Mid- and Far-Infrared Spectral Signatures of Mineral Dust from Low- to High-Latitude Regions: significance and implications

Di Biagio, Claudia; Bru, Elisa; Orta, Avila; Chevaillier, Servanne; Baldo, Clarissa; Bergé, Antonin; Cazaunau, Mathieu; Lafon, Sandra; Nowak, Sophie; Pangui, Edouard; Andreae, Meinrat O.; Dagsson-Waldhauserova, Pavla; Dintwe, Kebonyethata; Kandler, Konrad; King, James S.; Chaput, Amelie; Okin, Gregory S.; Piketh, Stuart; Saeed, Thuraya; Seibert, David; Shi, Zongbo; Williams, Earle; Sellitto, Pasquale; Formenti, Paola

doi:https://doi.org/10.5194/egusphere-2025-3512

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https://doi.org/10.5194/egusphere-2025-3512

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30 Jul 2025

| 30 Jul 2025

Mid- and Far-Infrared Spectral Signatures of Mineral Dust from Low- to High-Latitude Regions: significance and implications

Claudia Di Biagio, Elisa Bru, Avila Orta, Servanne Chevaillier, Clarissa Baldo, Antonin Bergé, Mathieu Cazaunau, Sandra Lafon, Sophie Nowak, Edouard Pangui, Meinrat O. Andreae, Pavla Dagsson-Waldhauserova, Kebonyethata Dintwe, Konrad Kandler, James S. King, Amelie Chaput, Gregory S. Okin, Stuart Piketh, Thuraya Saeed, David Seibert, Zongbo Shi, Earle Williams, Pasquale Sellitto, and Paola Formenti

Abstract. Mineral dust absorbs and scatters solar and infrared radiation, thereby affecting the radiance spectrum at the surface and top-of-atmosphere and the atmospheric heating rate. While half of the outgoing thermal radiation is emitted in the far infrared (FIR, 15–100 μm), knowledge of the optical properties and thermal radiative effects of dust is currently limited to the mid-infrared region (MIR, 3–15 μm). In this study we performed pellet spectroscopy measurements to evaluate the MIR and FIR contribution to dust absorbance and explore the variability and spectral diversity of the dust signature within the 2.5–25 μm range. Thirteen dust samples re-suspended from parent soils with contrasting mineralogy were investigated, including low and mid latitude dust (LMLD) sources in Africa, America, Asia, and Middle East, and high latitude dust (HLD) from Iceland. Results show that the absorbance of dust in the FIR up to 25 μm is comparable in intensity to that in the MIR. Also, spectrally different absorption (position and shape of the peaks) is observed for HLD compared to LMLD, due to differences in mineralogical composition. Corroborated with the few available literature data on absorption properties of natural dust and single minerals up to 100 μm wavelength, these data suggest the relevance of MIR and FIR interactions to the dust radiative effect for low to high latitude sources. Furthermore, the dust spectral signatures in the MIR and FIR could potentially be used to characterise the mineralogy and differentiate the origin of airborne particles based on infrared remote sensing observations.

How to cite. Di Biagio, C., Bru, E., Orta, A., Chevaillier, S., Baldo, C., Bergé, A., Cazaunau, M., Lafon, S., Nowak, S., Pangui, E., Andreae, M. O., Dagsson-Waldhauserova, P., Dintwe, K., Kandler, K., King, J. S., Chaput, A., Okin, G. S., Piketh, S., Saeed, T., Seibert, D., Shi, Z., Williams, E., Sellitto, P., and Formenti, P.: Mid- and Far-Infrared Spectral Signatures of Mineral Dust from Low- to High-Latitude Regions: significance and implications, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-3512, 2025.

Received: 25 Jul 2025 – Discussion started: 30 Jul 2025

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RC1:
'Comment on egusphere-2025-3512', Anonymous Referee #1, 04 Sep 2025
This paper articulates and fills a clear gap in the literature: the scarcity of measurements of mineral dust optical properties beyond a wavelength of ~15 um. The paper is also well written and the methodology rigorous, following previous very well-regarded work by the main authors. I do have two important comments about the framing of the paper that should be addressed before publication.
Major comments:
The authors take two samples of Icelandic dust to be representative of high-latitude dust. However, because Iceland is a volcanic island the mineralogical composition of its dust is likely to be quite different (more basaltic and darker) than that of other high-latitude sources, which are normally more silicate and quartz rich. The mineralogy of Icelandic dust might thus be an outlier among high-latitude dust sources and should not be taken as representative of high-latitude dust sources. I recommend that the authors replace “high latitude dust” with “Icelandic dust” in the title, abstract, and so forth. If the authors do want to take the Icelandic dust as representative then they should provide evidence that this is reasonable to do, for instance from a comparison of the mineralogy of Icelandic dust to that of other high-latitude sources.

The authors state at various locations in the manuscript that the optical properties in the far infrared region (beyond 15 um) are important to climate and remote sensing. However, conventional wisdom is that (dust) absorption (well) outside of the atmospheric window does not matter much because the atmosphere is very opaque beyond 15 um due to abundant absorption by water vapor. So I have a hard time imagining that the dust interaction with radiation would normally matter much for Earth’s radiation budget. Two exceptions worth highlighting are for dust in the upper troposphere and for dust in the Arctic (mentioned in the paper), but the dust concentration is very low in both locations. A clearer argument could be made for remote sensing, as long as it’s made clear that this would only work in the upper troposphere / stratosphere and the dry poles. If the authors do want to make the point that dust interactions with radiation beyond 15 um matter for Earth’s climate, I recommend they show (1) Earth’s outgoing radiation as a function of wavelength and (2) the height in the atmosphere at which the optical depth to TOA equals 1 (the emission height) for one or more standard profiles (e.g., mid-latitude summer). That can make at least a qualitative argument that the dust optical properties for wavelengths larger than 15 um matter for Earth’s radiation budget.

Minor comments:
Line 68: the 88% here seems very specific. My recollection of these papers also is that the error bar on the net DRE overlapped with zero, meaning that the dust-radiation interactions in the infrared could be >100% of those in the shortwave spectrum.

Many citations that are part of the sentence are listed with parentheses, which is a bit tedious to read. For instance, line 263 should probably be “The Sadrian et al. (2023) dataset…” instead of “The (Sadrian et al., 2023) dataset…”
Citation: https://doi.org/10.5194/egusphere-2025-3512-RC1
RC2:
'Comment on egusphere-2025-3512', Anonymous Referee #2, 13 Sep 2025
The paper discusses the importance of further measurements of dust spectral properties in the mid-infrared (~3–15 μm) and, particularly, in the far-infrared (~15–100 μm) regions, as they may have a substantial effect on total dust direct radiative forcing and climate model estimations. The study uses 13 dust samples from low-, mid-, and high-latitude regions, employing dust-KBr pellets to obtain absorbance spectra. The results show that the spectral signatures of low- and mid-latitude dust are in good agreement with past studies. However, dust collected from high-latitude regions exhibits different spectral characteristics due to its distinct composition, which is primarily dominated by amorphous minerals.
This study makes a valuable contribution by adding new dust spectral data from low-, mid-, and high-latitude regions to the global library of mineral dust spectra. As more remote sensing instruments are being developed to cover the mid- and far-infrared spectra and are planned to monitor aerosol particles, studies like this can significantly help reduce uncertainties in estimating the radiative effects of dust and in quantifying their abundance through remote sensing retrievals.

Comments:
For Figure 2: Since the absorbance spectra for the samples have already been offset, you can label the y-axis as “Absorbance (offset for clarity).” This is a more traditional approach and improves the overall clarity of the plot.

For line 245: where it is mentioned “A third band at 14 μm is potentially associated with anorthite or augite, which are the dominant minerals by mass, but no literature spectral data for these minerals are available at these wavelengths to confirm this attribution.”

You can find the transmission spectra of pure minerals (~ 2.5-25 um), including anorthite and augite, for comparison purposes in: Salisbury, J. W., Walter, L. S., Vergo, N., & Daria, D. M. (1991). Infrared (2.1–25 μm) spectra of minerals. Johns Hopkins University Press. ISBN: 978-0801844232.
That would be good if you could remove the white columns on both sides of Fig. 1.

Line 275: edit LLMD. It should be LMLD.
Citation: https://doi.org/10.5194/egusphere-2025-3512-RC2

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https://doi.org/10.5194/egusphere-2025-3512-supplement

Data sets

Absorbance spectra of Mineral Dust from Low- to High-Latitude Regions in the Mid- and Far-Infrared spectral range (2.5-25 µm) Claudia Di Biagio et al. https://doi.org/10.57932/905eff0b-d508-4aad-a422-5708e3132790

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Short summary

Spectroscopy measurements show that the absorbance of dust in the far-infrared up to 25 μm is comparable in intensity to that in the mid-infrared (3–15μm) suggesting its relevance for dust direct radiative effect. Data evidence different absorption signatures for high and low/mid latitude dust, due to differences in mineralogical composition. These differences could be used to characterise the mineralogy and differentiate the origin of airborne dust based on infrared remote sensing observations.


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