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

Abstract. Mineral dust is a major contributor to atmospheric aerosols and plays a complex role in Earth’s radiation budget. However, large uncertainties remain in quantifying its direct radiative effects (DRE), primarily due to poorly constrained absorption properties. Most estimates rely on remote sensing or laboratory studies, with few in situ measurements of freshly emitted dust. This study presents field data on dust optical properties collected during active dust emission in the Moroccan Sahara (September 2019). During high emission events, optical properties aligned with previous Saharan dust studies. Single scattering albedo (SSA) for PM2.5 (PM10) was 0.95 (0.94) at 370 nm and 0.97 (0.96) at 660 nm. Coarse particles contributed to negative scattering and SSA Ångström exponents (SSAAE), and absorption Ångström exponents (AAE) reached up to 2.5 (2.0), indicating strong wavelength-dependent absorption from iron oxides. The asymmetry parameter (g) was 0.7 (0.65) at 520 nm, while backscatter fraction (BF) was 0.11 (0.13), showing coarse dust’s impact on scattering. Mass absorption efficiency (MAE) decreased from 0.30 to 0.15 m2 g-1 at 370 nm with increasing particle size. Mass scattering efficiency (MSE) shifted toward longer wavelengths for larger particles. A key result is our consistent retrieval of the imaginary refractive index (k) across wavelengths, accounting for dust’s irregular shape. Retrieved k increased linearly with particle asphericity, rising from 0.0011 for spheres to 0.0016 for triaxial ellipsoids at 520 nm—a 60 % enhancement. These findings highlight the need for realistic and consistent particle shapes and k in satellite retrievals and climate models.

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Publication Details

Journal
Atmospheric chemistry and physics
Published
2026-10-14
DOI
https://doi.org/10.5194/acp-26-14531-2026
Citations
1
Primary Topic
Atmospheric aerosols and clouds
Type
article
Field-Weighted Citation Impact
3.21

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article

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

Martina Klose, Agnesh Panta, Jesús Yus-Díez, Adolfo González-Romero et al.
1 citations
Atmospheric chemistry and physics
Atmospheric aerosols and clouds
3.21
article

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

Martina Klose, Agnesh Panta, Jesús Yus-Díez, Adolfo González-Romero, Andrés Alástuey, Carlos Pérez García‐Pando, Marco Pandolfi, Xavier Querol, Cristina González-Flórez, Marı́a Gonçalves Ageitos, Jerónimo Escribano, Cristina Reche, Konrad Kandler, Matic Ivančič, Martin Rigler, Vicenzo Obiso
article en
1 citations

Abstract

Abstract. Mineral dust is a major contributor to atmospheric aerosols and plays a complex role in Earth’s radiation budget. However, large uncertainties remain in quantifying its direct radiative effects (DRE), primarily due to poorly constrained absorption properties. Most estimates rely on remote sensing or laboratory studies, with few in situ measurements of freshly emitted dust. This study presents field data on dust optical properties collected during active dust emission in the Moroccan Sahara (September 2019). During high emission events, optical properties aligned with previous Saharan dust studies. Single scattering albedo (SSA) for PM2.5 (PM10) was 0.95 (0.94) at 370 nm and 0.97 (0.96) at 660 nm. Coarse particles contributed to negative scattering and SSA Ångström exponents (SSAAE), and absorption Ångström exponents (AAE) reached up to 2.5 (2.0), indicating strong wavelength-dependent absorption from iron oxides. The asymmetry parameter (g) was 0.7 (0.65) at 520 nm, while backscatter fraction (BF) was 0.11 (0.13), showing coarse dust’s impact on scattering. Mass absorption efficiency (MAE) decreased from 0.30 to 0.15 m2 g-1 at 370 nm with increasing particle size. Mass scattering efficiency (MSE) shifted toward longer wavelengths for larger particles. A key result is our consistent retrieval of the imaginary refractive index (k) across wavelengths, accounting for dust’s irregular shape. Retrieved k increased linearly with particle asphericity, rising from 0.0011 for spheres to 0.0016 for triaxial ellipsoids at 520 nm—a 60 % enhancement. These findings highlight the need for realistic and consistent particle shapes and k in satellite retrievals and climate models.

Atmospheric chemistry and physicsVol. 26(19)
Karlsruhe Institute of Technology (DE), Institució Catalana de Recerca i Estudis Avançats (ES), European Centre for Medium-Range Weather Forecasts (GB), University of Nova Gorica (SI), Barcelona Supercomputing Center (ES), Technische Universität Darmstadt (DE), Danish Meteorological Institute (DK), Institute of Environmental Assessment and Water Research (ES), Aerosol d.o.o. (Slovenia) (SI), Universitat Politècnica de Catalunya (ES)
Deutsche Forschungsgemeinschaft, AXA Research Fund, Ministerio de Economía y Competitividad, Helmholtz Association, European Research Council, H2020 European Research Council, HORIZON EUROPE Marie Sklodowska-Curie Actions, HORIZON EUROPE Climate, Energy and Mobility, NextGenerationEU
Climate action
Openalex Percentile: Top 16%
Atmospheric aerosols and clouds
3.21
DOIOpen Access PDFOpenAlex
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