Iron Speciation in Different Saharan Dust Advections and Effect of the Procedural Blank on the Results From X-ray Absorption Spectroscopy and Selective Leaching Experiments

Chiara Petroselli,Beatrice Moroni,Marco Grotti,David Cappelletti,Roberta Selvaggi,Francesco D’Acapito,Stefano Crocchianti,Francesco Soggia

doi:10.3390/atmos11070735

Abstract

In this work, we applied X-ray Absorption Spectroscopy (XAS) and selective leaching experiments for investigating iron speciation in different dust advections collected on different unwashed quartz fiber filters. XAS analysis evidenced a predominance of Fe(III) in 6-fold coordination for Saharan dust and a trend towards Fe(II) and 4-fold coordination in the order: Saharan dust, mixed Saharan, and non-Saharan aerosol samples. The role of the sampling substrate was evaluated explicitly, including in the analysis a set of blank filters. We were able to pinpoint the possible contribution to the overall XAS spectrum of the residual Fe on quartz as the concentration decrease towards the blank value. In particular, the filter substrate showed a negligible effect on the structural trend mentioned above. Furthermore, selective leaching experiments evidenced a predominance of the residual fraction on Fe speciation and indicated the lowest Fe concentrations for which the blank contribution is <20% are 1 μ g for the first three steps of the procedure (releasing the acid-labile, reducible and oxidizable phases, respectively) and 10 μ g for the last step (dissolving the insoluble residuals).

Highlights

The Sahara desert is a significant dust source, emitting 50% of global mineral dust [1]
Iron speciation is investigated in different dust advections collected at the Monte
The aerosol mass concentrations and the total iron concentrations decrease from pure Saharan dust to non-Saharan samples reflecting the decrease in Saharan dust contribution to the sampled aerosol

Summary

Introduction

The Sahara desert is a significant dust source, emitting 50% of global mineral dust [1]. Mineral dust sources supplies approximately 95% of the globally averaged iron budget [2,3] and. North African deserts are the main emitting areas Due to their proximity and to the preferred transport routes, Saharan dust is deposited in the Mediterranean basin [4] and in the North Atlantic ocean [5,6]. It is worth noting that dust ability to impact on phytoplankton activity is mainly linked to iron solubility and, in turn, to its bioaccessibility (see [9] for definition). Being the most abundant transition metal in atmospheric aerosols [10], iron solubility in aerosols is highly variable (0.05 to 80% [11]) and depends on the aerosol source and on atmospheric and cloud processing [12].

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Conclusion