Saharan Dust Storm Rained Tiny Mineral Spheres on Switzerland for the First Time

On the afternoon of 6 February 2021, researchers standing outside a laboratory in Fribourg, Switzerland, noticed something odd: a faint, gritty patter that sounded almost like light rain, yet the sky was producing no ordinary rainfall. What was falling were iberulites, micrometer- to submillimeter-sized spherical aggregates of Saharan desert minerals, and the event marked the first time these curious particles had ever been reported north of 42 degrees latitude. According to a new study published in the journal Aerosol Research, the fall accompanied one of the strongest Saharan dust events measured in Switzerland since systematic monitoring began at the Jungfraujoch high-altitude research station in 2001, when PM10 concentrations exceeded 700 micrograms per cubic meter, far above the Swiss daily limit of 50.

Iberulites were first described in southern Spain, where they get their name, and have since been identified in Mallorca and Tenerife. They are typically between 50 and 300 micrometers in diameter and display a distinctive internal architecture: a coarse-grained core of quartz and other detrital minerals surrounded by a dense, fine-grained shell rich in phyllosilicate clays. Their mineral inventory, which includes silicates, carbonates, sulfates, halides, oxides, and even biological material such as plant fragments, diatom silica shells, plankton, and possibly viruses, points to the Sahara and Sahel as the ultimate source regions. Some minerals, like gypsum and alunite-jarosite, appear to form in the atmosphere itself during transport, likely through reactions involving atmospheric pollutants.

Two competing mechanisms have been proposed to explain how these spheres assemble. The first is in-cloud scavenging, in which cloud droplets coalesce around dust particles that serve as condensation nuclei, concentrating mineral matter inside a growing drop. The second is below-cloud scavenging, in which falling drops or ice particles collide with dust particles suspended in the air beneath the cloud, sweeping them up as they descend. Both processes end with the droplet drying out, leaving behind a compacted mineral sphere. Distinguishing between the two requires knowing exactly what the atmosphere was doing while the particles formed, which is precisely what made the Swiss event so scientifically valuable.

The dust that reached Switzerland on 6 February originated on the Hauts Plateaux region near the Moroccan-Algerian border. Satellite imagery from the SEVIRI instrument on EUMETSAT’s MSG satellite captured the plume’s birth, while HYSPLIT back-trajectory calculations traced the air masses from the Sahara across the Mediterranean to the Swiss Plateau. Weather stations in the Atlas Mountains recorded hurricane-force gusts of 117 kilometers per hour on 5 February, well above the threshold for dust emission. A low-pressure system over the Mediterranean then lifted the dust in a warm conveyor belt, an atmospheric river of mineral particles that rode the polar jet stream toward central Europe in a matter of hours.

Crucially, the meteorological conditions along the dust’s descent were exceptionally well documented. Radiosonde balloons launched from Payerne twice daily provided vertical profiles of temperature and humidity, while a Raman lidar, ceilometers, and Doppler radar profiled the dust layer itself. The sounding revealed a mixed-phase cloud layer near 5000 meters where relative humidity reached 97 percent and temperatures dipped below the frost point, ideal conditions for growing plate-like ice crystals. Below that, relative humidity plunged from about 70 percent to 40 percent within the first 1000 meters beneath the cloud, and temperatures hovered near freezing, far colder than the conditions reported for Iberian iberulite falls.

The key evidence came from comparing particle size distributions. At the Jungfraujoch station, sitting at 3580 meters in the Bernese Alps, instruments measured the size distribution of the dust cloud itself, which peaked near 0.15 micrometers. Inside the collected iberulites, however, the constituent particles peaked at 2.5 micrometers, and the distribution showed a striking deficit of particles between roughly 0.1 and 0.3 micrometers. This depletion is a fingerprint known as the Greenfield gap, first described in 1957. Particles in this intermediate size range are too large to be captured efficiently by Brownian diffusion and too small to have enough inertia to punch through the airflow streaming around a falling hydrometeor, so they simply follow the flow lines around it and escape capture.

Both in-cloud and below-cloud scavenging should produce some version of this gap, but the effect is far more pronounced for below-cloud scavenging, and the gap observed in the Swiss iberulites matched the narrower gap expected for frozen hydrometeors rather than raindrops. Quantitative checks reinforced the picture. Segmentation of electron microscope images of a polished iberulite section revealed about 598 particles in a single slice, with an average equivalent diameter of 2.0 micrometers, implying roughly 22,000 particles in the whole sphere. A theoretical calculation of how many 2-micrometer particles a 1-millimeter ice particle would sweep up during a 1000-meter fall through air carrying 60 micrograms of dust per cubic meter yielded about 26,000 collisions, remarkably close to the observed count.

The physics of falling ice also explains why frozen hydrometeors make such effective collectors. Snowflakes and ice crystals flutter and zigzag as they descend, lengthening their trajectories and raising the probability of collision with dust particles. At temperatures slightly below and above 0 degrees Celsius, frozen hydrometeors develop a sticky quasi-liquid surface film that boosts capture efficiency further, and laboratory and field studies consistently show that snow scavenges aerosol particles more effectively than rain on an equivalent water-content basis. Because the relative humidity below the cloud was so low, sublimational cooling likely kept the hydrometeors frozen far deeper into the dust layer than the nominal freezing level would suggest, meaning the dust was collected by ice, not liquid water.

Once a loaded hydrometeor melted or, more likely, evaporated in the bone-dry air below the cloud, the drying droplet behaved much like the droplets in an industrial spray dryer used in the ceramic, food, and pharmaceutical industries. Evaporation drives thermophoretic forces that push particles toward the droplet surface, while surface-tension gradients set up microcirculation cells that carry smaller and less dense grains outward faster than coarse ones. The result is the characteristic core-shell structure: fine phyllosilicates concentrated in the outer shell, coarse quartz grains in the core. Notably, the Swiss iberulites lacked the sulfur-rich rims seen on their Iberian counterparts, which are attributed to industrial sulfur emissions near the dust source and along the transport path, a difference consistent with formation over Switzerland, where the air masses crossed no major industrial zones at cloud level.

The Swiss iberulites were also larger than their Iberian relatives, averaging 113 micrometers across with a range of 43 to 365 micrometers, which the authors attribute to the superior scavenging efficiency of frozen hydrometeors. Perhaps the most consequential finding is what came after: follow-up analyses of bulk aerosol samples collected during other Swiss dust events revealed that more than half contained iberulites. If these spheres are this common in central Europe, they have almost certainly been collected, counted as ordinary dust, and overlooked by researchers worldwide. Determining how much Saharan dust is actually deposited as iberulites, and checking for them in dust archives across the globe, could meaningfully refine models of dust deposition, with implications for climate forcing, nutrient delivery to ecosystems, and the health impacts of desert dust that affects more than 150 countries.

Subject of Research: Formation mechanism of iberulites, spherical Saharan dust aggregates, observed during a Saharan dust event in Switzerland

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

Article References: Grobety, B., Favreau, P., Rausch, J., Jaramillo-Vogel, D., Steinbacher, M., & Neururer, C. (2026). Iberulite fall and formation mechanism during a Sahara dust event in Switzerland in February 2021. Aerosol Research, 4(2), 485-500. https://doi.org/10.5194/ar-4-485-2026

Image Credits: AI Generated

DOI: 10.5194/ar-4-485-2026

Keywords: iberulites, Saharan dust, aerosols, below-cloud scavenging, Greenfield gap, Switzerland, atmospheric science, mineral dust, ice crystals, dust transport, Jungfraujoch, particle size distribution