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. The mineralogy and mixing state of dust particles originating from theAfrican continent influences climate and marine ecosystems in the NorthAtlantic due to its effect on radiation, cloud properties and biogeochemicalcycling. However, these processes are difficult to constrain because of largetemporal and spatial variability, and the lack of in situ measurements ofdust properties at all stages of the dust cycle. This lack of measurements isin part due to the remoteness of potential source areas (PSAs) and transportpathways but also because of the lack of an efficient method to report themineralogy and mixing state of single particles with a time resolutioncomparable to atmospheric processes, which may last a few hours or less.Measurements are equally challenging in laboratory simulations where dustparticles need to be isolated and characterised in low numbers whilstconditions are dynamically controlled and monitored in real time. This isparticularly important in controlled expansion cloud chambers (CECCs) whereice-nucleating properties of suspended dust samples are studied in cold andmixed phase cloud conditions. In this work, the mineralogy and mixing state of the fine fraction ( µm )in laboratory-suspended dust from PSAs in north Africa were made usingnovel techniques with online single-particle mass spectrometry (SPMS) andtraditional offline scanning electron microscopy (SEM). A regionaldifference in mineralogy was detected, with material sourced from Moroccocontaining a high number fraction of illite-like particles in contrast toSahelian material which contains potassium- and sodium-depleted clay mineralslike kaolinite. Single-particle mixing state had a much greater localvariation than mineralogy, particularly with respect to organic–biologicalcontent. Applying the same methods to ambient measurement of transported dustin the marine boundary layer at Cabo Verde in the remote North Atlanticenabled the number fractions of illite/smectite clay mineral (ISCM),non-ISCM and calcium-containing particles to be reported at a 1  h timeresolution over a 20-day period. Internal mixing of silicate particles withnitrate, chlorine and organic–biological material was also measured andcompared to that in the suspended soils. The results show SPMS and SEM techniques are complementary and demonstratethat SPMS can provide a meaningful high-resolution measurement ofsingle-particle mineralogy and mixing state in laboratory and ambientconditions. In most cases, the differences in the mineralogical compositionbetween particles within a soil sample were small. Thus, particles were notcomposed of discrete mineral phases. In ambient measurements, the ISCM andnitrate content was found to change significantly between distinct dustevents, indicating a shift in source and transport pathways which may not becaptured in offline composition analysis or remote sensing techniques.

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