Infrared Transmission Spectra and Complex Refractive Indices of Surface Soils from Global Dust Entrainment Regions

Spectral data for the paper “Variations in Infrared Complex Refractive Index Spectra of Surface Soils from Global Dust Entrainment Regions” submitted to the journal of Atmosphere Manuscript ID: atmosphere-2291559.

Longwave infrared continuum removed transmission spectra, real (n) and imaginary (k) indices of refraction as well as subtractive Kramers-Kronig (SKK) MATLAB code presented in that paper are included in this Zenodo repository.

The continuum removed transmission spectral data are stored in an Excel file format in columns and the first column is the wavelengths (µm).   

Real (n) and imaginary (k) indices of refraction for each sample are stored in a separate file where the first the column is the wavelengths (µm), and the second and third are k and n respectively.

LWIR transmission: LWIR transmission was measured in the wavelength range between ~ 2.5 and 25 µm using a benchtop Nicolet 380 Fourier Transform Infrared (FTIR) spectrometer. First, we made soil-KBr pellets using ~ 0.5 mg of soil and ~ 200 mg of KBr blended using a clean mortar and pestle for 2 to 3 minutes to ensure uniform dispersion of mineral particles in the matrix. Pellet production was performed with the means of an evacuable KBr Die Kit and a CrushIR Digital Hydraulic Press from PIKE Technologies (Madison, WI, USA). The mixture was first transferred to the Die Kit which was then pressed under vacuum for about 4 to 5 minutes at a pressure of ~ 10 t cm-2, forming a hard disk 13 mm in diameter. Due to the hygroscopic nature of KBr, we pulled a vacuum on the pellet die for approximately 3 to 4 minutes prior to compression, and we immediately placed the pellets in a desiccant box, and then measured transmission within one or two hours of pellet creation.

The pellet was attached to a self-adhesive sampling card and placed into the transmission holder. The resulting transmission spectrum for the sample is recorded with the dust-KBr mixture measurement being ratioed to that for an empty chamber, or blank reference. To avoid contamination of transmission spectra with ambient gases, the instrument was initially purged with dry air for at least 5 minutes, prior to each transmission collection. To increase the quality of the spectra and improve the SNR, the numbers of scans averaged were 200 for the blank reference and 100 for the samples.

We ratioed transmission spectra to a blank KBr spectrum and then removed the continuum. The continuum removal (CR) transmission spectrum was employed to derive imaginary index of refraction (k) using Beer-Bouguer-Lambert law and k = βaλ/4π. Real index of refraction (n) spectrum was consequently derived from the k spectrum with a subtractive Kramers-Kronig (SKK) technique.  

The spectral range of 4–25 μm was chosen to present the transmission spectra as well as real (n) and imaginary (k) indices of refraction since absorption in the region from 2.5 to 3.5 μm is mainly due to water and is not necessarily diagnostic.

Here, we define any acronyms that may be present in the names of uploaded Excel files or their contents.

*LWIR = Longwave infrared

*T = Transmission

*CR = Continuum Removed