title: Characterisation of freshly emitted mineral dust aerosols determined by scanning electron microscopy: insights from in situ measurements at source locations
Creators: name: Panta, Agnesh
Publisher: name: UNSPECIFIED
Year: 2023

description: Mineral dust is a key component of the Earth system, affecting the radiation balance, cloud properties, biogeochemical cycles and impacting atmospheric circulation, air quality, aviation, and solar energy generation. Understanding of these processes is currently limited by a lack of in situ measurements resulting in a large uncertainty in the chemical, microphysical, optical, and radiative properties of mineral dust. This thesis presents an analysis of dust measurements obtained during the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) campaigns which took place in August-September 2019 over Southern Morocco and August-September 2021 over Iceland. Samples are collected for offline analysis using SEM/EDX and are used to provide insights into the chemical composition, particle size distribution (PSD), mixing state, and the shape and morphology of airborne dust. It focuses on the description and comparison of dust samples from northern Africa (Morocco) and high-latitude region (Iceland) which are the two distinct source regions. Moroccan dust particles are mineralogically dominated by (alumino) silicates such as clay minerals, quartz, and feldspar with varying amounts of carbonates, and iron-bearing minerals. An exhaustive analysis of the PSD and potential mixing state of different particle types is made, 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 oxidehydroxides 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 an estimation of about a 10th is 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 %. The median aspect ratio is rather constant across particle size and mineral groups, although slightly higher aspect ratios for internally mixed particles is observed. Icelandic dust particles are heterogeneous in size, shape, and elemental composition, with particle group medium aluminium silicates (most probably glass-like) making up the largest fraction across all size ranges (35 % - 93 % by volume). Sulfate particles were found on few of the samples, suggesting volcanic emission contribution to the aerosol load. Fe-rich and Ti-rich particles represented 3.5 % and 1.8 % respectively of the particle volume and were present mainly in the fine fraction. Particle median aspect ratio is observed to be size dependent and increases with particle size. The overall composition was consistent between the main studied area and the regional outflow areas except for one specific location which was rather less affected by the local emission. This work highlights that at an individual particle level Moroccan (Saharan) dust and Icelandic dust have some key differences. Icelandic dust shows elevated levels of Fe and Ca and slightly reduced Al compared to Moroccan dust. A major difference was the almost absence of K in Icelandic dust particles, whereas it is detected in the Moroccan dust particles. Further, the Fe contribution to the single particles was observed to be higher compared to the Moroccan dust with median Fe index of 0.1 for Icelandic dust and 0.06 for Moroccan dust. The median aspect ratio of the particles varied between 1.34 to 1.67, increasing as particle size increased. This is in contrast with Moroccan dust where the median aspect ratio was 1.46 and showed little dependence with size. However, the overall shape distribution is observed to be rather similar between Icelandic and Saharan dust. Single-particle analysis in this work shows that dust particles are mostly internally mixed, predominantly aspherical, and morphologically complex. Many Earth system models still assume globally uniform mineralogical composition or oversimplified compositional and physical parameters and therefore introduce errors in the assessment of regional forcing of dust. The detailed information on the chemical composition and morphology 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.