vol.18 (13), p.10089-10122
• Main Authors: Rodriguez-Navarro, Carlos, di Lorenzo, Fulvio, Elert, Kerstin
• Format: Article
• Language: English
• Subjects: Activated clay; Aggregates; Analysis; Atmospheric models; Atmospheric particulates; Bioavailability; Biogeochemistry; Calcite; Carbonates; Carcinogens; Chemical properties; Chlorite; Clay; Clay minerals; Cyclones; Deserts; Distribution; Dust; Dust cycle; Dust particles; Dust storms; Electron microscopy; Extreme weather; Fibers; Goethite; Health hazards; Hematite; Illite; Imagery; Iron; Kaolinite; Microscopy; Mineralogy; Minerals; Modelling; Nanoparticles; Physicochemical processes; Physicochemical properties; Radiation; Radiative forcing; Rain; Saharan dust; Satellite imagery; Satellites; Scanning electron microscopy; Smectites; Soil; Thermal radiation; Transmission electron microscopy
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-18-10089-2018

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 northern African cyclone that affected the southern Iberian Peninsula during 21–23 February 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) southern/central Algeria, northern Mali, and northwestern Niger, and (ii) northern Algeria, southern Tunisia, and northwestern Libya. 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 (55 % of the total Fe) and typically form nanogranular aggregates covered or interspersed with amorphous/poorly crystalline iron oxyhydroxide nanoparticles (ferrihydrite), which account for  ∼  18 % 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 the 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 the source of the  ∼  20 % soluble (bioavailable) iron in the studied desert dust. The dust particles are a potential health hazard, specifically the abundant and potentially carcinogenic iron-containing palygorskite fibers. Ultimately, we show that diffe