How Relevant Is It to Use Mineral Proxies to Mimic the Atmospheric Reactivity of Natural Dust Samples? A Reactivity Study Using SO2 as Probe Molecule

Darya Urupina,Frederic Thevenet,Manolis N Romanias

doi:10.3390/min11030282

Abstract

The experimental investigation of heterogeneous atmospheric processes involving mineral aerosols is extensively performed in the literature using proxy materials. In this work we questioned the validity of using proxies such as Fe2O3, FeOOH, Al2O3, MgO, CaO, TiO2, MnO2, SiO2, and CaCO3 to represent the behavior of complex mixtures of minerals, such as natural desert and volcanic dusts. Five volcanic dusts and three desert dusts were compared to a number of metal oxides, commonly used in the literature to mimic the behavior of desert dusts in the ability to form sulfites and sulfates on the surface exposed to SO2 gas. First, all samples were aged at room temperature, atmospheric pressure, under controlled experimental conditions of 175 ppm SO2 for 1 h under 30% of relative humidity. Second, they were extracted with 1% formalin and analyzed by High-Performance Liquid Chromatography (HPLC) to quantify and compare the amount of sulfites and sulfates formed on their surfaces. It was evidenced that under the experimental conditions of this study neither one selected pure oxide nor a mixture of oxides can adequately typify the behavior of complex mixtures of natural minerals. Therefore, to evaluate the real-life impact of natural dust on atmospheric processes it is of vital importance to work directly with the natural samples, both to observe the real effects of desert and volcanic dusts and to evaluate the relevancy of proposed proxies.

Highlights

Airborne mineral particulate of desert and volcanic origin represents an important component of the atmosphere
Calcium carbonate and goethite are not observed in volcanic ash but they are found in desert dusts and discussed in the literature; they were included to better explain the behavior of natural samples [56]
Three clay minerals show contrasting behavior: sulfites are only observed on the surface of illite, while sulfates are observed for kaolinite and illite, but not for montmorillonite

Summary

Introduction

Airborne mineral particulate of desert and volcanic origin represents an important component of the atmosphere. From 1000 to 3000 million tons of soil dust are mobilized by high winds in the arid and semiarid regions of the world, such as Gobi and Sahara [1]. These aerosols can travel thousands of kilometers, crossing oceans and continents. At the average of 33 million tons per year, volcanic particles represent an additional, seemingly minor source of mineral aerosol; high variability of volcanic eruptions can increase the contribution to 10,000 million tons on a year of high volcanic activity [3]. As an example, during the 2010 Eyjafjallajökull eruption, 90 μm particles were found as far as 1300 km from the vent [8]

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Conclusion