Analysis of Mineral Aerosol in the Surface Layer over the Caspian Lowland Desert by the Data of 12 Summer Field Campaigns in 2002–2020

In-situ knowledge on characteristics of mineral aerosols is important for weather and climate prediction models, particularly for modeling such processes as the entrainment, transport and deposition of aerosols. However, field measurements of the dust emission flux, dust size distribution and its chemical composition under realistic wind conditions remain rare. In this study, we present experimental data over annual expeditions in the arid and semi-arid zones of the Caspian Lowland Desert (Kalmykia, south of Russia); we evaluate characteristics of mineral aerosol concentration and fluxes, estimate its chemical composition and calculate its long-distance transport characteristics. The mass concentration in different years ranges from several tens to several hundred of μg m–3. The significant influence of wind velocity on the value of mass and counting concentration and on the proposed entrainment mechanisms is confirmed. An increased content of anthropogenic elements (S, Sn, Pb, Bi, Mo, Ag, Cd, Hg, etc.), which is characteristic for all observation points in the south of the European Russia, is found. The trajectory analysis show that long-range air particles transport from the Caspian Lowland Desert to the central regions of European Russia tends to increase in the recent decades.

Strong links between Saharan dust fluxes, monsoon strength, and North Atlantic climate during the last 5000 years

Despite the multiple impacts of mineral aerosols on global and regional climate and the primary climatic control on atmospheric dust fluxes, dust-climate feedbacks remain poorly constrained, particularly at submillennial time scales, hampering regional and global climate models. We reconstruct Saharan dust fluxes over Western Europe for the last 5000 years, by means of speleothem strontium isotope ratios (87Sr/86Sr) and karst modeling. The record reveals a long-term increase in Saharan dust flux, consistent with progressive North Africa aridification and strengthening of Northern Hemisphere latitudinal climatic gradients. On shorter, centennial to millennial scales, it shows broad variations in dust fluxes, in tune with North Atlantic ocean-atmosphere patterns and with monsoonal variability. Dust fluxes rapidly increase before (and peaks at) Late Holocene multidecadal- to century-scale cold climate events, including those around 4200, 2800, and 1500 years before present, suggesting the operation of previously unknown strong dust-climate negative feedbacks preceding these episodes.

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

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 spectroscopic view of mineral aerosol surface aging under atmospheric conditions

<p>Atmospheric mineral aerosols have direct and indirect influence on the climate system. So far, atmospheric interactions studies have mainly focused on pristine samples, despite the fact that aerosol particles may age under natural atmospheric conditions. For example, multiple freeze-melt or evaporation-condensation cycles of an aerosol-containing cloud droplet can change the surface chemistry of the aerosol particle, the droplet ionic strength and pH. These changes have a large impact on the ice nucleation ability of the aerosol particles. We probe the water structure and surface chemistry at water-mineral interface using surface spectroscopic techniques, particularly supercooled nonlinear spectroscopy [1-4]. We found that successive freeze-melt cycles disrupt the dissolution equilibrium, substantially changing the surface chemistry, giving rise to variations ice nucleation ability of the surface [4]. Along the aging process, the restructuring of the water molecules at the surface upon cooling changes. This was found to be correlated to the ice nucleation ability of the surface. We present here a spectroscopic overview on aging of selected mineral surfaces (Al<sub>2</sub>O<sub>3</sub>, Silica, Mica and PbO). We found that the pH, ionic strength, time in contact with water and number of freezing-melting events influence the aging dynamics and hence the ice nucleation ability.</p><p> </p><ol><li>Abdelmonem, A., Direct Molecular-Level Characterization of Different Heterogeneous Freezing Modes on Mica – Part 1. Atmos. Chem. Phys., 2017. <strong>17</strong>(17): p. 10733-10741.</li> <li>Abdelmonem, A., et al., Surface-Charge-Induced Orientation of Interfacial Water Suppresses Heterogeneous Ice Nucleation on α-Alumina (0001). Atmos. Chem. Phys., 2017. <strong>17</strong>(12): p. 7827-7837.</li> <li>Lützenkirchen, J., et al., A set-up for simultaneous measurement of second harmonic generation and streaming potential and some test applications. Journal of Colloid and Interface Science, 2018. <strong>529</strong>: p. 294-305.</li> <li>Abdelmonem, A., et al., Cloud history changes water-ice-surface interactions of oxide mineral aerosols (e.g. Silica). Atmos. Chem. Phys. Discuss., 2019. <strong>2019</strong>: p. 1-17.</li> </ol><p> </p>

Cloud history can change water–ice–surface interactions of oxide mineral aerosols: a case study on silica

Mineral aerosol particles nucleate ice, and many insights have been obtained on water freezing as a function of mineral surface properties such as charge or morphology. Previous studies have mainly focused on pristine samples despite the fact that aerosol particles age under natural atmospheric conditions. For example, an aerosol-containing cloud droplet can go through freeze–melt or evaporation–condensation cycles that change the surface structure, the ionic strength, and pH. Variations in the surface properties of ice-nucleating particles in the atmosphere have been largely overlooked. Here, we use an environmental cell in conjunction with nonlinear spectroscopy (second-harmonic generation) to study the effect of freeze–melt processes on the aqueous chemistry at silica surfaces at low pH. We found that successive freeze–melt cycles disrupt the dissolution equilibrium, substantially changing the surface properties and giving rise to marked variations in the interfacial water structure and the ice nucleation ability of the surface. The degree of order of water molecules, next to the surface, at any temperature during cooling decreases and then increases again with sample aging. Along the aging process, the water ordering–cooling dependence and ice nucleation ability improve continuously.

Enhanced heterogeneous uptake of sulfur dioxide on mineral particles through modification of iron speciation during simulated cloud processing

Iron-containing mineral aerosols play a key role in the oxidation of sulfur species in the atmosphere. Simulated cloud processing (CP) of typical mineral particles, such as illite (IMt-2), nontronite (NAu-2), smectite (SWy-2) and Arizona Test Dust (ATD) is shown here to modify sulfur dioxide (SO2) uptake onto mineral surfaces. Heterogeneous oxidation of SO2 on particle surfaces was firstly investigated using an in situ DRIFTS apparatus (diffuse reflectance infrared Fourier transform spectroscopy). Our results showed that the Brunauer–Emmett–Teller (BET) surface area normalized uptake coefficients (γBET) of SO2 on the IMt-2, NAu-2, SWy-2 and ATD samples after CP were 2.2, 4.1, 1.5 and 1.4 times higher than the corresponding ones before CP, respectively. The DRIFTS results suggested that CP increased the amounts of reactive sites (e.g., surface OH groups) on the particle surfaces and thus enhanced the uptake of SO2. Transmission electron microscopy (TEM) showed that the particles broke up into smaller pieces after CP, and thus produced more active sites. The “free-Fe” measurements confirmed that more reactive Fe species were present after CP, which could enhance the SO2 uptake more effectively. Mössbauer spectroscopy further revealed that the formed Fe phases were amorphous Fe(III) and nanosized ferrihydrite hybridized with Al ∕ Si, which were possibly transformed from the Fe in the aluminosilicate lattice. The modification of Fe speciation was driven by the pH-dependent fluctuation coupling with Fe dissolution–precipitation cycles repeatedly during the experiment. Considering both the enhanced SO2 uptake and subsequent promotion of iron dissolution along with more active Fe formation, which in turn led to more SO2 uptake, it was proposed that there may be a positive feedback between SO2 uptake and iron mobilized on particle surfaces during CP, thereby affecting climate and biogeochemical cycles. This self-amplifying mechanism generated on the particle surfaces may also serve as the basis of high sulfate loading in severe fog–haze events observed recently in China.

Cloud history changes water–ice–surface interactions of oxide mineral aerosols (e.g. Silica)

Mineral aerosol particles can act as ice nucleators, and many insights have been obtained on water freezing as a function of mineral surface properties such as the charge or morphology. Previous studies have mainly focused on pristine samples, despite the fact that under natural atmospheric conditions, aerosol particles age. For example, an aerosol-containing cloud droplet can go through different freeze-melt cycles, so that not only the aerosol surface structure may change, but also the ionic strength and pH of the cloud droplet. The potential variation of the surface properties of an ice nucleating particle during its residence in the atmosphere has been largely overlooked. Here, we use an environmental cell in conjunction with nonlinear spectroscopy (second-harmonic generation) to study the effect of freeze-melt processes on aqueous chemistry at silica surface at low pH. We found that the successive freeze-melt cycles disrupt the dissolution equilibrium, substantially changing the surface properties, giving rise to marked variations in the interfacial water structure and the ice nucleation ability of the surface. The degree-of-order of water molecules, next to the surface at a specific temperature, decreases and then increases again with sample aging. The water ordering–cooling dependence and ice nucleation ability improve continuously.