Meteorological data were collected from radiosonde balloons launched by the Federal Office for Meteorology and Climatology of Switzerland (MeteoSwiss) station in Payerne (46.817° N, 6.933° E; Fig. 1) twice daily at 11:00 and 23:00 UTC. Payerne is located 17 km west and 47 km north of the collection sites in Fribourg and Saint-Légier, respectively. The station is equipped with a Raman lidar, ceilometers, and a Doppler radar to measure precipitation, aerosol concentrations, and wind profiles. On 6 February, the ceilometer profile series from Payerne showed gaps in the morning and smaller gaps in the afternoon due to ground fog. The area is also covered by two weather radars: one on La Dole Mountain (46.425° N, 6.099° E), west of Saint-Légier and near Geneva, and the other on Plaine Morte (46.383° N, 7.500° E; Fig. 1), in the central Bernese Alps southeast of the two collection sites.

The size distributions of fine- and coarse-mode aerosols in the dust cloud were measured at the high-altitude alpine station Jungfraujoch (JFJ, 7.98° N, 46.55° E; 3580 m a.s.l.; Fig. 1) in the Bernese Alps. Its remote location, far from local human sources, makes it ideal for monitoring background atmospheric conditions. Jungfraujoch lab hosts a comprehensive long-term in situ monitoring program for aerosols and trace gases as part of the World Meteorological Organization (WMO) and is included in the Swiss National Air Pollution Monitoring Network (Hüglin and Grange, 2021). Aerosol measurements were conducted using a custom-built scanning mobility particle size spectrometer (SMPS) (Jurányi et al., 2011; Wiedensohler et al., 2012) and a white-light optical particle size spectrometer (FIDAS, Palas GmbH, Germany) (Fischer and Hüglin, 2024; Brem et al., 2023).

The dust RGB product, based on three infrared channels from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT's second-generation MSG-11 satellite (Schmetz et al., 2002), was used to identify the source of dust arriving over western Switzerland in the afternoon of 6 February (Schepanski et al., 2009). The dust detection method is based on the decrease in the brightness temperature recorded by SEVIRI when dust is present in the atmosphere. Moderate dust loading can decrease the brightness temperature by 5–10 K (Sokolik, 2002). The plume's path was tracked using satellite images following the approach described by Ashpole and Washington (2013). The visually identified dust routes were confirmed using forward- and backward-trajectory calculations with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, developed by the NOAA's Air Resources Laboratory (Stein et al., 2015). HYSPLIT uses archived three-dimensional meteorological data from ground and satellite observations, along with short-term forecasts. For this study, the Global Forecast System dataset from the National Centers for Environmental Prediction at 0.25° resolution was used to perform back-trajectory calculations. In all, 18 back-trajectory calculations have been performed: 3 arrival altitudes × 6 arrival times. The SEVIRI images and the backward trajectories trace the dust's origin for an arrival time over Fribourg at 2000 m above ground level, between 12:00 and 18:00 UTC on 6 February, roughly coinciding with the period of highest cloud density, to the Algerian–Moroccan border. The dust source meteorological data were obtained via the Real-time Environmental Applications and Display System (READY) from the archived Global Data Assimilation System (Rolph et al., 2017). The data were collected for the center of the main dust source observed in the satellite image, located at 33.8° N, 1.5° W.

Both single particles and iberulites were isolated from samples collected on the ground in Saint-Légier and Geneva, as well as from the aluminum foil in Fribourg. A selection of iberulites was gently placed onto a 1/2 in. (1.27 cm) pin stub (Ted Pella, Redding, CA, USA) and examined using an Olympus SZX12 binocular microscope (Tokyo, Japan). All single particles collected on the foil in Fribourg were dispersed onto a carbon-covered 1 in. (2.54 cm) pin stub and analyzed using automated scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) single-particle analysis (computer-controlled SEM, CCSEM). During CCSEM, particles are identified based on their back-scattered electron (BSE) contrast, and images, along with EDX spectra, are acquired. For this, an FEI Sirion FEG microscope equipped with an Oxford X-MAX EDX detector with a 150 mm² sensor size, a four-quadrant BSE detector, and AZtecFeature software (Oxford Instruments) was used at the Department of Geoscience of the University of Fribourg (Fribourg, Switzerland). Analyses were conducted at 20 kV. Particles were identified by simple gray-value (BSE intensity) thresholding. EDX analyses were performed for each identified particle. The AZtec software automatically applied a ZAF correction (PAP version of the f(ρz) formalism; Pouchou and Pichoir, 1985) – considering atomic number (Z), absorption (A), and fluorescence (F) effects – to the raw data for elemental concentrations. Quantification was based on factory-calibrated standards. The ZAF correction assumes homogeneous samples with smooth surfaces perpendicular to the beam and thicknesses greater than the excitation volume – conditions not met by these particles due to their rough surfaces and small sizes. However, studies (Kandler et al., 2018; Meier et al., 2018; Weinbruch et al., 1997) have shown that for particles larger than 1 µm, the relative standard deviation of the corrected major-element concentrations is approximately 2 %. For minor elements, the deviation is higher. Since only particles > 1 µm in diameter were included in further data processing, ZAF-corrected concentrations were used without additional corrections. Several blank analyses of empty substrates have shown that, under the applied analytical conditions, the substrate surface contains, in an area of ca. 1 mm², a maximum of five particles on unexposed substrates. These particles are organic in nature and most likely hair follicles.

Images from both the optical microscope and the SEM were used to characterize the particle size distribution of the individual constituents of the iberulites, as well as their microstructure, using Avizo software (Thermo Fisher). Selected iberulites were placed in a 1 in. (2.54 cm) round container and filled with impregnation resin. The resulting cylindrical samples were polished on the side containing the iberulites and then examined by electron microscopy. A BSE image of a polished iberulite section was also segmented using the Avizo software (Thermo Fisher), enabling the counting of individual particles and the determination of their equivalent spherical diameter dve, defined as the diameter of a sphere with the same volume as the particle with irregular shape (Kandler et al., 2018; Ott and Peters, 2008): 1dve=1P264πB3, where B is the projected area the particle occupies on the sample substrate, and P is the particle perimeter. Due to the resolution of the segmentation process, only particles larger than 0.05 µm can be detected and counted. To compare the SMPS with the SEM-derived PSDs, the mobility diameter dm obtained by SMPS must first be converted to the volume-equivalent diameter dve (DeCarlo et al., 2004): 2dve=CcdveCcdmχdm=CFdm, with Cc(dm)/Cc(dve) the inverse Cunningham slip correction factor ratio between the mobility and volume-equivalent diameters; CF the conversion factor; and the dynamic shape factor x, 3χ=FPFve, which is defined as the ratio of the drag force on the nonspherical particle FP to the drag force on its volume equivalent sphere Fve when both move at the same relative velocity with respect to the gas (Hinds, 1999). The electrical mobility is the velocity ve of a sphere (x=1) with the same velocity as the measured particle with n the number of charges on the particles and e the elementary unit of charge: 4ve=neCcdm3πηdm=neCcdve3πηχdve. From Eq. (4), the conversion Eq. (2) follows. Cc(dve) is unknown. The ratio Cc(dm)/Cc(dve) has, to our knowledge, been measured only for salt particles smaller than 600 nm. The latter (extrapolated for particles > 600 nm) was used to convert the volume-equivalent diameter into the mobility diameter. It is always less than one, increases with size, and decreases with axis ratio. The axis ratios b/a (ARs, the ratio of the longest dimension b to the orthogonal width) and shape factors appear to depend on size, flow regime (Alexander et al., 2016), and orientation (Cheng et al., 1988), ranging from near unity for smaller particles to a value of 3 for the largest particles. The average AR inside the iberulites is 1.46, close to 1.4 reported for dust particles in the 2 to 10 µm range (Ansmann et al., 2011; Rocha-Lima et al., 2018; Jeong, 2024). Overall, CF is mostly smaller than 1.0. The converted volume-equivalent diameters are smaller than the mobility diameters.

A loose, single sample of dust particles, lacking iberulites but collected simultaneously with the iberulites that fell in Fribourg, was analyzed using a Rigaku Ultima IV diffractometer equipped with a position-sensitive detector. Diffractograms were recorded from 5 to 70° 2θ in step-scan mode (0.02° per step and 1 min per degree). The copper X-ray tube was operated at 40 kV and 40 mA. The Cu Kα radiation was filtered through a nickel foil. Quantitative phase analysis via Rietveld refinement (Bish and Post, 1993) was performed on the recorded patterns using the PDXL2 software package (Rigaku). The refined instrumental parameters included the zero shift, sample displacement, and peak shape, modeled with a pseudo-Voigt profile. Structure parameters for the phases in the sample were sourced from the International Center for Diffraction Data database; the scale factor, unit-cell parameters, isotropic overall temperature factor, and in some cases the preferred orientation parameter (phyllosilicates, calcite) were the only structural parameters refined using the March–Dollase formalism (Dollase, 1986). Phase fractions were determined through the scale factors. Atomic positions and site occupancies remained fixed during refinement. Multiple iterations were performed until the process converged.

For air masses arriving at 2000, 2500, and 3000 m a.s.l. over Fribourg in the early afternoon of 6 February, HYSPLIT back-trajectory calculations were performed for hourly arrival times from 09:00 to 15:00 UTC.

The dust originated in northwestern Africa (Fig. 2), between the southern Moroccan border and northwestern Algeria. Satellite images showed a dust plume forming in a region called “Hauts Plateaux” (also known as “Hautes Plaines”) between the pre-Sahara and the Tell Atlas (Fig. 3). The air masses arriving over the Hauts Plateaux during the early afternoon of 5 February were at altitudes ranging from 1500 to 3500 m a.s.l., which is within the planetary boundary layer (PBL; calculated with the READY software; Rolph et al., 2017), spanning from 2600 to 4500 m. Average surface wind velocity (at 10 m above ground level) increased from 40 km h⁻¹ to approximately 70 km h⁻¹.

Weather stations in the Atlas Mountains recorded hurricane-force gusts (e.g., 117 km h⁻¹ on 5 February 2021 in Midelt, Morocco; Fig. 2) (Hoshyaripour, 2021). These gusts exceeded the threshold wind speed for dust emissions given for western Algeria (Marticorena et al., 1997). The dust plume was transported northeastward toward the Mediterranean Sea, where a low-pressure system developed. The associated warm conveyor belt lifted the dust to higher altitudes and carried it with the polar jet across the Mediterranean in an “atmospheric river” toward central Europe (Francis et al., 2022; Rautela et al., 2024). Source sensitivities (the probability that an air mass originates from a given source along a certain path) for Saharan dust (fine mode) indicate that the most common pathway is from the Sahara, over the Mediterranean, directly to the Alps (Collaud Coen et al., 2026).

On the afternoon of 6 February 2021, the Swiss Plateau – including Fribourg – experienced high-altitude mixed-phase clouds (altocumulus) that eventually gave way to periods of clear skies. Ground temperature measurements and T / RH profiles from the radiosonde released at 11:00 UTC in Payerne showed relatively high temperatures for 6 February in western Switzerland (4 °C at the ground and a 0 °C level at 2800 m a.s.l.), along with a typical inversion (Fig. 4). Nevertheless, compared with conditions during previous IFs over the Iberian Peninsula (Diaz-Hernandez and Sanchez-Navas, 2016), the temperature was much lower. The vertical profile of relative humidity (RH) was complex: RH decreased from 97 % at ground level to 48 % at 1000 m, then increased to 61 % at 3200 m a.s.l. (Fig. 4). RH remained between 48 % and 55 % up to 4300 m a.s.l., where it began to increase to 97 %, resulting in a 100 m thick layer between 5000 and 5100 m a.s.l. with temperatures below the frost point. At this altitude, altocumulus clouds were reported.

The Payerne ceilometer time profile (Fig. 5) shows a cloud base at 5000 m a.s.l. at midnight, with reflections above about 2500 m (the 0 °C level), which may be due to aerosol (SDE) or ice particles. Reflections from ice particles and dust are generally difficult to distinguish. Early in the morning, the cloud base lowered to 4000 m, and the reflections extended down to 2500 m. Below 2500 m a.s.l., the temperature is above 0 °C and rises rapidly; therefore, reflections below 2500 m a.s.l. in the afternoon are very unlikely to be from ice particles. The below-cloud reflections appear streaky in the ceilogram. After a large gap in ceilometer recordings between 09:00 and 11:00 UTC due to ground fog blocking the laser, a reflection wall extending to the ground appeared. These strong reflections from dust particles persisted throughout the day. The meteorological cloud base lowered from 4000 to 2000 m during the evening. Around 22:30 UTC, three short periods with very strong reflections were observed; during these times, precipitation was recorded on the ground. After 23:00 UTC, the reflections became less intense.

Lidar measurements were taken between 15:00 and 17:00 UTC, during the signal's peak, and the average was used to derive a mass concentration profile (Fig. 6) across the dust plume. The altitude range of 4000–5000 m served as the “no aerosol” reference. The profile was constructed using a lidar ratio of 0.57 m² sr⁻¹ ± 9 sr at 532 nm (Haarig et al., 2022), an Ångström coefficient of 0.5 for long-range-transported dust in northern Europe (Ansmann et al., 2001), and a mass extinction ratio at 532 nm of 0.47 ± 0.04 m² g⁻¹ (Nemuc et al., 2013). The profile showed three peaks. The highest concentration, 81 µg m⁻³, occurred at 1675 m. The second-largest concentration was recorded just above the ground, and a third small peak was located just above the cloud base at approximately 3500 m.

Giant particles were previously observed after long-range transport of Saharan dust. Rapid transport, high turbulence, and electrical forces are thought to counteract the particles' weight. In our case, the large “particles” were not individual particles but multimineral aggregates with spherical to slightly ellipsoidal shapes, known as iberulites after their first recording on the Iberian Peninsula (Fig. 7) (Díaz-Hernández and Párraga, 2008). The cores of the iberulites consist of coarser mineral grains (Figs. 8e, f and 9) that have accumulated without a visible cement matrix. Apart from a thin, dense surface layer, the iberulite cores show no internal structure, such as layering or preferred grain orientation. The iberulites collected in Saint-Légier and Fribourg (n=425) are larger than those from the Iberian Peninsula, with an average diameter of 113 µm (range: 43–365 µm). Most iberulites have smooth surfaces with few large surface pores (Fig. 8a–d), most likely due to particles ejected from the aggregates. Spherical surface depressions, formed due to vortices in the air around the rear end of falling water drops (Ren et al., 2020), as observed on Iberian iberulites (Díaz-Hernández and Párraga, 2008), are very rare among the sampled iberulites. Some iberulites were still wet.

During periods when JFJ is cloud-free, above the PBL, and free of Saharan dust, the number PSD has two modes: a stronger one near 0.05 µm and a weaker one between 0.15 and 0.4 µm (Fig. 10, green bars). The PSD measured at 11:00 UTC on 6 February, when JFJ was within the Saharan dust plume, is monomodal, with a maximum near 0.150 µm (Fig. 10, yellow bars). It shows, similar to PSDs observed during previous Saharan dust episodes at JFJ (Chou et al., 2011), much higher concentrations than measurements without SDEs. The PSD of particles within the iberulites is monomodal, with a maximum at 2.5 µm (Fig. 10, blue bars), much larger than that for Saharan dust sampled at JFJ (e.g., 0.2 µm). For these dimensions, the PSD of the particles inside the iberulites shows rather a minimum.

The number of particles Nsec (598) in the iberulite section shown in Fig. 9a was determined after segmentation of the image, shown in Fig. 9b, using Aviso. The average equivalent diameter deq calculated with Eq. (1) is 2.0 µm, and the total number of particles in the iberulite shown in Fig. 9a is, using Eq. (6), 22 000! As a comparison, the number of collisions of an ice particle with a diameter of 1 mm along a 1000 m trajectory in air charged with 60 µg cm⁻³ of dust with particles 2.0 µm in diameter and with an average density of 2.0 g cm⁻³, assuming an average CE of 0.4, is calculated using Eq. (5): 26 000. Although both values are only an approximation due to the high variability in the variables entering the equations, it is nevertheless remarkable that they are of the same order of magnitude.

Coarse-mode particles arriving at JFJ have been well documented in previous studies of particle volume distributions in dust plumes, with a mode in number PSDs at 1.4 µm (Schwikowski et al., 1995) and in particle surface area distributions at 40 µm (Brunner et al., 2021). Saharan dust clouds transported over long distances, such as to Barbados during the SALTRACE campaign (Weinzierl et al., 2017), to AERONET sites in the eastern Caribbean (Velasco-Merino et al., 2018), over the Mediterranean Sea during the ChArMEx/ADRIMED campaign (Denjean et al., 2016), and over Cape Verde during the SAMUN2 campaign (Weinzierl et al., 2011), have similar PSDs. The largest particles observed at the Caribbean sites are well above the maximum diameter expected for settling under Stokes' law during transport. The modes observed at JFJ and in the campaigns cited above resemble the PSDs observed in the source regions (Panta et al., 2023; Bencherif et al., 2022; Dubovik et al., 2006) but are lower in concentration. The presence of particles with diameters > 10 µm inside the iberulites is thus not exceptional.

X-ray diffraction of individual particle samples collected on the foil in Fribourg showed that the dust contained phyllosilicates (illite, muscovite, and kaolinite, totaling 35.1 wt %), followed by carbonates (28.0 wt % calcite, 4.0 wt % dolomite), quartz (23.3 wt %), and K-feldspar (1 wt %–2 wt %). The concentrations of gypsum and halite were below the detection limit. Analysis of individual particles within an iberulite confirms the XRD results. EDS analyses on the polished iberulite section have a resolution of 1–2 µm, depending on section thickness and local composition. Compositions of particles < 1 µm should therefore be interpreted with caution. Trends from the ideal muscovite composition toward the calcite composition, as well as toward the quartz compositions, in the (Mg + Fe + Si + K - Al - Ca) ternary diagram indicate that the particles are mixtures of the three phases (Fig. 11a). The ternary diagram (Mg + Fe + Si + K - S - Ca), with Mg + Fe + Si + K as the “silicate” corner and Ca as the “calcite” corner (Fig. 11b), shows measurements mainly along the joint between the silicate and calcite corners, indicating that the analyses are from particle aggregates containing both minerals and some measurements scattered in the triangle silicates–calcite–gypsum, indicating mixtures among these phases or the presence of sulfur on the particle surfaces. This lack of sulfur in the rims of the iberulites sampled in this campaign differentiates them from the iberulites of the Iberian Peninsula (Díaz-Hernández and Párraga, 2008; Párraga et al., 2021).

Dust from potential source areas has been characterized by bulk chemistry (Kandler et al., 2007) and mineralogy (Scheuvens et al., 2013a). Mineralogical fingerprints include phyllosilicates, such as the ratio of illite to kaolinite concentrations (I / K) (Caquineau et al., 1998; Chester and Johnson, 1971; Scheuvens et al., 2013b). In northern Africa, the general trend is an increase in I / K from south to north and from east to west. The value for the present single particles collected in Fribourg is approximately 2.9, consistent with values reported from the western Sahara (e.g., > 2 in NW Algeria and Morocco), which is the source area for the SDE determined from back-trajectory calculations and satellite data. High carbonate content (> 10 wt %, calcite > dolomite) is also a fingerprint of dust from northwestern Africa (Avila et al., 1997). Similar to illite, carbonate content decreases continuously toward the south (Paquet et al., 1984). High carbonate content is characteristic of dust emitted from regions within and adjacent to the Atlas Mountains (Grousset et al., 1992), the source for the present SDE determined by back-trajectory calculations and satellite imagery.

The layer at 5000 m altitude with supersaturated atmospheric conditions (0.11 % supersaturation) in a mixed-phase cloud is most likely the source of the ice/snowflake precipitation that led to the scavenging process by frozen hydrometeors. With an RH of 97 %, the highest value measured by the balloon sonde, and assuming a liquid water content > 0.01 g m³, the fraction of water bound in ice particles relative to the total water content is about 80 % (Korolev et al., 2017). The conditions are ideal for the crystallization of plate-like ice particles (Fukuta and Takahashi, 1999; Hallett and Bailey, 2009; Libbrecht, 2005). The temperature at the upper boundary of the dust layer was below 0 °C and became positive around 2800 m. However, melting of the frozen hydrometeors most likely began at a much lower level due to low relative humidity, in which the water vapor density of the surrounding air is lower than the equilibrium water vapor density of frozen hydrometeors at the freezing point; therefore, sublimation of water vapor occurs from the surface of the latter. Because of this sublimation process, latent cooling can push the surface temperature of the hydrometeor below 0 °C (Matsuo and Sasyo, 1981; Heymsfield et al., 2015). The width of the non-melting layer in the atmosphere increases nearly linearly with decreasing relative humidity, from 120 m at 90 % to 700 m at 50 %. The relative humidity at the top level of the main Sahara dust layer is around 50 % and remains constant down to an altitude of 1700 m a.s.l. It is therefore probable that the melting level is displaced far into the massive Saharan dust layer and that the encounter between dust and hydrometeors occurred when the latter were frozen. Although difficult to distinguish from aerosol particles, the reflections just below the cloud base may originate from frozen hydrometeors. Both solid ice crystals and snow crystals can capture particles. Several studies on BCS (Kyrö et al., 2009; Paramonov et al., 2011) indicate that frozen hydrometeors are more effective scavengers of aerosol particles than rain on an equivalent water-content basis. The fluttering, zigzagging fall style of frozen hydrometeors (Hashino et al., 2016; Kajikawa, 1992; Tagliavini et al., 2021; McCorquodale and Westbrook, 2021; Stringham et al., 1969) prolongs their trajectories, thereby increasing the probability of collisions with aerosol particles and, consequently, CE. CE also increases with temperature and relative humidity, although there is a discrepancy between experimental/natural data and theory (Santachiara et al., 2023). The highest CE has been recorded for mixed precipitation types at temperatures slightly below and above 0 °C (Miller and Wang, 1989). The temperatures between the likely origin of the frozen hydrometeors and 2000 m a.s.l. are in this range.

At these temperatures, frozen hydrometeors develop a stickier liquid layer on their surfaces. The increase in CE at higher RH is attributable to increased collector capacitance (Paramonov et al., 2011). Thus, the conditions for effective scavenging were met. The Greenfield gap for frozen hydrometeor fall situations was smaller, spanning 0.09–0.3 µm, compared with the gap observed for rain. This range is also observed in the iberulites collected in the present fieldwork, supporting the role of frozen hydrometeors as dust scavengers and indicating that BCS was at work. The increased CE of frozen hydrometeors is probably also responsible for the larger diameters observed in Swiss iberulites than in Iberian iberulites. When frozen hydrometeors exit the non-melting zone, they melt or evaporate, exhibiting a wide range of melting behaviors (Hallett and Oraltay, 2005). All frozen hydrometeors form thin liquid films on the surface and become stickier (Knight, 1979; Leinonen and Lerber, 2018; Hallett and Oraltay, 2005). Large frozen hydrometeors increase in size through coalescence in the melting layer (McFarquhar et al., 2007). The behavior of scavenged aerosol particles within melting hydrometeors has not been analyzed to date. Most likely, they are incorporated into the newly formed raindrops. Within and below the melting layer, due to the very low relative humidity (45 % at 1000 m, at the bottom of the massive dust layer), raindrops will dry very rapidly. A technological analog of this process is the (spray) drying of a powder suspension, which is used, e.g., in the ceramic, food, and pharmaceutical industries (Santos et al., 2018). The difference between the drying of a droplet in a spray dryer and that of a rain droplet falling out of a cloud is the temperature difference between the droplet and the drying airstream, which is around 200 °C in a spray dryer. Droplets in a spray dryer relax into spherical shapes, analogous to raindrops. The latent heat of water evaporation induces a strong heat flux from the droplet surface into the gas stream. This, in turn, drives thermophoretic particle displacement toward the droplet surface and a specific microcirculation in a layer near the surface, driven by the surface-tension gradient (Iskandar et al., 2003). This microcirculation entrains particles toward the droplet surface, leading to size- and density-based segregation. Smaller and/or less dense particles move faster toward the droplet surface, forming a surface layer. Coarser particles are concentrated in the core. Such a shell–core structure was observed in the present iberulite and has also been reported in the Iberian examples (Díaz-Hernández and Párraga, 2008; Párraga et al., 2021), with fine-grained phyllosilicates concentrated in the shell and coarse-grained quartz grains in the core. The sulfur-rich rims observed in Iberian iberulites, but lacking in our iberulite, were explained by the condensation of gaseous sulfuric acid on the iberulite rind and the absorption of atmospheric SO₂ attributed to industrial emissions in northern Africa. If the present iberulites are formed through BCS above western Switzerland with few heavy industries, such rims are not expected. Even if ICS occurred before the dust cloud arrives over Switzerland should be the iberulite formation mechanism, the trajectory of the air masses arriving over Fribourg crosses only over Marseille (France), a region with heavy industries. The air masses crossed the Algerian sea border between the industrial hotspots of Algier and Arzew–Mostaganem (Etkind et al., 2026) and at 2000 m a.s.l. over Marseille, with a PBL height which in wintertime is around 1000 m (Riandet et al., 2023), i.e., below the dust cloud level.