Mineralogy and mixing state of north African mineral dust by online single-particle mass spectrometry
Authors: Marsden Nicholas A. , Ullrich Romy , Möhler Ottmar , Eriksen Hammer Stine , Kandler Konrad , Cui Zhiqiang , Williams Paul I. , Flynn Michael J. , Liu Dantong , Allan James D. , Coe Hugh 
Published: February 2019

Abstract. 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 illite-like 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 calcium-containing particles to be reported at a 1 h time
resolution over a 20-day period. Internal mixing of silicate particles with
nitrate, chlorine and organic–biological material was also measured and
compared to that in the suspended soils. The results show SPMS and SEM techniques are complementary 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 differences in the mineralogical composition
between particles within a soil sample were small. Thus, particles were not
composed of discrete mineral phases. In ambient measurements, 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 offline composition analysis or remote sensing techniques.