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Insights into the single-particle composition, size, mixing state, and aspect ratio of freshly emitted mineral dust from field measurements in the Moroccan Sahara using electron microscopy
Agnesh Panta, Konrad Kandler, Andrés Alastuey, Cristina González-Flórez, Adolfo Gonzalez-Romero, Martina Klose, Xavier Querol, Cristina Reche, Jesús Yus-Díez, Carlos Pérez García-Pando, 2023, original scientific article

Abstract: Abstract. The chemical and morphological properties of mineral dust aerosols emitted by wind erosion from arid and semi-arid regions influence climate, ocean, and land ecosystems; air quality; and multiple socio-economic sectors. However, there is an incomplete understanding of the emitted dust particle size distribution (PSD) in terms of its constituent minerals that typically result from the fragmentation of soil aggregates during wind erosion. The emitted dust PSD affects the duration of particle transport and thus each mineral's global distribution, along with its specific effect upon climate. This lack of understanding is largely due to the scarcity of relevant in situ measurements in dust sources. To advance our understanding of the physicochemical properties of the emitted dust PSD, we present insights into the elemental composition and morphology of individual dust particles collected during the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) field campaign in the Moroccan Sahara in September 2019. We analyzed more than 300 000 freshly emitted individual particles by performing offline analysis in the laboratory using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectrometry (EDX). Eight major particle-type classes were identified with clay minerals making up the majority of the analyzed particles both by number and mass, followed by quartz, whereas carbonates and feldspar contributed to a lesser extent. We provide an exhaustive analysis of the PSD and potential mixing state of different particle types, focusing largely on iron-rich (Fe oxide-hydroxides) and feldspar particles, which are key to the effects of dust upon radiation and clouds, respectively. Nearly pure or externally mixed Fe oxide-hydroxides are present mostly in diameters smaller than 2 µm, with the highest fraction below 1 µm at about 3.75 % abundance by mass. Fe oxide-hydroxides tend to be increasingly internally mixed with other minerals, especially clays, as particle size increases; i.e., the volume fraction of Fe oxide-hydroxides in aggregates decreases with particle size. Pure (externally mixed) feldspar represented 3.2 % of all the particles by mass, of which we estimated about a 10th to be K-feldspar. The externally mixed total feldspar and K-feldspar abundances are relatively invariant with particle size, in contrast to the increasing abundance of feldspar-like (internally mixed) aggregates with particle size with mass fractions ranging from 5 % to 18 %. We also found that overall the median aspect ratio is rather constant across particle size and mineral groups, although we obtain slightly higher aspect ratios for internally mixed particles. The detailed information on the composition of freshly emitted individual dust particles and quantitative analysis of their mixing state presented here can be used to constrain climate models including mineral species in their representation of the dust cycle.
Keywords: mineral dust, Moroccan Sahara, electron microscopy
Published in RUNG: 13.05.2024; Views: 324; Downloads: 3
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Combined fit to the spectrum and composition data measured by the Pierre Auger Observatory including magnetic horizon effects
Juan Manuel González, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution

Abstract: The measurements by the Pierre Auger Observatory of the energy spectrum and mass composition of cosmic rays can be interpreted assuming the presence of two extragalactic source populations, one dominating the flux at energies above a few EeV and the other below. To fit the data ignoring magnetic field effects, the high-energy population needs to accelerate a mixture of nuclei with very hard spectra, at odds with the approximate �−2 shape expected from diffusive shock acceleration. The presence of turbulent extragalactic magnetic fields in the region between the closest sources and the Earth can significantly modify the observed CR spectrum with respect to that emitted by the sources, reducing the flux of low-rigidity particles that reach the Earth. We here take into account this magnetic horizon effect in the combined fit of the spectrum and shower depth distributions, exploring the possibility that a spectrum for the high-energy population sources with a shape closer to �^(−2) be able to explain the observations. We find that a large inter-source separation �s and a large magnetic field RMS amplitude within the Local Supercluster region, such that �rms ≃ 100 nG (40 Mpc/�s) √︁25 kpc/�coh, are needed to interpret the data within this scenario, where �coh is the magnetic field coherence length.
Keywords: ultra-high energy cosmic rays, Pierre Auger Observatory, mass composition, extragalactic magnetic fiels
Published in RUNG: 22.01.2024; Views: 579; Downloads: 6
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Variability in sediment particle size, mineralogy, and Fe mode of occurrence across dust-source inland drainage basins : the case of the lower Drâa Valley, Morocco
Adolfo Gonzalez-Romero, Cristina González-Flórez, Agnesh Panta, Jesús Yus-Díez, Cristina Reche, Patricia Córdoba, Natalia Moreno, Andrés Alastuey, Konrad Kandler, Martina Klose, 2023, original scientific article

Abstract: The effects of desert dust upon climate and ecosystems depend strongly on its particle size and size-resolved mineralogical composition. However, there is very limited quantitative knowledge on the particle size and composition of the parent sediments along with their variability within dust-source regions, particularly in dust emission hotspots. The lower Drâa Valley, an inland drainage basin and dust hotspot region located in the Moroccan Sahara, was chosen for a comprehensive analysis of sediment particle size and mineralogy. Different sediment type samples (n= 42) were collected, including paleo-sediments, paved surfaces, crusts, and dunes, and analysed for particle-size distribution (minimally and fully dispersed samples) and mineralogy. Furthermore, Fe sequential wet extraction was carried out to characterise the modes of occurrence of Fe, including Fe in Fe (oxyhydr)oxides, mainly from goethite and hematite, which are key to dust radiative effects; the poorly crystalline pool of Fe (readily exchangeable ionic Fe and Fe in nano-Fe oxides), relevant to dust impacts upon ocean biogeochemistry; and structural Fe. Results yield a conceptual model where both particle size and mineralogy are segregated by transport and deposition of sediments during runoff of water across the basin and by the precipitation of salts, which causes a sedimentary fractionation. The proportion of coarser particles enriched in quartz is higher in the highlands, while that of finer particles rich in clay, carbonates, and Fe oxides is higher in the lowland dust emission hotspots. There, when water ponds and evaporates, secondary carbonates and salts precipitate, and the clays are enriched in readily exchangeable ionic Fe, due to sorption of dissolved Fe by illite. The results differ from currently available mineralogical atlases and highlight the need for observationally constrained global high-resolution mineralogical data for mineral-speciated dust modelling. The dataset obtained represents an important resource for future evaluation of surface mineralogy retrievals from spaceborne spectroscopy.
Keywords: mineral dust, aerosols, geology
Published in RUNG: 12.01.2024; Views: 886; Downloads: 3
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Insights into the size-resolved dust emission from field measurements in the Moroccan Sahara
Cristina González-Flórez, Martina Klose, Andrés Alastuey, Sylvain Dupont, Jerónimo Escribano, Vicken Etyemezian, Adolfo Gonzalez-Romero, Yue Huang, Konrad Kandler, Jesús Yus-Díez, 2023, original scientific article

Abstract: Atmospheric mineral dust consists of tiny mineral particles that are emitted by wind erosion from arid regions. Its particle size distribution (PSD) affects its impact on the Earth's system. Nowadays, there is an incomplete understanding of the emitted dust PSD and a lot of debate about its variability. Here, we try to address these issues based on the measurements performed during a wind erosion and dust emission field campaign in the Moroccan Sahara within the framework of FRAGMENT project.
Keywords: aerosol particles, mineral dust, emission processes, climate
Published in RUNG: 23.10.2023; Views: 1129; Downloads: 5
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Search for primary photons at tens of PeV with the Pierre Auger Observatory
Nicolás González, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution

Abstract: The observation of primary photons with energies around 10[sup]16 eV would be particularly interesting after the discovery of Galactic gamma-ray sources with spectra extending into the PeV range. Since photons are connected to the acceleration of charged particles, searches for photons enhance the multi-messenger understanding of cosmic-ray sources as well as of transient astrophysical phenomena, while offering wealthy connections to neutrino astronomy and dark matter. Additionally, diffuse photon fluxes are expected from cosmic-ray interactions with Galactic matter and background radiation fields. Previously, the energy domain between 1 PeV and 200 PeV was only explored from the Northern Hemisphere. The Pierre Auger Observatory is the largest astroparticle experiment in operation and, thanks to its location, has a sizable exposure to the Southern sky, including the Galactic center region. In this contribution, we present the first search for photons from the Southern hemisphere between 50 and 200 PeV exploiting the Auger data acquired during ∼4 yr of operation. We describe the method to discriminate photons against the dominating hadronic background; it is based on the measurements of air showers taken with the low-energy extension of the Pierre Auger Observatory composed by 19 water-Cherenkov detectors spanning ∼ 2km[sup]2 and an Underground Muon Detector. The search for a diffuse flux of photons is presented and its results are interpreted according to theoretical model predictions. This study extends the range of Auger photon searches to almost four decades in energy.
Keywords: Pierre Auger Observatory, cosmic rays, photons, water-Cherenkov detectors, underground muon detectors
Published in RUNG: 26.09.2023; Views: 956; Downloads: 6
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Ultra-fast silicon detectors for CERN’S high-luminosity large hadron collider : written report
Adrián González Briones, 2022, research project (high school)

Keywords: standard silicon detectors, ionized particles, hadrontherapy, particle physics
Published in RUNG: 15.06.2022; Views: 1565; Downloads: 0
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