Minor effect of physical size sorting on iron solubility of transported mineral dust

Abstract

Abstract.Observations show that the fractional solubility of Fe (FS-Fe, percentage of dissolved to total Fe) in dust aerosol increases considerably from 0.1 % in regions of high dust mass concentration to 80 % in remote regions where concentrations are low. Here, we combined laboratory geochemical measurements with global aerosol model simulations to test the hypothesis that the increase in FS-Fe is due to physical size sorting during transport. We determined the FS-Fe and fractional solubility of Al (FS-Al) in size-fractionated dust generated from two representative soil samples collected from known Saharan dust source regions using a customized dust re-suspension and collection system. The results show that the FS-Fe is size-dependent and ranges from 0.1-0.3 % in the coarse size fractions (>1 µm) to ∼0.2-0.8 % in the fine size fractions (<1 µm). The FSAl shows a similar size distribution to that of the FS-Fe. The size-resolved FS-Fe data were then combined with simulated dust mass concentration and size distribution data from a global aerosol model, GLOMAP, to calculate the FS-Fe of dust aerosol over the tropical and subtropical North Atlantic Ocean. We find that the calculated FS-Fe in the dust aerosol increases systematically from ∼0.1 % at high dust mass concentrations (e.g., >100 µg m −3 ) to ∼0.2 % at low concentrations (<100 µg m −3 ) due to physical size sorting (i.e., particle gravitational settling). These values are one to two orders of magnitude smaller than those observed on cruises across the tropical and sub-tropical North Atlantic Ocean under an important pathway of Saharan dust plumes for simiCorrespondence to: Z. B. Shi (z.shi@bham.ac.uk) lar dust mass concentrations. Even when the FS-Fe of submicrometer size fractions (0.18-0.32 µm, 0.32-0.56 µm, and 0.56-1.0 µm) in the model is increased by a factor of 10 over the measured values, the calculated FS-Fe of the dust is still more than an order of magnitude lower than that measured in the field. Therefore, the physical sorting of dust particles alone is unlikely to be an important factor in the observed inverse relationship between the FS-Fe and FS-Al and the atmospheric mineral dust mass concentrations. The results suggest that processes such as chemical reactions and/or mixing with combustion particles are the main mechanisms to cause the increased FS-Fe in long-range transported dust aerosols.

Chemistry and mineralogy of clay minerals in Asian and Saharan dusts and the implications for iron supply to the oceans

“…In addition, the dust-derived Fe supply to low-nitrate low-chlorophyll (LNLC) regions of the oceans has been shown to control dinitrogen fixation (Moore et al, 2009;Schlosser et al, 2014). Therefore, interest has grown in recent years regarding atmospheric aerosol transport, inputs to the surface ocean, and the subsequent dissolution of Fe from aerosols (Desboeufs et al, 2001;Jickells and Spokes, 2001;Hand et al, 2004;Guieu et al, 2005;Meskhidze et al, 2005;Baker and Jickells, 2006;Buck et al, 2006Buck et al, , 2010Cwiertny et al, 2008;Journet et al, 2008;Mahowald et al, 2009;Shi et al, 2009Shi et al, , 2011Aguilar-Islas et al, 2010;Baker and Croot, 2010;Fu et al, 2010;Johnson et al, 2010;Paris et al, 2010;Trapp et al, 2010;Formenti et al, 2011;Rubin et al, 2011;Takahashi et al, 2011;Sholkovitz et al, 2012).…”

“…Iron dissolution from aerosols has been represented by fractional Fe solubility ( %FeS), and varies strongly depending on the aerosol source Sholkovitz et al, 2012). Sholkovitz et al (2012) compiled total Fe loading (Fe T ) and %FeS for a global-scale set of aerosol samples, and found a hyperbolic trend in the %FeS as a function of Fe T , which was explained by the mixing of mineral dusts of high Fe T and low %FeS and anthropogenic aerosols of low Fe T and high %FeS. However, mineral dust is an important supply of bioavailable Fe to the remote ocean, particularly during dust events originating from desert sources.…”

Scaling properties of rainfall and desert dust in the Canary Islands

“…Additionally, large concentrations of aeolian emissions could lead to problems in transport operations (Baddock et al, 2013). Mineral dust deposition on the ocean surface represents an input of micronutrients and, as a consequence, partially controls the productivity of marine ecosystems (Shi et al, 2011). A detailed list of environmental consequences and hazards to human population caused by dust storms, as well as an excellent review of causes and effects of dust storms, can be found in Goudie and Middleton (2006).…”

Characterization of Traffic-Related Particulate Matter Emissions in a Road Tunnel in Birmingham, UK: Trace Metals and Organic Molecular Markers

“…The MOUDI samples were then extracted using HF and HNO 3 as described in Shi et al (2011). To extract the total element fraction, the samples were placed in Teflon bombs and HF and HNO 3 were added (1:3) and the bombs were heated for 24 hours at 150°C.…”