TEM analysis of the internal structures and mineralogy of Asian dust particles and the implications for optical modeling
Mineral dust interacts with incoming/outgoing electromagnetic radiation in the atmosphere. This interaction depends on the microphysical properties of the dust particles, including size, mineral composition, external morphology, and internal structure. Ideally all of these properties should be accounted for in the remote sensing of dust, the modeling of single-scattering properties, and radiative effect assessment. There have been many reports on the microphysical characterizations of mineral dust, but no investigations of the internal structures of individual dust particles. We explored the interiors of Asian dust particles using the combined application of focused ion beam thin-slice preparation and high-resolution transmission electron microscopy. The results showed that individual dust particles consisted of numerous mineral grains, which were organized into several types of internal structure: single and polycrystalline cores of quartz, feldspars, calcite, and amphibole often with oriented clay coatings; individual clay agglomerates of nano-thin clay platelets showing preferred to random orientations common with coarser mineral inclusions; and platy coarse phyllosilicates (muscovite, biotite, and chlorite). Micron to submicron pores were scattered throughout the interior of particles. Clays in the coatings and agglomerates were dominated by nano-thin platelets of the clay minerals of illite–smectite series including illite, smectite, and their mixed layers with subordinate kaolinite and clay-sized chlorite. Submicron iron oxide grains, dominantly goethite, were distributed throughout the clay agglomerates and coatings. Unlike the common assumptions and simplifications, we found that the analyzed dust particles were irregularly shaped with birefringent, polycrystalline, and polymineralic heterogeneous compositions. Accounting for this structural and mineralogical makeup may improve the remote sensing retrieval of dust and the evaluation of radiation effects, but will also require sophisticated single-scattering modeling. In particular, the observed internal structures of dust particles such as clay coatings, preferred orientation, embedded grains in clays, and pores, have the potential to considerably impact on the light scattering by dust particles. The distribution and size of structural components with contrasting dielectric properties, such as iron oxides, should also be explicitly accounted for.