Iberulite fall and formation mechanism during a Sahara dust event in Switzerland in February 2021

Abstract. Sahara dust events can promote the formation of micrometer‑sized spherical mineral aggregates known as iberulites as they are usually found on the Iberian Peninsula. Iberulites have not, to our knowledge, been reported from Central Europe. Two formation mechanisms – in- and below scavenging – have been given. Strong iberulite fall (IFs) were observed in Western Switzerland in February 2021 when large amounts of dust were transported from the Moroccan-Algerian border towards Central Europe during a Saharan Dust Event (SDE). In contrast to the previous IFs observed on the Iberian Peninsula, this IF occurred under low-temperature conditions e.g. near the freezing point in the cloud and at the surface. The relative humidity in the first 1000 m below the clouds decreased from 70 to 40 %. and the comparison between particle-size distribution (PSD) within the iberulites and the dust PSD in the free atmosphere revealed a Greenfield gaprelative to the dust in the free atmosphere. The meteorological conditions, the microstructure of the iberulites, and the presence of a Greenfield gap point to below-cloud scavenging as the most likely formation mechanism. Further discoveries of iberulite in Swiss SDE samples suggest that the phenomenon is widespread and not only limited to regions close to the dust source.