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

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.

A review of coarse mineral dust in the Earth system

“…To improve on our understanding of how uptake coefficients evolve during dust atmospheric cycle, and to specifically constrain these coefficients for the coarse and super-coarse fractions which account for most of the available reactive surface, we adopt here Tang et al (2017)'s recommendation, and we further extend it. Tang et al (2017) and Urupina et al (2021) suggested the investigation of the reactivity of the authentic dust sources (or mineralogical equivalent soils), instead of those of individual minerals, to account for the right proportions and competitive effects of the different mineral phases. Indeed, as an example, Abou-Ghanem et al ( 2020) showed that natural TiO2 minerals do not behave like the commercially available TiO2 that was extensively used by Ndour et al (2009Ndour et al ( , 2008.…”

Surface Distribution of Sulfites and Sulfates on Natural Volcanic and Desert Dusts: Impact of Humidity and Chemical Composition

“…An increase in the sulfite surface concentration is observed as a function of Ti and Fe content. Formation of sulfites on the surface of pure oxides of titanium and iron was observed earlier . While studying natural samples, it is logical to try to look for the trends including the highest amount of elements as they are a better fit to represent a complex mixture.…”

“…As can be seen from Table , the composition of desert and volcanic dusts is dominated by silicon (Si) and aluminum (Al), followed by other elements, such as iron (Fe), calcium (Ca), magnesium (Mg), and Ti (titanium). The bulk composition of all of the discussed samples has been provided earlier …”

Photo-enhanced uptake of SO2 on Icelandic volcanic dusts

“…Maters et al measured the uptake of SO 2 on natural volcanic ash and synthetic volcanic glass samples under tropospherically relevant conditions of SO 2 concentration and temperature, but under dry conditions. 23 Recently, we have investigated for the first time the transformation of SO 2 on Icelandic v-dust; 24,25 we expand this work here by evaluating the uptake of SO 2 on v-dusts in the dark and under UV light irradiation. In particular, the aging of v-dusts by SO 2 is investigated under atmospherically relevant conditions, i.e.…”

“…Maters et al measured the uptake of SO 2 on natural volcanic ash and synthetic volcanic glass samples under tropospherically relevant conditions of SO 2 concentration and temperature but without relative humidity. 23 Recently, we have investigated for the first time the transformation of SO 2 on Icelandic v-dust; 24,25 we expand this work here by evaluating the uptake of SO 2 on vdusts in the dark and under UV light irradiation. In particular, the aging of v-dusts by SO 2 is investigated under atmospherically relevant conditions, i.e., at atmospheric pressure, an SO 2 concentration of 75 ppb, relative humidity (RH) in the 0-72% range, and in the dark and under different UV photon fluxes, performing long laboratory experiments over timescales of typically 24 hours.…”