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.