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

Abstract

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.

Composition and origin of PM<sub>2.5</sub> aerosol particles in the upper Rhine valley in summer

“…These instruments were used to provide real time information on size and chemical composition for individual particles and bulk samples, respectively. The LAAPTOF is a commercially available single particle mass spectrometer and has been well described in recent publications (Ahern et al, 2016;Gemayel et al, 2016;Gemayel et al, 2017;Marsden et al, 2018;Marsden et al, 2016;Marsden et al, 2019;Ramisetty et al, 2018;Reitz et al, 2016;Shen et al, 2018;Shen et al, 2019). In brief, aerosol particles with a size range of 70 nm to 2.5 m vacuum aerodynamic diameter (dva) are sampled with a flowrate of ~80 cm 3 min -1 , focused and accelerated via an aerodynamic lens (ADL).…”

Quantifying bioaerosol concentrations in dust clouds through online UV-LIF and mass spectrometry measurements at the Cape Verde Atmospheric Observatory

“…The transmission efficiency of the inlet as a function of size has also been accounted when considering size distributions for particles up to 10 µm. Inlet penetration as a function of size has also been considered based on the Andersen 321A PM 10 inlet (McFarland, 1984).…”

Quantifying Bioaerosol Concentrations in Dust Clouds through Online UV-LIF and Mass Spectrometry Measurements at the Cape Verde Atmospheric Observatory

“…Analysis of the LAAP-ToF data acquired during the ICE-D intensive campaign has been presented by Marsden et al (2019), in which the mineralogy and mixing state of North African dust transported to the Cape Verde Islands is described in detail with reference to dust samples collected on the ground from potential source areas in North Africa. Using a combination of cluster analysis and sub-compositional analysis, that work showed that differences in mineralogical composition of ground https://doi.org/10.5194/acp-2020-157 Preprint.…”