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Mineral Dust as Micronutrient Supplier
For the HNLC Southern Ocean, dust deposition is supposed to be an important source of micronutrients. Hence, the key issue for the marine ecosystem is trace elements that dust contains rather than dust particles themselves. Elemental composition and bioavailability of elements are two key issues of dust that determine the amount of elements assimilated by marine ecosystem.
Elemental composition of mineral dust
As the product of wind erosion of soils, mineral dust contains several chemical elements that could be important for the biological processes of marine ecosystem. Measuring the dust elemental compositions is important to estimate the emission inventory of trace elements from dust source areas and evaluate the biological impact of dust input (Baker et al., 2003; Zhang et al., 2015). Chemical compositions of dust can differ from its parent soil since the dust materials contain only the fine fraction of soil particles. In bulk soils, large particles, especially the sandy fraction, are dominated by quartz and calcium containing materials (e.g. calcite and gypsum), whereas smaller particles contain clay minerals, feldspars, quartz, micas, carbonates and iron oxides (Journet et al., 2014; Schütz and Rahn, 1982). Quartz contains mainly Si; Clay minerals are mainly composed of Si and Al; Fe content is found principally in clays, feldspars and iron oxides; Ca and Mg exist mostly in gypsum, calcite and dolomite (Journet et al., 2014). The variation of chemical compositions with mineral species results in finally size dependence of elemental concentrations in desert soils (Eltayeb et al., 1993; Eltayeb et al., 2001; Castillo et al., 2008; Schütz and Rahn, 1982; Miller et al., 1972). For example, Schütz and Rahn (1982) studied African and American soils and found that elemental concentrations for most elements, except for Si, increase to the highest when the particle size decreases to 20 μm. This increase is greater in higher weathered and more winnowed soils, and is negligible in humus-rich soils. Eltayeb et al. (1993) found a nearly constant concentration of Al, K, Sr and Rb, a positive fractionation for the elements Ca, Ti, Mn, Fe, Y, and a negative fractionation for Si and Zr in aerosol fraction of Namibian soils. Therefore, both major elements and trace elements exhibit preferential partitioning with size fractions. As a result of different fractionation behavior of elements, elemental composition dust may differ from the chemical composition of parent soil.
Previous modelling studies generally take the average iron concentration of Earth’s crust (3.5%) (Taylor and McLennan, 1995) as the iron content in dust (e.g. Duce and Tindale, 1991; Luo et al., 2008). However, spatial heterogeneities of dust elemental composition have been shown in former studies. For example, Formenti et al. (2008) found an average iron content equaling to 8.6 ± 0.2% (mean ± std) for local dust and 7.6 ± 0.6% for transported dust in Banizoumbou, Niger. Different iron content in dust from local source and dust from remote sources reflects regional variability of iron content. In addition, both dust sources showed much higher iron compositions than the values used by dust models. Iron composition of dust fallout measured by Gaiero et al. (2007) at four sites in Patagonian coast, despite the fact that dust deposited closing source areas is different from dust transported for long distance, also showed higher iron concentrations (4.3 ± 0.6%) than values used by models. Spatial variability of dust elemental composition must be taken into account to better evaluate the emission inventory of trace elements associated with dust.
Bioavailability of trace elements in dust
After deposited into seawater, only a fraction of trace elements in dust is bioavailable for the marine biota, where “being bioavailable” means being effective in causing a biological effect on the phytoplankton. For elements like Fe, the processes making iron bioavailable are complicated and different forms of iron are bioavailable, but all bioavailable iron are in dissolved phase including colloidal phase or soluble phase (Barbeau et al., 2001; Rich and Morel, 1990; Fitzsimmons and Boyle, 2014). Although not all forms of dissolved iron are bioavailable (Visser et al., 2003), the iron bioavailability is generally evaluated by the common known “fractional solubility” (hereafter “solubility”) that is defined as the percentage ratio of dissolved amount to the total amount.
Factors controlling the solubility of micronutrients in mineral dust: the case of iron
For marine ecosystem, the bioavailability of micronutrients associated with dust depends on multiple factors: 1) the mineralogical composition of source dust, 2) the chemical processing history of dust during atmospheric transport, 3) the deposition process of dust into the ocean, 4) the composition of seawater (Baker and Jickells, 2006; Baker et al., 2006b; Desboeufs et al., 1999; Journet et al., 2008; Paris and Desboeufs, 2013; Gierlus et al., 2012; Losno et al., 1991; Schulz et al., 2012; Shi et al., 2012). The following content will discuss the impact of these factors with a focus on iron.
Mineralogical composition of dust may influence the solubility of elements in dust. As indicated by Journet et al. (2008), solubility of iron-containning minerals can vary by different orders of magnitude, particularly iron contained by clays generally show much higher solubility than iron (hydr-)oxide. Despite the variation of solubility among Ca-containing minerals (Krueger et al., 2004; Chou et al., 1989), some minerals such as calcium carbonate in dust can act as alkalinity buffers and neutralize the acidic conditions in cloud droplets or rainwater during atmospheric transport (Losno et al., 1991; Loye-Pilot et al., 1986) and hence prevent the enhancement of solubility by atmospheric acid processing that is presented below.
Atmospheric processing is suggested to result in greater uncertainty of bioavailability of iron in dust (Shi et al., 2012). Previous studies generally found fractional iron solubility less than 0.5% for non-atmospheric processed dust but ranging from 0.1% to ~90% for transported aerosol (Mahowald et al., 2005; Hand et al., 2004; Chen and Siefert, 2004; Baker and Jickells, 2006; Sedwick et al., 2007; Heimburger et al., 2013a). The difference of solubility suggests an enhancement of dust solubility by atmospheric processing (Shi et al., 2012). During the atmospheric transport, dust particles can incorporate into cloud droplets as cloud condensation nuclei (CCN) or as interstitial particles (Andreae and Rosenfeld, 2008; Gierlus et al., 2012). Cloud water is an effective medium for heterogeneous chemical reactions. Nitrate and sulfate produced by the oxidation process by H2O2, O3, O2 and NO2 acidify the cloud waters (Rosenfeld et al., 2014). Enhanced acidity of cloud water finally enhanced the elemental solubility of dust particles (Spokes and Jickells, 1995; Spokes et al., 1994; Desboeufs et al., 2001; Desboeufs et al., 2005). In addition to the acid processing in cloud, acid processing as wet particles outside cloud can also enhance the iron solubility of dust (Spokes and Jickells, 1995; Spokes et al., 1994; Desboeufs et al., 2001; Desboeufs et al., 2005; Shi et al., 2015). After the evaporation of cloud droplets, pH values of the water content in dust particles could decrease to 2 or even lower (Meskhidze et al., 2003; Zhu et al., 1992). As indicated by the recent study of Shi et al. (2015), the highly acidic wet particles outside cloud resulting from the evaporation of cloud droplets is the main process increasing readily dissolved iron during atmospheric processing. Furthermore, the exposition of dust particles to the solar ration in the presence of acidic solutions can also enhance the solubility of iron due to the photoreduction reaction (Hand et al., 2004; Fu et al., 2010) that converts the relatively insoluble Fe(III) into the more soluble Fe(II) (Kieber et al., 2005). Fu et al. (2010) found that dust in HCl solution showed higher increase of iron solubility under irradiation compared to the dark reaction.
Common methods of elemental solubility estimation
Elemental solubility measurements generally include a leaching procedure to extract the dissolved phase and a separation procedure to separate the dissolved phase from the particulate phase. Solubility measurements could be affected by multiple experimental parameters including the choice of leaching solvent, the time of contact between dust materials, and the separation procedure.
Table 1 summarizes several Fe solubility estimations in previous studies carried on samples of different origins or sample types including rainwater and surface snow. For aerosol samples or proxies of dust (e.g. fine fraction of soil), the choice of leach solution to measure elemental solubility depends on purposes of measurements (Shi et al., 2012).
Ultrapure deionized water is commonly used to measure the elemental solubility in solutions without affecting factors such as non-acidified cloud water (Aguilar-Islas et al., 2010; Buck et al., 2010 Winton et al., 2014). Because the buffer capacity of pure water is very limited, the pH of solutions can change to different extent resulting from the dissolution of minerals such as carbonate (Aghnatios et al., 2014). The use of weak buffers such as ammonium acetate (pH = 4.7) (Sarthou et al., 2003; Baker et al., 2006) may avoid the problem of pH modification and provide comparable results between different mineral aerosols. The pH4.7 can also simulate the dissolution at pH condition similar to rainwater. To simulate the acid processing during the atmospheric transport, previous studies often use acidified solutions and found a strong impact of pH on iron solubility (Desboeufs et al., 2005; Desboeufs et al., 1999; Baker et al., 2006b; Spokes and Jickells, 1995; Edwards and Sedwick, 2001). For example, Spokes and Jickells (1995) tested the solubility of Saharan aerosol successively under pH8, pH2, and pH5.5 and observed a variation of iron solubility from 0.1% (pH8) to 4.7 (pH2) and finally to 0.3% (pH5.5). Leaching solution could also be purified, modified or synthetic seawaters to investigate the dissolution capacity of aerosol at the pH of seawater (approximately pH8.2) and with the presence of organic ligands in marine aquatic environment (Aguilar-Islas et al., 2010; Wu et al., 2007). For samples like rainwater or meltwater of snow, dust materials they contained are already in liquid phase. Accumulated dust deposition in annual surface snow samples in oceanic region is released immediately into seawater after melting. Rainwater containing the dust materials is also ready to enter into seawater. Therefore, dissolved phase is separated immediately after the collection of rainwater (Buck et al., 2010; Heimburger et al., 2013a) or the melting of snow(Edwards and Sedwick, 2001; Winton et al., 2014).
Table of contents :
Remerciements
Table of content
Introduction
Bibliography
Chapter 1 Background, Significance and Approaches of Research
1. Dust Emission processes
2. Sources, Transport and Deposition of Mineral Dust to the Southern Ocean
2.1. Distribution and contribution of dust sources in the Southern Ocean
2.1.1. Distribution of dust sources
2.1.2. Contribution of dust sources in the Southern Ocean
2.2. Dust transport and deposition in the Southern Ocean
3. Mineral Dust as Micronutrient Supplier
3.1. Elemental composition of mineral dust
3.2. Bioavailability of trace elements in dust
3.2.1. Factors controlling the solubility of micronutrients in mineral dust: the case of iron
3.2.2. Common methods of elemental solubility estimation
4. Research Topics and Strategies
4.1. Research topics
4.2. Research Strategies
4.2.1. Long-term dust concentration measurements in Patagonia
4.2.2. Spatial heterogeneity of source dust elemental compositions
4.2.3. Some aspects of the solubility of continental dust
Bibliography
Chapter 2 Long-term dust concentration measurements in Patagonia
Introduction of Chapter
Abstract
1. Introduction
2. Materials and methods
2.1. Aerosol sampling location and methods
2.2. Elemental analysis
2.3. Chemical compositions of the crustal fraction of the aerosol
2.4. Air mass back trajectories
2.5. Wind simulation and meteorological records
3. Results and discussion
3.1. Chemical composition of the dust fraction
3.2. Atmospheric concentration of sea salt and mineral dust
3.3. Seasonal pattern of the aerosol concentration
3.4. Meteorological dependence of seasonal dynamics of dust concentrations and emission
4. Conclusions
Acknowledgements
References List
Supporting Information for the article
XRF measurement conditions and calibration lines
Conclusions of Chapter 2
Chapter 3 Spatial Heterogeneity of source dust compositions
Introduction of Chapter
Abstract
1. Introduction
2. Study area
2.1. Patagonia Desert
2.2. Namibia: Namib Desert and Kalahari Desert
3. Materials and methods
3.1. Soil-derived aerosol generation
3.2. Soil sample collection
3.3. Elemental analysis
3.4. Principal component analysis of compositional data
3.5. Accumulation factor and enrichment factor of dust relative to parent soil
4. Results and discussion
4.1. Elemental composition of soil and aerosol
4.1.1. Element concentration of topsoil and soil-derived dust in Patagonia and Namibia
4.1.2. Spatial variation of elemental composition in regional scale
4.1.3. Robust principle component analysis
4.2. Variation of elemental composition from bulk soil to aerosol
5. Conclusion
Acknowledgements
References List
Supporting Information for the article
Soil tablets preparation and measurement
Soil-derived aerosol analysis: the “thin layer method”
Conclusions of Chapter 3
Chapter 4 Contribution to Bioavailability Study of Mineral Dust from Patagonia and Namibia
Introduction of Chapter
Abstract
1. Introduction
2. Materials and methods
2.1. Mineral aerosol samples
2.2. Dissolution experiments of aerosol sample
2.3. Centrifugation separation of suspension
2.4. Chemical analysis
3. Results and discussion
3.1. Comparison of solubility values between centrifugation and filtration
3.2. Variation of solubility with elements and its dependence on pH
3.3. Dependence of solubility on types of dust sample
4. Conclusion
5. Prospect
Acknowledgements
References List
Supporting Information
Conclusions of Chapter 4
Conclusions and prospects
Appendix
Appendix 1. Super clean protocol
Appendix 2. Classification of cleanroom (ISO 14644-1)
Appendix 3. XRF instrument (PANalytical, Epsilon 3XL) and XRF analysis
Appendix 4. Illustration of aerosol sampling station in Río Gallegos, Patagonia
Appendix 5. Atmospheric concentration of Si, Al, Fe, Na, dust and sea salt measured in Río Gallegos
Appendix 6. Dust generation by SyGAVib: the condition set
Appendix 7. Map of elemental composition for a) Patagonian soils, b) Namibian soils, c) Patagonian dust, d) Namibian dust.
Appendix 8. Elemental concentration in soils (SP: Soil from Patagonia; SN: Soil from Namibia)
Appendix 9. Elemental concentration in dust (DP: Dust from Patagonia; DN: Dust from Namibia)
Appendix 10. Coordinates of soil samples and total mass of dust generated from soil (DP: Dust from Patagonia; DN: Dust from Namibia)
Appendix 11. Photo of soil sampling
Appendix 12. Mass of elements, differential Solubility and pH after leaching
List of figures
List of tables
