Mineralogy and mixing state of North African mineral dust by on-line single-particle mass spectrometry

Nicholas A. Marsden, Romy Ullrich, Ottmar Möhler, Stine Eriksen Hammer, Konrad Kandler, Zhiqiang Cui, Paul I. Williams, Michael J. Flynn, Dantong Liu, James D. Allan, Hugh Coe
2018 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> The mineralogy and mixing state of dust particles originating from the African continent influences climate and marine ecosystems in the North Atlantic due to its effect on radiation, cloud properties and biogeochemical cycling. Single-particle mineralogy and mixing state is particularly important in many processes but is difficult to predict because of large temporal and spatial variability and the lack of in-situ measurements of dust properties during emission,
more » ... during emission, transport and deposition. This lack of measurements is in part due to the remoteness of potential source areas (PSA) and transport pathways, but also because of the lack of an efficient method to report the mineralogy and mixing state of single particles with a time resolution comparable to atmospheric processes.</p> <p>In this work, the mineralogy and mixing state of the fine fraction (<<span class="thinspace"></span>2.5<span class="thinspace"></span>&amp;mu;m) in laboratory suspended dust from the Sahara and Sahel were made using novel techniques with on-line single-particle mass spectrometry (SPMS) and traditional off-line scanning electron microscopy (SEM). A regional difference in mineralogy was detected, with material sourced from Morocco contained a high number fraction of illite like particles in contrast to Sahelian material which contains potassium and sodium depleted clay minerals like kaolinite. Applying the same methods to ambient measurement of transported dust in the marine boundary layer at Cabo Verde in the remote North Atlantic enabled the number fractions of illite/smectite clay mineral (ISCM), non-ISCM, and calcium containing particles to be reported at a 1 hour time resolution over a 20 day period alongside internal mixing with nitrate, sulphate and organic/biological material. The ISCM and nitrate content was found to change significantly between distinct dust events, indicating a shift in source and transport pathways which may not be captured in off-line composition analysis or remote sensing techniques.</p> <p>The results show SPMS and SEM techniques are complimentary and demonstrate that SPMS can provide a meaningful high resolution measurement of single-particle mineralogy and mixing state in laboratory and ambient conditions. In most cases, the mineralogy varies continuously between particles rather than a collection of discrete mineral phases. These techniques will be useful in resolving the complexity of mineral dust transport and in obtaining atmospherically relevant test material for laboratory experiments of dust properties.</p>
doi:10.5194/acp-2018-725 fatcat:j5iondmrovadllyee2p3yv4zc4