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 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.