Mineralogy and mixing state of north African mineral dust by online single-particle mass spectrometry

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Marsden, Nicholas, Coe, Hugh, Allan, James, Williams, Paul, Liu, Dantong
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2019
• Subjects: North Atlantic
• Online Access: https://research.manchester.ac.uk/en/publications/12c036d5-57da-4070-821e-fc7d13f2269b; https://doi.org/10.5194/acp-19-2259-2019

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. However, these processes are difficult to constrain because of large temporal and spatial variability, and the lack of in situ measurements of dust properties at all stages of the dust cycle. This lack of measurements is in part due to the remoteness of potential source areas (PSAs) 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, which may last a few hours or less. Measurements are equally challenging in laboratory simulations where dust particles need to be isolated and characterised in low numbers whilst conditions are dynamically controlled and monitored in real time. This is particularly important in controlled expansion cloud chambers (CECCs) where ice-nucleating properties of suspended dust samples are studied in cold and mixed phase cloud conditions. In this work, the mineralogy and mixing state of the fine fraction ( < 2:5 μm) in laboratory-suspended dust from PSAs in north Africa were made using novel techniques with online single-particle mass spectrometry (SPMS) and traditional offline scanning electron microscopy (SEM). A regional difference in mineralogy was detected, with material sourced from Morocco containing a high number fraction of illitelike particles in contrast to Sahelian material which contains potassium- and sodium-depleted clay minerals like kaolinite. Single-particle mixing state had a much greater local variation than mineralogy, particularly with respect to organic– biological content. 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 ...