the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 May 2026
Source-Dependent Optical and Mineral Signatures of Dust Outbreaks over the Mediterranean
Abstract. Dust events frequently affect the Mediterranean Basin, however, the evolution of their optical and microphysical properties during transport remains poorly characterized. This study examines four major dust outbreaks in 2021–2022 affecting the Mediterranean, originating from the Eastern, Western, and Central Sahara and the Middle East. Combining ground-based AERONET sun photometers (24 stations), satellite (IASI, MODIS MIDAS) dust optical depth (DOD) data, and HYSPLIT back-trajectories, we track these events across multiple Mediterranean sites. Results reveal clear regional differences in dust optical properties, such as aerosol optical depth, single scattering albedo, and asymmetry factor, arising from source regions and transport processes. Saharan events are dominated by coarse, scattering mineral dust, while the Middle East event featured finer, more absorbing particles, likely influenced by anthropogenic sources. MIDAS DOD-to-AOD ratios indicate that only one East-Central Saharan event maintained high dust fractions (DOD-to-AOD > 0.8), suggesting relatively pure dust, while other events exhibited stronger spatial variability, with the Middle East event showing the lowest ratios, reflecting enhanced mixing with anthropogenic or marine aerosols. A regional case study in Cyprus using in situ elemental and absorption
measurements shows that Middle East dust, despite lower mass concentrations, exhibits stronger absorption than Saharan dust.
METAL-WRF mineralogical simulations indicate broadly similar dominant mineral fractions (silicates and calcium-rich minerals) across events, suggesting that optical variability was mainly driven by dust-to-total aerosol ratio and mixing state rather than mineralogy. UAV-based composition data further validate modeled variability, although discrepancies in aluminum and magnesium highlight limitations in current dust representations.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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- RC1: 'Comment on egusphere-2026-1956', Anonymous Referee #1, 31 May 2026 reply
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- 1
This manuscript investigates the source-dependent optical, microphysical, and mineral signatures of four major dust outbreaks affecting the Mediterranean Basin during 2021–2022. The study combines a wide range of observational and modeling tools, including AERONET sun-photometer retrievals, MIDAS/MODIS dust optical depth, IASI dust products, HYSPLIT back-trajectories, lidar-based vertical information, ground-based in situ observations in Cyprus, UAV-based composition measurements, and METAL-WRF mineralogical simulations. The topic is scientifically relevant as mentioned in the manuscript that the Mediterranean is strongly affected by dust from both Saharan and Middle Eastern sources, and the optical properties of transported dust are essential for assessing radiative effects, air quality impacts, and model representation of dust aerosol. The manuscript is generally well organized and contains a valuable multi-platform analysis. A major issue is that some conclusions regarding the relative roles of mineralogy, mixing state, and anthropogenic influence are plausible but not yet fully demonstrated. Therefore, I would recommend publication if the following comments could be properly addressed after moderate revision.
Major Comments
#1. One of the key conclusions of the manuscript is that the dominant mineral fractions simulated by METAL-WRF are broadly similar across events, and therefore the observed optical variability is mainly driven by dust-to-total aerosol ratio and mixing state rather than mineralogical differences. This conclusion is not fully demonstrated. The mineralogical analysis is available mainly for selected events and stations, whereas the optical analysis covers a broader set of AERONET sites and events. Moreover, METAL-WRF mineralogy is itself subject to uncertainties in source mineral maps, emission size distribution, transport, and deposition. The authors should more clearly distinguish between what is directly supported by the combined observations and what is inferred from the model. If possible, it would be helpful to provide a more quantitative comparison between mineral composition, DOD-to-AOD ratio, FMF, SSA, and ASY across the events. Or the conclusion should be phrased more cautiously, emphasizing that the current evidence suggests, rather than definitively proves, that aerosol mixing and dust fraction dominate the observed optical variability.
#2. Another issue is that the manuscript states the Middle East event is characterized by finer and more absorbing particles, likely influenced by anthropogenic sources. This interpretation is reasonable and is supported by higher FMF, lower DOD-to-AOD ratios, lower ASY, and stronger absorption observed in Cyprus. Nevertheless, the evidence remains partly circumstantial. The manuscript would be strengthened if the authors provided additional support for the anthropogenic influence, such as co-variation with black carbon, organic aerosol, sulfate/nitrate, CO, or known emission regions along the transport pathways. It is also necessary to briefly discuss the source region and sector (e.g., oil industry) of anthropogenic pollution. It will be better if other possibilities, including source-specific mineral absorption, aging, coating by secondary aerosols, or mixed dust–pollution–marine aerosol states, could be discussed more systematically.
#3. The study focuses on four major dust outbreaks in 2021–2022, selected from widespread events affecting multiple AERONET stations. This is a reasonable design, but the conclusions are sometimes written in a way that may imply more general source-dependent behavior. Since only one Middle East event and a limited number of Saharan events are analyzed, it is important to clarify whether the findings are representative of broader source-region differences or mainly characteristic of these selected episodes. The authors should add a short discussion on the limitations of the case-study approach and specify which conclusions are robust across the selected events and which require confirmation with a larger event climatology.
Specific Comments
#1. The criterion for AERONET inversion quality is described as sky error < 6%, whereas the figure caption for Fig. 4 mentions sky error < 5%. Please check and make the threshold consistent throughout the text and figure captions.
#2. The station ordering by distance from source is useful, but for event D the manuscript acknowledges that the sequence is not straightforward because multiple sub-events occurred. In this case, the interpretation of evolution with distance should be treated cautiously.
#3. In Fig. 3, the manuscript should briefly discuss the uncertainty of assigning aerosol types based only on AOD–AE space, particularly in mixed dust–pollution and dust–marine aerosol conditions.
#4. The Cyprus case study is one of the strongest parts of the manuscript. It would be helpful to make the comparison between events A, C, and D more explicit in terms of dust mass, absorption coefficient, and elemental abundance. A compact summary table may improve clarity.
#5. In Fig. 11, the text states that event C exhibits the highest absorption “∼50% across the entire spectral range,” but the unit and meaning of this percentage are not clear. Please revise this sentence and specify whether this refers to a relative increase compared with Saharan events.
#6. The METAL-WRF analysis indicates broadly similar mineral fractions among events, but the comparison with UAV-based observations shows discrepancies in Al and Mg. These discrepancies should be discussed in more detail, because they may affect the conclusion about mineralogical similarity.
#7. The manuscript should more clearly explain how the dust-layer altitude used for METAL-WRF mineralogical averaging was selected and how sensitive the mineral fractions are to this vertical selection.
#8. Line#217 “at a vertical spacing of ±”, seems missing a number here.