| 31 Aug 2026
FTIR spectroscopy of desert dust: implications for complex refractive index spectra and dust sample diversity
Abstract. Mineral dust is a dominant natural aerosol that has a strong influence on Earth’s radiative budget. However, its radiative impacts remain poorly constrained due to limited knowledge of its complex refractive index (CRI), especially in the thermal infrared (TIR). We present a new application of Fourier Transform Infrared (FTIR) Attenuated Total Reflectance (ATR) spectroscopy to derive CRI spectra for six dust samples in the TIR, 2.5–25 μm. The CRI was estimated through two distinct methods. The first utilizes the Beer–Lambert Law to determine the imaginary component of the CRI, κ(λ), whereupon the real component, n(λ), is retrieved via the Kramers–Kronig relations. The second implements a direct ATR reflectance inversion approach where n(λ) and κ(λ) are simultaneously retrieved by fitting modeled Fresnel reflectance to measured ATR spectra. This methodology circumvents uncertainties associated with many sample preparation methods, thus enabling a more direct characterization of natural samples. Comparisons with literature CRI spectra for the six samples in question (and for mineral dust broadly) provide context for interpreting results from both retrieval approaches. For each sample, the two methods capture the same major absorption features as available literature does, but differ in retrieved absorption magnitude and long-wavelength behavior. Additionally, ATR measurements revealed significant variability in CRI spectral shape and magnitude, reflecting sample diversity linked to geographic origin. These results provide new constraints on desert dust optical properties and highlight sample-to-sample variability that can inform climate and radiative transfer models.
1. Representativeness of the ATR sample
The manuscript emphasises that ATR-FTIR preserves the physical and mineralogical characteristics of the dust sample. However, I am not sure to what extent the optical properties retrieved from particles in direct contact with the ATR crystal are representative of airborne dust. Could particle–crystal contact, packing density, aggregation, or particle morphology affect the retrieved CRI? The manuscript itself notes differences between size fractions and attributes some of these differences to sample–crystal contact. It would be useful if the authors could discuss whether the retrieved CRI should be considered an intrinsic material property, or rather an effective property of the ATR sample configuration, and how transferable these values are to atmospheric aerosols.
2. Uncertainty and robustness of the Fresnel inversion
The Fresnel-based approach avoids the wavelength-independent penetration-depth assumption of the Beer–Lambert method, but the inversion itself seems to be an ill-posed problem. The retrieved CRI may therefore depend on the initial conditions, parametrisation, and optimisation procedure. Could the authors provide some quantitative assessment of how sensitive the retrieved CRI is to these choices? In particular, do different initial guesses or parametrisations converge to the same solution? This would give a better indication of how well constrained the reported CRI values actually are.
3. Effective CRI of heterogeneous mineral mixtures
The dust samples contain mixtures of several mineral phases with different optical properties. I therefore wonder how the retrieved CRI should be interpreted for these heterogeneous samples. Is it intended to represent an effective optical property of the mineral mixture? If so, how might it depend on the relative mineral abundances, particle size, or mixing state? I think this is particularly relevant if the reported values are intended to be used as optical constants for atmospheric dust.
4. Relevance for climate models
The manuscript motivates the retrieval of dust CRI partly by its importance for climate and radiative-transfer modelling. However, it is not entirely clear to me how much the differences in CRI reported here would actually matter in an atmospheric context. Could the authors provide a simple sensitivity estimate showing how the different retrieved CRIs affect quantities such as extinction, absorption, single-scattering albedo, or radiative forcing? Even a relatively simple or order-of-magnitude estimate could help demonstrate the relevance of the differences for climate modelling.