Researchers at ETH Zurich, led by Ryan Vella and Ulrike Lohmann, used the ICON-HAM global aerosol–climate model, which explicitly simulates aerosol microphysics, cloud droplet and ice crystal formation, and the interaction of both clouds and aerosols with radiation. They compared two ten-year simulations. In the control run, the sea surface followed present-day conditions; in the so-called hosing run, they superimposed sea surface temperature and sea ice anomalies taken from a fully coupled EC-Earth3 simulation in which freshwater was poured into the North Atlantic at a rate of 0.3 Sverdrups poleward of 50° N. That freshwater forcing cut the simulated AMOC from roughly 17.5 to 7.0 Sverdrups — a weakening of about 60 percent. Crucially, all anthropogenic and biomass-burning emissions were held fixed, so any change in atmospheric aerosols had to arise purely from altered winds, transport and rainfall.

The imposed slowdown produced a stark hemispheric temperature asymmetry. Surface temperatures dropped by more than 4 kelvin over the subpolar North Atlantic, and averaged over the Northern Hemisphere the cooling reached about 2.4 kelvin — nearly twice the global mean of roughly 1.2 kelvin. The annual-mean surface freezing line marched about 761 kilometres southward, from around 58° N to 51° N. Global precipitation fell by about 0.10 millimetres per day, with the reduction concentrated in the Northern Hemisphere at 0.29 millimetres per day, and the tropical rain belt — the Intertropical Convergence Zone — shifted southward, drying the North Atlantic, western Europe and much of the northern tropics while wetting regions to the south of the equator.

That reorganisation of winds and rain transformed the atmosphere’s particle load. Weakened westerlies over the subpolar North Atlantic cut sea-salt emissions there by roughly 63 percent, but the dominant change came from the Sahara. Steepened temperature gradients between the cooled mid-latitudes and the relatively unchanged tropics strengthened low-level winds over the subtropical Atlantic and western Sahara, boosting Saharan dust emissions by 81.0 milligrams per square metre per day — a 56 percent increase — and sending a thicker plume westward across the tropical North Atlantic. Globally, the total aerosol column burden rose by 38 percent, almost entirely in the Northern Hemisphere, where the mean burden climbed 41 percent.

A second, subtler mechanism lengthened the lifetime of existing particles. Because Northern Hemisphere rainfall declined, wet scavenging — the rainout of particles from the atmosphere — became less efficient, so fine sulfate and carbonaceous aerosols lingered longer aloft. Together, the enhanced dust and the extended lifetimes raised the availability of cloud condensation nuclei in the lower and middle troposphere between the equator and about 40° N, and increased the concentration of dust particles capable of nucleating ice — the ice-nucleating particles — mainly between 10 and 30° N where the Saharan plume is most prominent.

Clouds responded in two distinct ways. Thermodynamically, the colder and more stable lower troposphere over the subpolar North Atlantic favoured persistent stratiform cloud decks: low cloud cover increased by roughly 4 to 9 percent across the region, with local anomalies above 12 percent, while liquid water path rose by up to 24 grams per square metre over the North Atlantic and 37 grams per square metre over the North Pacific. But this local brightening was the exception within a broader loss of cloud water. North of 40° N, total water path fell over 82 percent of the area, by 11.6 percent on average and by about a third over the Nordic and Barents Seas, because the colder, more ice-covered surface supplied less moisture to the atmosphere.

The microphysical story emerged when the team sorted clouds by temperature over 40–90° N. In mixed-phase clouds — the realm between 0 and −35 °C where supercooled droplets and ice crystals coexist — dust ice-nucleating particle concentrations rose by 27.2 percent and in-cloud ice crystal number climbed 32.1 percent. The extra crystals grew at the expense of supercooled droplets through the Wegener–Bergeron–Findeisen process, in which ice, having a lower saturation vapour pressure than liquid water, drains vapour from nearby droplets. The result: mixed-phase clouds held 8.1 percent less water and became optically thinner. Warm clouds behaved differently, gaining 6.4 percent in total water path as the increased droplet numbers suppressed the collision-coalescence process that converts cloud droplets to rain. Cirrus ice crystal numbers, by contrast, fell by 7.1 percent despite abundant dust, because the stabilised atmosphere suppressed the deepest convective towers and shifted the coldest cloud tops toward warmer temperatures where homogeneous freezing nucleates fewer crystals.

When all these regional effects were tallied with a radiative-kernel decomposition, the global-mean cloud contribution turned out surprisingly small: a net warming of just +0.21 watts per square metre at the top of the atmosphere. Subpolar low-cloud brightening — a cooling effect — was largely cancelled by optical thinning of mixed-phase and ice clouds and by opposing tropical cloud changes tied to the southward ITCZ shift. The large apparent cloud radiative effect anomaly of +0.86 watts per square metre was mostly clear-sky masking: brighter ice, snow and dust hidden beneath clouds, not a genuine change in cloud properties. This contrasts with earlier studies that found cloud changes acting as a net positive feedback on AMOC-induced cooling — a difference the authors attribute to their resolution of aerosol–cloud microphysics.

The dominant cooling terms lay elsewhere. The expansion of subpolar sea ice and Northern Hemisphere snow cover reflected an additional 0.83 watts per square metre of sunlight back to space — of which 0.68 watts came from sea ice and 0.15 from continental snow — and the enhanced Saharan dust itself scattered and absorbed radiation, contributing an effective radiative forcing of −0.23 watts per square metre. Together with the modest cloud term, these channels summed to −0.86 watts per square metre, reinforcing rather than offsetting the surface cooling. Notably, this figure excludes the longwave response to the cooling, such as the Planck and lapse-rate responses, which the decomposition did not separate.

The study carries clear caveats. Because sea surface temperatures and sea ice were prescribed rather than coupled, the diagnosed cloud and aerosol effects cannot feed back on the ocean or the AMOC itself — a significant constraint given that the largest identified term is the sea-ice albedo response, which in the real world interacts with the very ice and ocean state held fixed here. With emissions pinned at 2015 levels and no concurrent greenhouse warming, the results isolate a mechanistic pathway rather than a realistic future trajectory, and as a single-model study the magnitudes, and in some regimes the signs, may depend on the ICON-HAM parameterisations. Still, the message is striking: a slowdown of the Atlantic conveyor can boost Saharan dust by more than half and stretch aerosol lifetimes through suppressed rainfall alone, and those particles then sculpt clouds from within — dehydrating mixed-phase clouds through dust-triggered ice formation even as they brighten low marine clouds. Climate models that prescribe aerosol effects rather than simulating them cannot capture this coupling, and its omission may help explain why projections of the climate response to future AMOC change vary so widely.

Subject of Research: Aerosol and cloud responses to a simulated weakening of the Atlantic Meridional Overturning Circulation

Article Title: Aerosol–cloud interactions influence the climate response to AMOC weakening

Article References: Vella, R., & Lohmann, U. (2026). Aerosol–cloud interactions influence the climate response to AMOC weakening. Atmospheric Chemistry and Physics, 26(20), 14939-14959. https://doi.org/10.5194/acp-26-14939-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14939-2026

Keywords: AMOC, aerosol-cloud interactions, Saharan dust, mixed-phase clouds, sea ice, radiative forcing, ICON-HAM, climate modelling, North Atlantic, ITCZ, ice-nucleating particles, freshwater hosing

The measurement campaign, known as EXPORT — short for EXperiments with the Portable Observatory for aeRosol Transport — was led by Andebo Waza, Konrad Kandler, Franz Kanngießer, Stephanie Fiedler, and colleagues from institutions including the GEOMAR Helmholtz Center for Ocean Research Kiel and Technische Universität Darmstadt. Expedition M207 departed Belém, Brazil, on January 3, 2025, and concluded in Mindelo, Cabo Verde, on February 11, 2025, tracing a northbound course directly through the dust corridor. Expedition M208 then began and ended in Mindelo, running from February 15 to March 17, 2025, keeping the ship and its instruments close to the African continent where dust outbreaks are most intense. Together, the two cruises captured continuous aerosol and meteorological observations across three distinct, multi-day Saharan dust events: the first over the tropical mid-Atlantic, and the latter two nearer the African coast.

What makes the EXPORT dataset unusual is its combination of ground-based in-situ sampling and remote sensing of lofted dust layers, all from a moving platform in a region where such measurements are rare. At the heart of the in-situ package was an optical particle counter, or OPC, which measured particulate matter concentrations and particle size distributions for diameters up to roughly 35 micrometers — a size range that includes the coarse dust grains responsible for much of the atmospheric dust mass burden. Complementing the optical counts, the team deployed a flat plate sampler to collect deposited particles, which were then analyzed using scanning electron microscopy coupled with energy-dispersive X-ray analysis, or SEM-EDX. This technique allowed the researchers to determine particle size, morphology, and elemental composition, from which they derived deposition rates — a key quantity for estimating how much dust, and the nutrients it carries, actually reaches the ocean surface.

Above the ship, a battery of remote sensing instruments probed the vertical structure of the atmosphere. CALITOO and MICROTOPS sun photometers measured aerosol optical depth at several wavelengths, providing a column-integrated measure of how much sunlight the dust attenuated. A Vaisala CL61 ceilometer recorded fine-scale layering of aerosol plumes, including depolarization measurements that help distinguish irregularly shaped mineral dust particles from spherical marine aerosols such as sea spray. A wind lidar profiled wind speeds aloft, an optical disdrometer characterized precipitation, and a broadband radiation sensor tracked the radiative effects of the dust. Nearly 90 radiosonde launches — balloon-borne instrument packages that profile temperature, humidity, and winds — stitched these observations into a three-dimensional picture of the dust-laden atmosphere.

The scientific stakes are considerable. Saharan dust influences climate in multiple, competing ways: it scatters and absorbs solar radiation, alters cloud formation by serving as ice-nucleating particles, and fertilizes ocean ecosystems with iron and other micronutrients. Yet climate and weather models still struggle to represent dust transport accurately, in part because ground-truth observations over the remote tropical Atlantic are so sparse. Most long-term dust records come from land-based stations in Africa or from satellite retrievals, which infer column properties but cannot resolve the vertical distribution, particle microphysics, or surface deposition directly. Ship-based campaigns like EXPORT fill precisely this gap, offering calibration points for satellites and validation data for reanalysis products and forecast models.

Indeed, the EXPORT team documented their measurements with a first comparison against reanalysis datasets and the METEOR’s own meteorological sensors, an early sanity check that will help future users judge how well existing models capture the observed dust events. The three dust outbreaks sampled during the cruises serve as illustrative case studies: one plume encountered far from any landmass in the tropical mid-Atlantic, and two others observed closer to West Africa, where fresh outbreaks loft dense dust into the Saharan Air Layer — the hot, dry, elevated air current that carries dust across the basin. Comparing in-situ size distributions and concentrations against model output during these events can reveal where models overestimate or underestimate coarse-particle transport, a persistent problem because large dust grains settle out faster than many models assume.

The winter timing of the campaign is itself scientifically meaningful. Between December and March, the Saharan dust belt shifts southward, and dust transport toward the Amazon becomes most pronounced. This seasonal dust delivery is thought to supply phosphorus and other nutrients that sustain the rainforest’s productivity, compensating for losses from leaching and runoff. Capturing this season with direct measurements — rather than extrapolating from other times of year — matters for quantifying the ocean and land fertilization budgets, for understanding how dust radiative forcing modulates the tropical Atlantic energy balance, and for constraining how dust may respond to changing land use and climate in North Africa.

All of the EXPORT datasets are now publicly available through the PANGAEA data repository, spanning optical particle counter measurements, flat plate sampler collections, sun photometer retrievals, Maritime Aerosol Network observations, radiation and ceilometer profiles, radiosonde soundings, wind lidar data, and disdrometer records for both cruises. The ship’s own meteorological data are archived separately through the dship.o2a-data.de portal operated by Universität Hamburg. This open release means that atmospheric modelers, satellite validation teams, and aerosol researchers worldwide can immediately begin mining the data — comparing it against dust reanalyses, testing transport schemes, or refining retrieval algorithms for spaceborne lidars and imagers.

For a field that has long relied on sparse island stations and satellite proxies, a two-month, instrument-dense transect through the heart of the winter dust corridor is a rare and valuable resource. As the preprint moves through peer review at Earth System Science Data, the EXPORT measurements stand ready to sharpen estimates of how much African dust crosses the Atlantic, at what heights, in what particle sizes, and with what consequences for climate, clouds, and ecosystems on both sides of the ocean.

Subject of Research: Trans-Atlantic transport of Saharan mineral dust measured from a research vessel during two 2025 expeditions

Article Title: Trans-Atlantic aerosol transport from January to March 2025 – measurements from two expeditions on a research vessel

Article References: Waza, A., Kandler, K., Kanngießer, F., Gottschall, J., Leuzinger, A., Speith, S., Sudarchikova, N., Wahl, S., Hepert, W., & Fiedler, S. (2026). Trans-Atlantic aerosol transport from January to March 2025 – measurements from two expeditions on a research vessel. https://doi.org/10.5194/essd-2026-733

Image Credits: AI Generated

DOI: 10.5194/essd-2026-733

Keywords: Saharan dust, mineral dust, aerosol transport, tropical Atlantic, R/V METEOR, research expedition, aerosol optical depth, ceilometer, radiosondes, SEM-EDX, dust deposition, Earth System Science Data

The measurement site, a small terminal depression called L’Bour near M’Hammid El Ghizlane, sits at the transition between the southern foothills of the Atlas Mountains and the open Saharan Desert. It is an ideal natural laboratory for dust emission studies. Mornings were typically calm, but as solar heating destabilized the near-surface air in the afternoons, winds strengthened and frequent local saltation events injected fresh dust into the lower atmosphere. On two days, 4 and 6 September 2019, the campaign captured intense cold-pool outflows known as haboobs, dramatic walls of dust that provided the researchers with abundant material for their instruments. This diurnal rhythm of quiet mornings and dusty afternoons allowed the team to build a rich dataset spanning a wide range of emission intensities.

The instrumentation combined a multi-angle integrating nephelometer, which measured light scattering at three wavelengths and eight angular positions, with a seven-wavelength dual-spot aethalometer that tracked absorption from the near-ultraviolet to the near-infrared. A Fine Dust Aerosol Spectrometer recorded particle size distributions in 63 logarithmically spaced bins covering optical diameters from roughly 0.2 to 19 micrometers. During the first part of the campaign, the optical instruments sampled through a PM2.5 inlet; later they switched to a PM10 inlet, a change that profoundly influenced what the team observed, because most of the mass of mineral dust resides in the coarse fraction. All measurements were averaged into 15-minute intervals to smooth out instrumental noise while preserving the turbulent structures that carry dust aloft.

The headline optical results are strikingly consistent with previous Saharan dust studies, which is itself reassuring for the field. During dust events, the single scattering albedo, the ratio of scattering to total extinction, was 0.95 for PM2.5 particles at 370 nanometers, rising to 0.97 at 660 nanometers, with slightly lower values of 0.94 and 0.96 for the PM10 fraction. These numbers tell us that Saharan dust is predominantly a scatterer of sunlight, but that its absorption is far from negligible, especially at short wavelengths. The absorption Ångström exponent, a measure of how strongly absorption varies with wavelength, reached values of up to 2.5, far above the values near 1 that characterize soot-like anthropogenic particles. This steep spectral signature is the fingerprint of iron oxide minerals such as hematite and goethite, which absorb much more strongly in the ultraviolet and blue than in the red and near-infrared.

Particle size emerged as a dominant control on nearly every optical property the team measured. The asymmetry parameter, which describes whether light is preferentially scattered forward or backward, was 0.7 at 520 nanometers for PM2.5 dust, and the backscatter fraction was correspondingly low at 0.11, both hallmarks of large particles that funnel scattered light strongly into the forward direction. The scattering Ångström exponent dropped to negative values when coarse dust dominated, a well-known indicator of coarse-mode particles. Mass absorption efficiency fell from about 0.30 to 0.15 square meters per gram at 370 nanometers as particle size increased, showing that larger grains absorb less light per unit mass. Meanwhile, the mass scattering efficiency shifted its peak toward longer wavelengths for larger particles, exactly as scattering theory predicts when the size parameter, the ratio of particle circumference to wavelength, grows beyond the regime of wavelength-independent scattering.

The most consequential part of the study, however, goes beyond describing the dust. The researchers used their combined measurements to retrieve the imaginary part of the complex refractive index, the quantity usually written as k that directly controls how much light a particle absorbs. Retrieving k from field data is an exercise in conditional reasoning: the answer depends on assumptions about particle density, the real part of the refractive index, and, critically, the shape of the particles. The team therefore ran their inversion repeatedly under a menu of prescribed shape assumptions, including perfect spheres, biaxial spheroids using aspect ratios measured on individual particles by electron microscopy during the campaign, the spheroid distribution commonly used in AERONET satellite retrievals, triaxial ellipsoids, and a set of six irregular particle geometries from the MOPSMAP modeling database.

The outcome was unambiguous. For central assumptions about density and the real refractive index, the retrieved k at 520 nanometers ranged from approximately 0.00104 for spherical particles to approximately 0.00145 for triaxial ellipsoids, a difference of about 40 percent. Non-spherical shapes systematically demanded larger values of k to reproduce the same observed absorption-to-scattering ratio, and the more elongated or irregular the assumed particle, the higher the retrieved absorption. The team found a nearly linear relationship between k and a shape descriptor, the ratio of volume-equivalent to cross-section-equivalent particle radius, with the effect weakening at longer wavelengths. Particle density assumptions also mattered, because density controls how the measured size distribution is converted between optical and aerodynamic diameters, whereas the assumed real refractive index had only a weak influence across its plausible range.

To put their in-situ results in context, the researchers compared them with two decades of dust-filtered retrievals from the AERONET network, using stations across western and eastern North Africa, the Sahel, the Arabian Peninsula, central Asia, and India, including Ouarzazate, the station closest to their field site. The comparison showed qualitative agreement with the Ouarzazate climatology, especially when the more realistic non-spherical shape assumptions were used, with median values converging closely at 550 and 675 nanometers. But the analysis also exposed pronounced regional variability in dust absorption, with retrieved k increasing from western toward eastern North Africa and further still across Asia, likely reflecting regional differences in iron oxide content and mineralogy. Differences in particle size distributions between surface measurements and column-integrated retrievals, along with dust ageing and residual aerosol contamination, complicate any strict comparison, so the authors frame the AERONET analysis as context rather than validation.

Why does all this matter? Mineral dust is one of the most abundant aerosol types in the atmosphere by mass, and its direct radiative effect is estimated at a central value of minus 0.40 watts per square meter, but with an uncertainty of plus or minus 0.30 that leaves open the possibility of a substantially different sign or magnitude. Dust absorption is the least constrained ingredient in these calculations, and most existing measurements come either from laboratory chambers using resuspended soils or from remote sensing of dust that has already been transported, aged, and mixed with other aerosols. Direct measurements of dust at the point of emission remain scarce. By showing that the shape assumptions embedded in retrieval algorithms and climate models can shift inferred absorption by tens of percent, the study provides a concrete explanation for part of the persistent spread among dust absorption estimates, and it suggests a practical path forward: past datasets built on spherical assumptions could potentially be reinterpreted using the relationship between particle asphericity and retrieved k.

The authors are careful about the limitations of their work. Their retrieval is explicitly conditional, restricted to the PM2.5 dust-event period, and their observing system simulation experiments revealed that size-dependent distortions in the measured size distributions, arising from different sampling inlets and possible particle losses, cannot be fully eliminated even by basing the cost function on the ratio of truncated scattering to extinction. Future campaigns, they argue, should prioritize co-located and size-consistent measurements, well-characterized inlets, and independent constraints on particle density and morphology. Still, the message of this desert field campaign is clear and broadly resonant: the light absorbed by a billowing Saharan dust plume depends not only on what the dust is made of, but on what shape we assume its grains to be, and getting that shape right may be essential for pinning down one of climate science’s most stubborn uncertainties.

Subject of Research: Optical properties and shape-dependent refractive index retrieval of freshly emitted Saharan mineral dust

Article Title: Optical properties and shape-dependent retrievals of the imaginary part of the refractive index of freshly emitted Saharan dust

Article References: Yus-Díez, J., Escribano, J., Pandolfi, M., Alastuey, A., González-Flórez, C., Gonzalez-Romero, A., Gonçalves Ageitos, M., Ivančič, M., Klose, M., Kandler, K., Obiso, V., Panta, A., Reche, C., Rigler, M., Querol, X., & Perez García-Pando, C. (2026). Optical properties and shape-dependent retrievals of the imaginary part of the refractive index of freshly emitted Saharan dust. Atmospheric Chemistry and Physics, 26(19), 14531-14557. https://doi.org/10.5194/acp-26-14531-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14531-2026

Keywords: Saharan dust, mineral dust aerosol, refractive index, light absorption, single scattering albedo, particle shape, iron oxides, AERONET, Morocco, dust emission, radiative forcing, aerosol optics

The research, led by Anthony La Luna of the University of Maryland Baltimore County together with Zhibo Zhang, Qianqian Song, Hongbin Yu, John E. Yorks, and Ping Yang, tackles a deceptively simple question: what happens to the light-scattering properties of dust as particles grow from much smaller than a wavelength to much larger than one? The answer, derived from first principles and confirmed with two independent particle-shape databases, is that dust obeys universal asymptotic laws at both extremes, and that those laws depend far less on the intricate shape of individual grains than scientists had assumed.

The team focused on three quantities that dominate dust typing in lidar remote sensing: the lidar ratio, which compares extinction to backscatter; the linear depolarization ratio, which measures how strongly particles scramble the polarization of reflected laser light; and the single-scattering albedo, the fraction of light extinguished that is scattered rather than absorbed. In the Rayleigh limit, where particles are tiny compared with the laser wavelength, the theory predicts that the lidar ratio scales as the inverse cube of the size parameter while the single-scattering albedo vanishes as its cube, with depolarization remaining near zero. Numerical regressions across all tested refractive indices recovered slopes of minus 2.88 plus or minus 0.08, in close agreement with the ideal minus 3 prediction.

The more surprising results emerge at the opposite extreme, in the geometrical-optics regime that governs super-coarse dust grains with radii exceeding ten micrometers. Using a decomposition of the scattering matrix into ray-tracing and diffraction contributions, the authors show that strong absorption extinguishes the internally refracted rays that normally generate cross-polarized backscatter. What survives is essentially first-surface Fresnel reflection, which preserves polarization. The consequence is dramatic: the depolarization ratio, long treated as the signature of dust, collapses toward zero, while the lidar ratio soars because backscatter is suppressed relative to extinction. The single-scattering albedo converges toward one half, slightly elevated by surface reflection to a predicted floor of roughly 0.55.

Critically, these asymptotic behaviors proved insensitive to particle geometry. The researchers compared sharp-edged, irregular hexahedra from the TAMUdust2020 database with smooth triaxial spheroids of aspect ratio 1.5, two shape models that could hardly be more different. In the intermediate size regime, the two geometries diverged substantially, with depolarization magnitudes differing by 0.1 to 0.4, a range that matters because operational lidars frequently sample coarse dust there. But in both limiting regimes, absorption forced the distinct shapes to converge on the same behavior, demonstrating that the suppression of depolarization is a fundamental consequence of volumetric absorption rather than an artifact of any particular model.

To connect single-particle physics to what lidars actually measure, the team averaged their calculations over realistic particle size distributions drawn from the FENNEC, AER-D, and AERONET datasets, spanning source-region Saharan dust through transported plumes. The framework naturally explains a long-puzzling observation: the inverse-v spectral pattern in which the depolarization ratio of dust peaks at 532 nanometers relative to 355 and 1064 nanometers. At the longest wavelength, iron-oxide absorption is negligible and depolarization tracks particle size, so coarse dust produces high values. At 355 nanometers, where iron oxides absorb most strongly, the depolarization signal is damped, dragging the ultraviolet value below the visible one.

The mineralogy enters through the complex refractive index, parameterized against laboratory measurements of dust from eight major source regions. Varying the iron-oxide volume fraction from zero to ten percent, a range spanning natural variability from the iron-poor Bodele depression to iron-rich Sahel sources, boosted the imaginary refractive index by up to an order of magnitude at ultraviolet wavelengths. A plume with the super-coarse FENNEC size distribution and a modest three percent iron-oxide fraction, well within typical Saharan values, yields a depolarization ratio of only about 0.18 at 355 nanometers alongside a lidar ratio that can exceed 300 steradians, far above the 40 to 44 steradians assumed for dust in standard CALIOP retrievals.

That combination, low depolarization with a high lidar ratio, is precisely the signature that classification algorithms associate with polluted or mixed aerosols rather than pure dust. When the authors fed their simulated plume into the CALIOP version 4 automated typing scheme, the algorithm assigned it to the Polluted Dust or Dusty Marine subtypes depending on surface type. The result is what the researchers call a lidar ambiguity: super-coarse, iron-rich dust optically mimics fine-mode aerosols, so retrievals that truncate the coarse mode, as AERONET inversions do at radii above roughly fifteen micrometers, can reproduce the measured signal while fundamentally misrepresenting the particle population. Super-coarse dust, whose underrepresentation already biases estimates of dust radiative cooling, may be systematically underestimated from space.

The study also offers a way out. Because depolarization responds to mineralogy most strongly at short wavelengths and to size most strongly at long ones, the ratio of depolarization at two wavelengths, the depolarization color ratio, becomes markedly elevated for coarse, absorptive particles. The authors propose that future spaceborne lidars measure multiwavelength depolarization so this metric can disentangle size from iron-oxide content. Coupled with spatially explicit mineralogy maps from NASA’s EMIT imaging spectrometer, used as priors in a forward scattering model, the framework could close the loop between surface composition and atmospheric dust transport.

For a field preparing to interpret a new generation of satellite lidars, the message is stark but actionable. Dust is not a single optical class; it is a continuum whose signature shifts with size and iron content in ways now governed by derivable physical law. Algorithms that assume a fixed dust lidar ratio or an ever-high depolarization plateau risk mislabeling the very particles that dominate extinction in aged plumes. Recognizing that the biggest, darkest desert grains can masquerade as urban haze may force a rethinking of aerosol typing from the ground up, and with it, a more honest accounting of how much giant dust the atmosphere truly carries.

Subject of Research: Asymptotic light-scattering behavior of absorbing mineral dust aerosols in lidar remote sensing across the Rayleigh and geometrical-optics regimes

Article Title: Asymptotic behavior of lidar scattering properties of absorbing dust aerosols across Rayleigh and geometrical-optics regimes: theory and implications

Article References: La Luna, A., Zhang, Z., Song, Q., Yu, H., Yorks, J. E., & Yang, P. (2026). Asymptotic behavior of lidar scattering properties of absorbing dust aerosols across Rayleigh and geometrical-optics regimes: theory and implications. Atmospheric Chemistry and Physics, 26(19), 14487-14506. https://doi.org/10.5194/acp-26-14487-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14487-2026

Keywords: mineral dust, lidar, depolarization ratio, lidar ratio, iron oxide, aerosol classification, geometrical optics, Rayleigh scattering, super-coarse dust, satellite remote sensing, refractive index, Saharan dust

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

CONIOPOL, whose name blends ‘coniology’ (the study of atmospheric dust) with ‘polarization’, was developed by Quentin Laffineur, Alexander Mangold, and Andy W. Delcloo and is described in the journal Atmospheric Measurement Techniques. The algorithm runs on the Vaisala CL61, a single-wavelength automatic lidar ceilometer that fires a 910.55-nanometer infrared laser straight up and records the light scattered back by particles in the air. What sets the CL61 apart from ordinary ceilometers is its polarization capability: it measures both the parallel and cross-polarized components of the returning signal. Spherical particles such as liquid droplets preserve the laser’s polarization, while irregular particles like mineral dust, volcanic ash, and ice crystals scramble it. That difference, quantified as the volume linear depolarization ratio, becomes the key that unlocks particle identification.

The algorithm weaves together three strands of information: the attenuated backscatter profile, the depolarization ratio, and the cloud-base height determined by the instrument itself. After careful signal conditioning, including noise filtering based on the signal-to-noise ratio and a two-dimensional median filter that suppresses isolated noisy pixels without blurring real atmospheric layers, a decision tree sorts every measurement into categories. A threshold on backscatter, determined statistically from a full year of data at about 3 x 10^-6 per meter per steradian, separates clouds and precipitation from aerosols. Depolarization values then split the targets further: values above 0.2 point to mineral dust, values above 0.37 suggest volcanic ash, and values below 0.1 indicate spherical particles such as smoke, marine salt, or liquid droplets.

One of the algorithm’s most distinctive features is its honesty about ambiguity. Rather than forcing every observation into a single pure category, CONIOPOL includes transitional classes that capture mixtures, such as dust blended with boundary-layer pollution or pollen mixed with continental aerosols. A two-dimensional sliding-window consistency filter then examines neighborhoods of five consecutive profiles and five adjacent range gates, spanning roughly 150 seconds and 24 meters, and suppresses isolated detections that do not form coherent structures. This approach acknowledges that the atmosphere is a continuum, not a set of neat boxes, and it makes the resulting classification maps far more robust against noise and instrument artifacts.

Calibration was a central challenge. The CL61 has no internal depolarization calibrator, so the team adopted a clever workaround borrowed from previous work on Doppler lidars: monitoring the depolarization signal at the base of liquid water clouds, whose spherical droplets provide a natural reference with a known, near-zero depolarization. Over the full year of observations, from February 2024 to January 2025, the median depolarization at liquid cloud bases stayed stable between 0.01 and 0.02, confirming that the instrument’s polarization channel did not drift. The backscatter calibration, anchored to a factory-calibrated reference using the well-established liquid-cloud calibration method, also remained stable throughout the study period.

To test whether the classifications made physical sense, the researchers compared them against the Copernicus Atmosphere Monitoring Service (CAMS) global forecast, which models aerosol mixing ratios for dust, sea salt, soot, organic carbon, and sulfate. Because the two datasets differ fundamentally in resolution and nature, the comparison was conditional rather than point-by-point. The results were striking: whenever CONIOPOL detected dust, CAMS dust mixing ratios were significantly elevated below about 3 kilometers, and the intermediate ‘aerosol mix’ class showed intermediate dust levels, exactly as expected for diluted or partially mixed dust intrusions. Similarly, smoke and marine detections coincided with enhanced modeled concentrations of soot, organic carbon, sulfate, and sea salt, particularly during summer when wildfire plumes crossed Europe at higher altitudes.

The year-long record also revealed clear seasonal rhythms. Continental aerosols dominated, detected in more than half of all hours and peaking near 85 percent in July and August, when photochemical production of secondary particles intensifies. Smoke and marine aerosols appeared 10 to 27 percent of the time, mostly confined below one kilometer, consistent with sea-salt aerosols trapped in the marine boundary layer. Dust appeared mainly in spring and autumn, with winter dust layers reaching above 3 kilometers. Ice crystals showed a pronounced seasonal cycle with maxima in late winter and autumn, while liquid cloud droplets were by far the most common hydrometeor, occurring 20 to 70 percent of the time depending on the month.

A showcase example from February 2024 illustrates the algorithm’s power. During a single night, CONIOPOL tracked an elevated dust plume near the top of the boundary layer, distinguished an upper cloud layer of ice crystals from a lower layer of supercooled liquid water, and even captured a snow-to-rain transition near 2,200 meters altitude. Precipitation classification relies on depolarization thresholds derived from the data themselves: values below 0.01 indicate fog, drizzle, or mist; between 0.01 and 0.23, liquid rain; and above 0.41, heavily rimed graupel. Compared with a conventional rain gauge, the algorithm detected precipitation 5 to 13 percent more often, a positive bias partly attributed to the difficulty of separating drizzle and fog from true precipitation.

Sensitivity tests confirmed the algorithm’s resilience. Perturbing the backscatter calibration by up to 10 percent left most classifications unchanged, while depolarization variability proved to be the dominant source of uncertainty, mainly shifting observations between neighboring categories near fixed thresholds rather than producing arbitrary reassignments. The authors caution that the thresholds were developed for regional background conditions and long-range transported aerosols, and that fresh plumes near strong sources, such as erupting volcanoes or intense wildfires, may behave differently. They also note that a single site and a single year cannot fully validate the approach.

Even with those caveats, the implications are considerable. Polarization-enabled ceilometers are cheap, rugged, and already deployed in operational weather networks across Europe and beyond. CONIOPOL demonstrates that they can deliver scientifically meaningful aerosol typing around the clock, bridging the gaps between sparse satellite overpasses, campaign-based research lidars, and surface air-quality stations. The team envisions extending the algorithm to other CL61 sites and combining its output with the ACTRIS research infrastructure and the new EarthCARE satellite mission. If that happens, the humble ceilometer on the roof of a weather station may become one of the most valuable eyes humanity has on the ever-changing chemistry of the sky.

Subject of Research: Real-time aerosol and hydrometeor classification using single-wavelength polarization lidar

Article Title: New operational perspective to identify aerosols and hydrometeors in real-time with a pioneering algorithm (CONIOPOL) based on single-wavelength polarization lidar (CL61)

Article References: Laffineur, Q., Mangold, A., & Delcloo, A. W. (2026). New operational perspective to identify aerosols and hydrometeors in real-time with a pioneering algorithm (CONIOPOL) based on single-wavelength polarization lidar (CL61). Atmospheric Measurement Techniques, 19(19), 6415-6446. https://doi.org/10.5194/amt-19-6415-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6415-2026

Keywords: aerosols, lidar, ceilometer, depolarization, CONIOPOL, cloud phase, precipitation, Saharan dust, wildfire smoke, air quality, CAMS, atmospheric monitoring

The core challenge the team set out to address is one of the most consequential blind spots in atmospheric science. Aerosols, the tiny solid and liquid particles suspended in the air, are far from uniformly mixed. Their optical and microphysical properties can change dramatically with height, and particles in the planetary boundary layer often behave very differently from those in the free troposphere above. These vertical contrasts govern how aerosols scatter and absorb sunlight, how they interact with clouds, and how long they persist in the atmosphere. Yet the tools available to profile them each carry significant drawbacks. Ground-based and satellite lidars suffer from signal attenuation in dense layers and depend on retrieval assumptions about aerosol type. Research aircraft provide the most direct measurements but at operational costs that rule out routine use. UAVs and tethered balloons are limited in vertical range and payload.

AeroSonde was designed to fill that gap with an approach that is almost aggressively simple. The system pairs a standard Vaisala RS41 radiosonde with a compact add-on unit weighing just 175 grams including its housing, measuring 90 by 60 by 40 millimeters. At its heart sits a Sensirion SPS30 particulate matter sensor, a device that uses a 660-nanometer laser diode, a photodiode detector, focusing optics, and a small integrated fan to measure particle mass concentrations in the PM1, PM2.5, PM4, and PM10 fractions, along with data in five particle size bins. A Raspberry Pi RP2040 microcontroller manages data acquisition and streams measurements at one hertz to a ground station over a LoRaWAN radio link operating in the EU 863 to 870 megahertz band. The team confirmed reliable data reception even when the probe drifted more than 100 kilometers horizontally and more than 25 kilometers vertically from the receiver.

Crucially, the researchers did not simply trust the sensor out of the box. Over 22 days during the summer of 2025, five SPS30 units were run side by side in a laboratory at the Institute of Geophysics, University of Warsaw, and then calibrated against reference-grade instruments: an Aurora 4000 polar nephelometer for scattering measurements and a TSI Laser Aerosol Spectrometer 3340 for particle size. The inter-sensor agreement was strikingly tight, with average Pearson correlation coefficients of 0.97 for PM10 and PM1 when one-hour averaging was applied, and mean relative differences of less than 9 percent. Using a non-linear regression model grounded in Lorenz-Mie scattering simulations, the team converted the sensor’s mass readings into estimates of the aerosol scattering coefficient, achieving a coefficient of determination of 0.96 against the nephelometer with a root mean square error of just 5.1 inverse megameters, or 3.8 percent.

The calibration also revealed honest limits. The scattering Ångström exponent, an indicator of particle size distribution, and the effective particle radius could be estimated from the sensor’s typical particle size parameter, but with substantially larger uncertainties, limiting their use to qualitative discrimination between fine-mode and coarse-mode particles. The team also verified that the sensor’s fan maintained a stable speed of roughly 6000 revolutions per minute across pressures from 1000 down to 50 hectopascals, confirming that airflow through the measuring chamber requires no correction at altitude, and that the sensor’s response time of under five seconds supports a vertical resolution of 15 to 20 meters at typical balloon ascent rates of 3 to 4 meters per second.

The field campaigns put all of this to the test under genuinely challenging conditions. Three launches from the SolarAOT Radiative Transfer Station in Strzyżów, a rural background site on Niebylecka Hill, captured two episodes of long-range transport of biomass-burning aerosol from North America in June and August 2025, and a Saharan dust intrusion in late August. Back-trajectory modeling with the LAGRANTO tool, driven by ECMWF wind fields, traced the smoke plumes across central Canada and the Atlantic, while the ICAP multi-model ensemble indicated smoke optical depths exceeding 0.1 over Poland, accounting for 61 percent of the total column loading. During the dust event, trajectories showed air masses arriving from northern Africa, with dust contributing 58 percent of the total aerosol optical depth.

The comparison against independent remote sensing was the decisive test. Because the balloon drifts horizontally during its one-to-two-hour ascent, the team developed a spatiotemporal matching procedure that aligns each AeroSonde measurement with the lidar profile at the time the sampled air parcel passed over the station, using wind data from the co-launched radiosonde. When the resulting scattering coefficient profiles were compared with aerosol extinction profiles retrieved from a Raymetrics lidar using two variants of the Klett inversion, the Pearson correlations ranged from 0.74 to 0.81, with the agreement holding across all altitude ranges from the boundary layer up to above 8 kilometers. The positive bias between extinction and scattering was consistent with the expected contribution of aerosol absorption. Column-integrated aerosol optical depth derived from the AeroSonde profiles agreed with AERONET sun photometer values to within 0.01 to 0.05 for the biomass-burning cases and 0.11 for the dust event.

The case studies also revealed a physics-driven subtlety that the authors address candidly. During the Saharan dust sounding, both scattering and optical depth were underestimated relative to the sun photometer and lidar. Several factors conspire here: mineral dust absorbs light, the SPS30 measures at a scattering angle of roughly 90 degrees where the dust phase function reaches a minimum, and the sensor’s design is better suited to fine-mode particles than to the coarse particles above 5 micrometers that dominate dust. The team argues that aerosol-type-dependent calibration against reference instruments is essential, particularly in dust-influenced environments, and points to future upgrades including a single-channel aethalometer to retrieve absorption directly and an optical particle sizer with an extended size range.

What makes the result genuinely exciting is what it unlocks. Unlike lidar, the AeroSonde is not hampered by the geometric overlap limitations that blind ground-based systems in the lowest few hundred meters, allowing more reliable characterization of boundary-layer aerosol contributions, which the measurements showed ranged from 1 to 36 percent of total column optical depth during the transport events. Unlike drones, the passive ascent avoids flow-induced sampling artifacts and can reach into the stratosphere, opening a window on volcanic plumes and stratospheric aerosol that UAV platforms cannot access. The expendable nature of the platform, each launch sacrifices the sensor, is the trade-off, but at forty dollars per unit the economics remain compelling compared with aircraft campaigns costing orders of magnitude more.

The researchers frame the work as a proof of concept, and the limited number of profiles means broader validation across more aerosol regimes is still needed. But the implications reach well beyond one Polish hilltop. Vertical profiles of this kind provide an independent check on reanalysis products and model simulations of aerosol transport, and combining AeroSonde scattering measurements with Raman lidar extinction retrievals could eventually yield single-scattering albedo profiles, a key quantity for climate forcing calculations. If a forty-dollar sensor, a microcontroller, and a weather balloon can deliver scientifically useful profiles of the particles that shape our climate, the era of dense, affordable, three-dimensional aerosol monitoring may be closer than anyone expected.

Subject of Research: Balloon-borne aerosol profiling using calibrated low-cost optical particle sensors

Article Title: A new balloon-borne system for measuring the vertical variability of aerosol optical properties using low-cost sensors

Article References: Broda, M., Zawadzka-Mańko, O., Chiliński, M. T., Nurowska, K., Kłapiński, S., Makuch, P., & Markowicz, K. M. (2026). A new balloon-borne system for measuring the vertical variability of aerosol optical properties using low-cost sensors. Atmospheric Measurement Techniques, 19(18), 5989-6014. https://doi.org/10.5194/amt-19-5989-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-5989-2026

Keywords: aerosols, balloon-borne measurements, low-cost sensors, SPS30, lidar, atmospheric profiling, biomass burning aerosol, Saharan dust, aerosol optical depth, planetary boundary layer, radiosonde, atmospheric measurement techniques

The Early-Middle Pleistocene Transition, or EMPT, is one of the great unsolved puzzles of Quaternary science. Between about 1.4 and 0.4 million years ago, glacial cycles lengthened and became asymmetric, with gradual ice build-up and abrupt terminations. Scientists attribute this shift to a combination of orbital forcing and internal feedbacks, including long-term cooling, declining atmospheric carbon dioxide, the stripping away of soft regolith beneath ice sheets and the resulting dynamics of thicker, more persistent ice. Crucially, the start of this transition coincides with the first known hominin occupations in Western Europe, at sites in Italy and the Iberian Peninsula dated to around 1.4 to 1.2 million years ago. Understanding the environmental backdrop against which the earliest Europeans lived makes the Mediterranean, a corridor for dispersal and a region exquisitely sensitive to climate change, an indispensable natural archive.

The team, led by Maé Catrain of the Muséum National d’Histoire Naturelle in Paris, focused on a depth interval of the ODP 976 core spanning 281.23 to 256.53 metres composite depth, with an age model anchored by nannofossil events, sapropel layers and correlation with global benthic oxygen isotope stacks. The pollen record, previously established by the same group, captures 31 plant families and 72 genera distributed across the classic altitudinal belts of Mediterranean vegetation, from thermo-Mediterranean olive and pistachio woodlands to supra-Mediterranean pine and fir forests. Alternations between temperate taxa such as deciduous oak and steppe indicators like Artemisia, Amaranthaceae and Ephedra trace the rhythm of warm-humid interglacials and cold-dry glacials across the studied interval.

To translate vegetation into numbers, the researchers applied five independent climate reconstruction methods to the pollen data: the modern analogue technique, weighted averaging partial least-squares regression, random forest, boosted regression trees and the climatic amplitude method. Each rests on a different mathematical foundation, from machine learning bootstrapping and boosting to the overlap of climatic tolerances of around 135 taxa calibrated on thousands of modern pollen spectra. Importantly, the climatic amplitude method can incorporate relict taxa that have since vanished from the Mediterranean, such as Tsuga, Zelkova and Cathaya, a decisive advantage for a period as ancient as the late Early Pleistocene. A non-parametric PERMANOVA test confirmed that, for most stages, the methods produced statistically consistent reconstructions of mean annual air temperature and precipitation, lending confidence to the overall picture.

The results reveal that MIS 37, between about 1245 and 1215 thousand years ago, was the warmest interval of the entire study period, with temperate forests reaching roughly 35 percent representation, steppes shrinking to about 10 percent, warm-water foraminifera peaking and sea surface temperatures climbing to around 16.5 degrees Celsius. This finding challenges the conventional view of MIS 31, often dubbed the super interglacial, as the warmest stage of the era. While MIS 31 is associated with maxima in obliquity, eccentricity and insolation and has been linked to unusual warmth at high latitudes, the Alboran record shows it as comparatively modest, closer to MIS 33 and MIS 35 than to the pronounced warmth of MIS 37. The result echoes earlier observations from Iberian margin cores U1385 and U1387, suggesting that the Atlantic exerted a moderating influence on the western basin.

On the glacial side of the ledger, MIS 34, from roughly 1141 to 1114 thousand years ago, emerges as the most severe cold phase, with steppe expansion to about 28 percent, forest regression to 10 percent, peaks of the polar-affinity foraminifera Neogloboquadrina pachyderma reaching 40 to 70 percent, and reconstructed annual air temperatures near 9 degrees Celsius, corroborated by heavy oxygen isotope values of up to 1.62 per mille. Sea surface temperatures in the glacial stages fell as low as 9.5 degrees, far below the modern value of 17.7 degrees for the region. The synchrony between steppe pollen and cold-water foraminifera indicates that Iberian steppe landscapes were driven not only by aridity but also by low temperatures, a subtle but important distinction for interpreting early hom habitats.

Perhaps the most striking discovery comes from the clay mineralogy. Analysed by X-ray diffraction on oriented slides, the clay assemblages are dominated by illite and smectite, with chlorite, kaolinite and minor palygorskite. Around 1140 thousand years ago, at the transition between MIS 35 and MIS 34, the record shows an abrupt shift: smectite rises sharply to average values near 42 percent while illite, kaolinite and chlorite decline, and the illite-to-kaolinite ratio climbs. Because illite dominates dust from the northern Sahara while kaolinite becomes prevalent toward the Sahel, the rising ratio points to intensified Saharan dust supply and developing aridity in North Africa. The simultaneous smectite shift, which cannot be explained by local rivers or sea-level changes, points to a reorganisation of oceanic advection, likely involving smectite-rich sediments recirculated through the Strait of Gibraltar by the Mediterranean outflow and the Western Alboran Gyre. Notably, this signal fades eastward and is absent from Ionian and Levantine records, suggesting the change is tied to Atlantic circulation and attenuates into the central Mediterranean.

The land-sea comparison across basins reveals a Mediterranean that was climatically fragmented. At Montalbano Jonico in southern Italy, interglacial forest development peaked in MIS 31 and 35 rather than MIS 37, and the foraminiferal assemblages are dominated by the cold-water, productivity-loving Turborotalita quinqueloba, indicating warmer but still cool central Mediterranean waters less influenced by cold Atlantic inflow. Within the Western Mediterranean itself, the marine ODP 976 record, the continental Palominas section in southern Spain and Monte San Giorgio in Sicily show broadly consistent temperature oscillations, but precipitation patterns diverge: MIS 34 was wet at Monte San Giorgio, MIS 36 wet at Palominas, while the Alboran record shows dry glacials throughout. These contrasts paint a picture of steep regional gradients shaped by the interplay of Atlantic, monsoonal and Mediterranean circulation systems.

One of the study’s most consequential conclusions concerns the relative roles of temperature and rainfall. Pollen-based temperature reconstructions systematically run colder than estimates derived from faunal assemblages at early hominin sites in southern Spain, such as Barranco León, Fuente Nueva 3, Quibas and Cueva Victoria, a discrepancy the authors attribute partly to the regional averaging inherent in marine pollen records, which integrate vegetation from coasts to mountains across both Spain and Morocco, and partly to methodological limitations when deciduous oak dominates the assemblage. Precipitation reconstructions, by contrast, align remarkably well between pollen and faunal archives, converging around 700 to 750 millimetres per year at several sites. This consistency suggests that vegetation and climate in the southwestern Mediterranean were governed more by variations in rainfall and drought than by temperature, a finding with clear implications for modelling the environments encountered by the first hominins to venture into Western Europe.

The team emphasises that this window, spanning MIS 37 to 31, captures only the opening act of the Early-Middle Pleistocene Transition, including the exceptionally long 75,000-year MIS 36-35 cycle that foreshadowed the coming 100,000-year world. Extending this kind of integrated, multi-method analysis across the full transition is the declared next step, both to disentangle the mechanisms behind the lengthening of climate cycles and to reconstruct the environmental stage on which early humans expanded into Europe. For now, the Alboran mud has delivered a rare, precisely dated snapshot of a world in climatic flux, where a sudden shift in dust and ocean circulation at 1140 thousand years ago marks the moment the Western Mediterranean began its long march toward the ice ages of the later Pleistocene.

Subject of Research: Paleoclimate and paleoenvironmental reconstruction in the Western Mediterranean during the late Early Pleistocene

Article Title: Paleoenvironmental and paleoclimatic reconstruction in the Western Mediterranean during the Late Early Pleistocene

Article References: Catrain, M., Fauquette, S., Peyron, O., Combourieu-Nebout, N., Lebreton, V., Fischer-Fries, M., Robles, M., Bout-Roumazeilles, V., Richard, P., Delattre, M., Dubost, L., Joannin, S., Viala, A., Altolaguirre, Y., Suc, J.-P., Lepelletier, J., & Moncel, M.-H. (2026). Paleoenvironmental and paleoclimatic reconstruction in the Western Mediterranean during the Late Early Pleistocene. Climate of the Past, 22(9), 1631-1653. https://doi.org/10.5194/cp-22-1631-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1631-2026

Keywords: Mediterranean climate, Early-Middle Pleistocene Transition, ODP Site 976, Alboran Sea, pollen analysis, planktonic foraminifera, clay mineralogy, paleoclimate reconstruction, Marine Isotope Stage 31, Saharan dust, hominin dispersal, Climate of the Past

At the heart of the study is a measurement called aerosol optical depth, or AOD, which quantifies how much sunlight is removed from a beam passing through the atmosphere by particles suspended in the air. An AOD of 0.1 describes relatively clean skies, while values above 1.0 indicate conditions so hazy that the sun itself can appear dimmed and diffuse. The researchers worked with daily retrievals from the MODIS instrument aboard NASA’s Terra satellite, which has been scanning the Earth since early 2000. Raw satellite AOD products, however, carry well-documented biases, particularly over the bright surfaces and complex aerosol mixtures of West Africa. To correct these errors, the team built a machine learning framework known as Gradient Boosting Quantile Regression, trained against ground truth from fourteen AERONET sun photometer stations spread across nine West African countries.

The correction proved dramatic. At the Nigeria-Ilorin AERONET site, the machine learning adjustment shrank the raw mean bias of MODIS retrievals from minus 0.252 to just minus 0.015, a near-perfect agreement on average. More importantly, the fraction of satellite observations falling within the expected error envelope of ground measurements jumped from a dismal 23.0 percent to 95 percent. When the team tested the model’s ability to generalize to locations it had never seen, using a leave-one-site-out validation strategy, the expected error agreement remained a respectable 74.5 percent. This bias-corrected, boundary-clipped daily record gives Nigeria something it has never had before: a trustworthy, continuous, quarter-century chronicle of its atmospheric particle loading, suitable for everything from climate model evaluation to public health epidemiology.

So what does 25 years of data actually show? Perhaps surprisingly, the national annual trend is essentially flat. The linear slope across the full record is minus 0.0020 per year, with a p-value of 0.226, meaning the change cannot be distinguished from random year-to-year variability. But seasonal decomposition told a subtler story. The dry season, when Saharan dust and harmattan winds dominate, shows a suggestive decline of minus 0.0043 per year, equivalent to roughly a 16 percent reduction over the study period. This trend sits just at the edge of conventional statistical significance, with a p-value of 0.059 after accounting for autocorrelation through lag-1 pre-whitening, so the authors are careful to describe it as indicative rather than definitive. The wet season shows only a marginal, non-significant decrease of minus 0.0012 per year. In other words, Nigeria’s aerosol environment has been remarkably stable, with at most a gentle cleaning of its dustiest months.

The seasonal rhythm itself, however, is unmistakable. The monthly climatology is predominantly unimodal, peaking in February at a mean AOD of 0.865, when dry northeasterly harmattan winds carry desert dust deep into the country, and bottoming out in November at 0.370, as the monsoon transition washes the atmosphere clean. That February peak is more than double the November minimum, a swing that shapes everything from solar power output to aviation visibility to respiratory disease patterns across the region. Previous research has linked these dust episodes to meningitis outbreaks in the Sahel, and the harmattan season has been implicated in altered transmission dynamics of respiratory infections, making the timing and intensity of this annual maximum a matter of direct public health consequence.

To bring order to the daily chaos of aerosol variability, the researchers applied an unsupervised K-means clustering algorithm to the full record, letting the data sort itself into distinct atmospheric states. Three regimes emerged. Clean days, with a mean AOD of 0.399, account for 57.7 percent of the record. Transitional days, averaging 0.647, make up 31.6 percent. And then there are the Extreme days, averaging a staggering AOD of 1.163, which occupy 10.7 percent of all days in the record. These extreme events are overwhelmingly a dry season phenomenon: they occur on 18.1 percent of dry season days but only 3.2 percent of wet season days. That asymmetry confirms quantitatively what residents of Kano or Maiduguri know viscerally: the harmattan is when the air becomes a hazard.

One of the most practically useful contributions of the study is its characterization of how often extreme aerosol events recur. Using empirical return period analysis, the team calculated that days with AOD exceeding 1.0 arrive on average every 12.5 days, based on 711 exceedances across the record, with a 95 percent confidence interval of 11.6 to 13.4 days. Thresholds above 1.5 are crossed roughly every 61 days, with 145 exceedances recorded. And truly exceptional events, with AOD above 2.0, occur about once every 6.06 years, though with only four such events in 25 years the confidence interval is wide, spanning 3.03 to 24.25 years. These recurrence benchmarks give atmospheric hazard planners, health officials, and solar energy operators a climatological yardstick for what counts as a normal bad day versus a once-in-a-decade crisis.

The spatial structure of Nigeria’s aerosol burden is equally striking. A global spatial autocorrelation analysis yielded a Moran’s I of 0.949, with a p-value of 0.001 and a z-score of 163.9, confirming that aerosol loading is not randomly distributed but intensely clustered in space. Local Indicators of Spatial Association, corrected for the false discovery rate, flagged 40.1 percent of grid cells as statistically significant hotspots or coldspots. The pattern reveals a stable High-High hotspot belt across Southern and Southwestern Nigeria, where dense populations, urban pollution, and biomass burning combine, and persistent Low-Low spots in the Northeast. This geographic fingerprint has remained stable across the record, suggesting that the underlying drivers, from dust transport corridors to emission sources, are entrenched features of the regional environment rather than transient anomalies.

The study also probed the atmosphere’s memory, asking whether today’s aerosol conditions predict tomorrow’s. The answer is a strong yes, at least over short horizons. The lag-1 daily autocorrelation is 0.75, decaying to 0.21 by lag 30, indicating that aerosol states persist for days to weeks before the signal fades. An ARIMA (2,0,2) time series model successfully captured these synoptic dynamics, achieving a pseudo R-squared of 0.58. This short-range predictability has real-world value: it means that once a major dust outbreak begins, forecasters can anticipate with reasonable confidence that hazy conditions will persist for several days, giving hospitals, airports, and power grid operators time to prepare.

Taken together, these findings establish a rigorous long-term atmospheric baseline for a country whose air quality has historically been monitored far less intensively than its population warrants. The stability of the national trend, punctuated by a possible dry season improvement, offers a measure of reassurance, but the sheer frequency of extreme events, one in every nine days nationally and nearly one in five during the dry season, underscores the scale of the ongoing exposure. As West Africa’s combustion emissions continue to grow and climate change reshapes dust mobilization across the Sahara, records like this one will serve as the indispensable reference point against which future change is measured. For Nigeria’s policymakers, the message is clear: the dustiest skies are predictable, clustered, and recurring on a known clock, and that predictability is itself a tool for protecting public health.

Subject of Research: Long-term satellite analysis of aerosol optical depth dynamics, seasonal regimes, and extreme dust events over Nigeria from 2000 to 2024

Article Title: A 25-year analysis of aerosol optical depth dynamics over Nigeria (2000–2024): spatial-temporal dynamics, seasonal regimes, and extreme event characterization

Article References: Ekpe, E. E., Shi, C., Yin, S., Wang, H., Zhao, Y., Letu, H., & Raihan, M. Z. U. (2026). A 25-year analysis of aerosol optical depth dynamics over Nigeria (2000–2024): spatial-temporal dynamics, seasonal regimes, and extreme event characterization. Air Quality, Atmosphere & Health, 19(10), Article 224. https://doi.org/10.1007/s11869-026-02111-4

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02111-4

Keywords: aerosol optical depth, Nigeria, MODIS Terra, machine learning bias correction, AERONET, Saharan dust, harmattan, air quality, spatial autocorrelation, extreme events, seasonal climatology, West Africa