Sahara dust event of 06.02.21 in Switzerland: Iberulite fall and formation mechanism

During the Saharan Dust Event (SDE) in February 2021, dust clouds were transported from the Moroccan-Algerian border to Central Europe. In Western Switzerland, large particles up to 300 µm in size falling to the ground were observed in situ. The large "particles" were multimineral spherical aggregates, termed iberulites (IF) after their first recording on the Iberian Peninsula (Díaz-Hernández et al., 2008). The cores contain coarser grains, lacking a visible cement matrix, and have a thin, dense surface layer of much smaller particles. The Particle Size Distribution (PSD) inside the iberulites is ±monomodal, with a maximum at 2.5 µm, i.e., much larger than the value, i.e., 0.2 µm, for Saharan dust sampled at the JungFrauJoch (JFJ) station in the main dust layer. The PSD of the particles inside the iberulites shows a minimum where the JFJ has a maximum, i.e., between 0.1 and 1.0 µm. 

The atmospheric conditions during the IF were well documented (meteorological station in Payerne close to Frribourg!). In contrast to the previous IFs observed in the Iberian Peninsula, this IF occurred under lower-temperature conditions, e.g., near the freezing point in the cloud and at the surface. Two mechanisms have been envisaged for aggregating a large number of dust particles  (Díaz-Hernández et al., 2008) 1. The coalescence of drops within a cloud increases the number of particles within a single growing drop (In-Cloud Scavenging, ICS), or 2. the particle concentration increases by collisions of drops with the latter below the cloud (Below-Cloud Scavenging, BCS). The BCS rate (= collection efficiency, CE) depends on particle size (Slinn, 1977).

Particles >1µm will be included by impaction, and CE is taken as 100%. However, for particles with radii between 0.3µm and 1µm, the CE is <<1. Particles with radii within the size span given above, despite being on collision trajectories, follow the flow lines and are sent around the latter, whereas very small particles (< 0.1µm) may be pushed by Brownian motion and deposited on the droplet's rear end (Brownian capture), and CE is also close to 1. For particles with radii between 0.3 and 1.0 µm, CE decreases by two orders of magnitude. This decrease in CE was first described by Greenfield(Greenfield, 1957) and is therefore referred to as Greenfield gap. The temperature at the upper boundary of the dust layer was below 0°C, and scavenging occurred by frozen hydrometeors, which are known to be better scavengers of aerosol particles than rain. The presence of a Greenfield gap in the iberulites collected in western Switzerland indicates that below-cloud scavenging is the probable formation mechanism

Díaz-Hernández, J. L. and Párraga, J., 2008., Geochimica et Cosmochimica Acta, 72, 3883–3906

Greenfield, S. M.,1957, Journal of Meteorology, 14, 115–125