07 Nov 2022
07 Nov 2022
Status: this preprint is currently under review for the journal ACP.

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 Panta1, Konrad Kandler1, Andres Alastuey2, Cristina González-Flórez3, Adolfo González-Romero2,3, Martina Klose4, Xavier Querol2, Cristina Reche2, Jesús Yus-Díez2,5, and Carlos Pérez García-Pando3,6 Agnesh Panta et al.
  • 1Atmospheric Aerosol Group, Institute of Applied Geosciences, Technical University of Darmstadt, Darmstadt, Germany
  • 2Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, 08034, Spain
  • 3Barcelona Supercomputing Center, Barcelona, Spain
  • 4Department Troposphere Research, Institute of Meteorology and Climate Research (IMK-TRO), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 5Grup de Meteorologia, Departament de Física Aplicada, Universitat de Barcelona, C/Martí i Franquès, 1, 08028, Barcelona, Spain
  • 6ICREA, Catalan Institution for Research and Advanced Studies, Barcelona, Spain

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 where clay minerals make up the majority of the analyzed particles by number, with carbonates and quartz contributing to a lesser extent. We provide an exhaustive analysis of the size distribution and potential mixing state of different particle types, focusing largely on iron-rich (Fe-oxi/hydroxides) and feldspar particles, which are key to the effects of dust upon radiation and clouds. Nearly pure or externally mixed Fe-oxi/hydroxides are present only in diameters smaller than 2 µm and mainly below 1 µm. Fe-oxi/hydroxides tend to be increasingly internally mixed with other minerals, especially clays, as particle size increases, i.e., the volume fraction of Fe-oxi/hydroxides in aggregates decreases with particle size. Pure (externally-mixed) feldspar grains represented 3.7 % of all the particles, of which we estimated about a quarter 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. 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 along with the 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.

Agnesh Panta et al.

Status: open (until 19 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-742', Anonymous Referee #1, 10 Dec 2022 reply

Agnesh Panta et al.

Agnesh Panta et al.


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Short summary
Desert dust is a major aerosol component of the Earth's system and affects climate. Dust properties are influenced by particle size, mineralogy, shape, and mixing state. This work characterizes freshly emitted individual mineral dust particles from a major source region using electron microscopy. Our new insights on the critical particle-specific information will contribute to better constraining climate models that consider mineralogical variations in their representation of the dust cycle.