Optical properties and shape-dependent retrievals of the imaginary part of the refractive index of freshly emitted Saharan dust
4 affiliations
- 1University of Nova Gorica (Nova Gorica, Slovenia)
- 2Institute of Environmental Assessment and Water Research (Barcelona, Spain)
- 3European Centre for Medium-Range Weather Forecasts (Reading, United Kingdom)
- 4Barcelona Supercomputing Center (Barcelona, Spain)
Abstract
Mineral dust is a major contributor to atmospheric aerosol mass and plays a complex role in Earth's radiation budget. However, large uncertainties remain in quantifying its direct radiative effects, partly because dust absorption is poorly constrained and in situ measurements of freshly emitted dust remain scarce. Here, we present high temporal resolution measurements of dust optical properties collected during active dust emission events in the Moroccan Sahara in September 2019. During dust emission periods, the optical properties aligned with previous Saharan dust studies. Single scattering albedo (SSA) for PM 2.5 (PM ₁₀ ) 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), consistent with stronger wavelength-dependent absorption by iron oxides. The asymmetry parameter ( g ) was 0.7 (0.65) at 520 nm, and the 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 m ² g ⁻¹ at 370 nm with increasing particle size. Mass scattering efficiency (MSE) shifted towards longer wavelengths for larger particles. We further retrieved the imaginary part of the complex refractive index, k , for the PM 2.5 dust-event period under prescribed assumptions for particle shape, particle density, and the real part of the refractive index. For the central density assumption, retrieved k at 520 nm ranged from 0.00104 for spheres to 0.00145 for triaxial ellipsoids, corresponding to a 40 % increase. These results show that particle-shape assumptions can substantially affect retrieved k , and therefore need to be considered when interpreting in situ, remote-sensing, and model estimates of dust optical properties.
Versions
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BibTeX
@article{yusdiez2026optical,
title = {{Optical properties and shape-dependent retrievals of the imaginary part of the refractive index of freshly emitted Saharan dust}},
author = {Jesús Yus-Díez and Jeronimo Escribano and Marco Pandolfi and Andrés Alastuey and Cristina González-Flórez and Adolfo Gonzalez-Romero and María Gonçalves Ageitos and Matic Ivančič and Martina Klose and Konrad Kandler and Vincenzo Obiso and Agnesh Panta and Cristina Reche and Martin Rigler and Xavier Querol and Carlos Perez García-Pando},
journal = {Atmospheric Chemistry and Physics},
year = {2026},
month = oct,
doi = {10.5194/acp-26-14531-2026},
url = {https://acp.copernicus.org/articles/26/14531/2026/},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Optical properties and shape-dependent retrievals of the imaginary part of the refractive index of freshly emitted Saharan dust
AU - Jesús Yus-Díez
AU - Jeronimo Escribano
AU - Marco Pandolfi
AU - Andrés Alastuey
AU - Cristina González-Flórez
AU - Adolfo Gonzalez-Romero
AU - María Gonçalves Ageitos
AU - Matic Ivančič
AU - Martina Klose
AU - Konrad Kandler
AU - Vincenzo Obiso
AU - Agnesh Panta
AU - Cristina Reche
AU - Martin Rigler
AU - Xavier Querol
AU - Carlos Perez García-Pando
PY - 2026
DA - 2026/10/14
JO - Atmospheric Chemistry and Physics
DO - 10.5194/acp-26-14531-2026
UR - https://acp.copernicus.org/articles/26/14531/2026/
AB - Mineral dust is a major contributor to atmospheric aerosol mass and plays a complex role in Earth's radiation budget. However, large uncertainties remain in quantifying its direct radiative effects, partly because dust absorption is poorly constrained and in situ measurements of freshly emitted dust remain scarce. Here, we present high temporal resolution measurements of dust optical properties collected during active dust emission events in the Moroccan Sahara in September 2019. During dust emission periods, the optical properties aligned with previous Saharan dust studies. Single scattering albedo (SSA) for PM 2.5 (PM ₁₀ ) 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), consistent with stronger wavelength-dependent absorption by iron oxides. The asymmetry parameter ( g ) was 0.7 (0.65) at 520 nm, and the 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 m ² g ⁻¹ at 370 nm with increasing particle size. Mass scattering efficiency (MSE) shifted towards longer wavelengths for larger particles. We further retrieved the imaginary part of the complex refractive index, k , for the PM 2.5 dust-event period under prescribed assumptions for particle shape, particle density, and the real part of the refractive index. For the central density assumption, retrieved k at 520 nm ranged from 0.00104 for spheres to 0.00145 for triaxial ellipsoids, corresponding to a 40 % increase. These results show that particle-shape assumptions can substantially affect retrieved k , and therefore need to be considered when interpreting in situ, remote-sensing, and model estimates of dust optical properties.
SN - 1680-7324
ER -