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

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.