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

Abstract

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, 10 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) 15 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 20 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 25 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.Atmos. Chem. Phys. Discuss., https://doi

Physical Weathering Intensity Controls Bioavailable Primary Iron(II) Silicate Content in Major Global Dust Sources

“…Sediments were freeze‐dried after sampling from the core. The dust deposited to this core is primarily from the Saharan dust plume (McGee et al, ), containing mostly quartz and oxidized secondary minerals such as Fe oxyhydroxides and clays (Caquineau et al, ; Rodriguez‐Navarro et al, ; Schroth et al, ; Sunda et al, ). North African dust is highly uniform in mineral composition, and clay‐mineral‐rich, indicating that it is highly chemically weathered as a whole (Prospero, ).…”

“…Solid‐phase Fe is transferred from a range of source areas to the ocean though riverine transport of sediment, sea ice transport of various particles, and eolian transport of dust, of which eolian transport of dust is most important to remote open‐ocean regions (Martínez‐García et al, ). Eolian processes are often assumed to transport secondary Fe(III)‐rich aluminosilicates and oxidized Fe(III) particles that are the products of chemical weathering, given the mineralogical composition of major low‐latitude dust sources (Hawkings et al, ; Raiswell et al, ; Rodriguez‐Navarro et al, ; Shi et al, ). However, both solid‐phase Fe(II) and Fe(III) species exist in the surface ocean (von der Heyden et al, ), and continental margins can supply primary Fe(II) silicates to the oceans through physical weathering of bedrock (Lam et al, ).…”

Hygroscopic Properties of Saline Mineral Dust From Different Regions in China: Geographical Variations, Compositional Dependence, and Atmospheric Implications

“…Furthermore, deposition of mineral dust is known to be an important source of Fe and P in open oceans, largely controlling biogeochemical cycles in these regions (Conway & John, 2014;Jickells et al, 2005;Li et al, 2017;Mahowald et al, 2011;Mahowald et al, 2018). Dust particles are mainly composed of minerals with low hygroscopicity, including quartz, feldspar, carbonate, iron oxides, illite, kaolinite, and montmorillonite (Engelbrecht et al, 2016;Formenti et al, 2011;Ito & Wagai, 2017;Journet et al, 2014;Nickovic et al, 2012;Rodriguez-Navarro et al, 2018;Shao et al, 2007). Therefore, fresh mineral dust particles are usually considered to be rather nonhygroscopic, and the single hygroscopicity parameters, κ, are typically found to be smaller than 0.01 (Herich et al, 2009;Koehler et al, 2009;Ma et al, 2010;Sullivan et al, 2009;Tang et al, 2016).…”

Wet Dust Deposition Across Texas During the 2012 Drought: An Overlooked Pathway for Elemental Flux to Ecosystems

“…Because dust‐in‐rain events deliver large pulses of dissolved nutrients to ecosystems, these sporadic events can have significant impacts on ecosystem processes (Arvin et al, ; Rodriguez‐Navarro et al, ). In humid forest ecosystems, wet dust‐related deposition of bioavailable nutrients can offset nutrient leaching losses, sustaining productivity in the later stages of ecosystem development (Chadwick et al, ; Eger et al, ; Gross et al, ; Yu et al, ).…”

“…Further, precipitation that coincides with dust events typically contains high concentrations of dissolved ions and particulate matter (PM; Ávila et al, ). Thus, even when dust‐in‐rain events are infrequent, they can comprise a major fraction of annual wet deposition for some key elements (Ávila et al, ; Prospero et al, ; Rodriguez‐Navarro et al, ). In sum, analyzing the chemical and elemental composition of wet dust deposition and quantifying its contribution to atmospheric deposition is necessary to better understand dust impacts on terrestrial and ocean biogeochemical cycles (Bergametti & Forêt, ; Jickells et al, ; Prospero et al, ).…”