Mobilisierung von Eisen in Vulkanasche während des Transports in Eruptionssäulen
Physik des Erdkörpers
Physik und Chemie der Atmosphäre
Zusammenfassung der Projektergebnisse
Volcanic ash fertilizes the iron-limited areas of the surface ocean through releasing soluble iron. As ash iron is mostly insoluble upon the eruption, it is hypothesized that heterogeneous in-plume and in-cloud processing of the ash promote the iron solubilization. Direct evidences concerning such processes remain lacking. In this study, I developed a 1-D numerical model to simulate the physicochemical interactions of the gas–ash–aerosol in volcanic eruption plumes including the iron mobilization processes and aqueous chemistry. For model validation, the surface and bulk composition of the four ash samples from Eyjafjallajökull eruption in May 2010 are analyzed. Based on the results, I answer the key questions raised in the proposal: What is the potential composition of the liquid film around the ash particle that forms in the cooled volcanic plume? The results show that the absorption of volcanic volatiles (SO 2, HCl and HF) onto the ash surface increases the acidity and dissolution rate. Higher liquid water content (LWC) favors higher absorption (up to 38% for SO2 and 55% for HF and HCl) and lower pH (1.9). The model results agree very well with the leaching measurements. The measured values often show no distinctive trend. Therefore, it is not possible to draw a robust conclusion out of the measurements alone. Nevertheless, the model results help the interpretation and unravel the underlying processes and parameters. Comparison of the model and measurements show that individual particles have been exposed to various LWCs and thus, underwent different atmospheric processing pathways. This is not a surprise when one considers the heterogeneous structure of the cloud and particles in the atmosphere. Which processes mainly control the iron speciation on the ash surface? High LWC causes higher acidity and faster iron mobilization that is visible in the first hour of the Eyjafjallajökull ash cloud transport. Afterward, as LWC decreases due to freezing, the iron mobilization rate decreases with acidity. The low pH conditions promote acid-mediated dissolution of the Fe phases present in the ash surface. In the first hour, both Fe(II) and Fe(III) increase due to the ash dissolution. Then the aqueous chemistry reduces the Fe(II) to Fe(III) mainly through sulfur (IV) to sulfur (VI) oxidation. This processes results in the enrichment of Fe at the ash surface, which is confirmed by XPS and sputter depth analysis measurements. What are the optimum source and atmospheric conditions for bio-available iron formation? Elevated HCl and HF concentrations in the volcanic plume and high LWC lead to low pH conditions that is favorable for rapid iron mobilization at the ash surface. Particle size distribution basically controls the surface area to mass ratio, which is a key parameter for gas-aerosol partitioning processes. Smaller particles show the highest acid absorption and the highest Fe mobilization rate. What is the contribution of different volcano types to the global annual bio-available iron budget? Our results show that elevated halogen content in the gas (HCl and HF) and reduced conditions in the magma (that essentially modulate the pH and iron redox state, respectively) seem to be the favorable conditions for ash iron mobilization. This is comparable with the results of the mineral dust studies that report a strong correlation between the water-soluble fraction of Fe-carrying aerosols, pH and the iron redox state. Thus, attributing the fertilization potential of the ash to the tectonic setting of a volcano is an inconsistent hypothesis.
Projektbezogene Publikationen (Auswahl)
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(2016) A numeral model to simulate the chemical processing of volcanic ejecta in eruption plumes and clouds. EGU GA 2016
Hoshyaripour, G. A., Hort, M., Brasseur, G.
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(2016) Chemical Transport of Volcanic Ash: Case Study of Eyjafjallajökull Eruption 2010. AGU FM 2016
Hoshyaripour, G. A., Hort, M.
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(2017) Volcanic ash particles hold clues to their history and effects, Eos, 98
Hoshyaripour, G. A.
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(2018) Characterizing the volcanic ash surface using laboratory experiments, analytical measurements and numerical models: case of Eyjafjallajökull eruption 2010. EGU GA 2018
Hoshyaripour, G. A., Bruns, M., Hartmann, J., Hellmann, R., Hort, M.
