Spectral Measurements of Parent Soils from Globally Important Dust Aerosol Entrainment Regions

Spectral data for the paper “Spectral Characterization of Parent Soils from Globally Important Dust Aerosol Entrainment Regions” submitted to the journal of JGR: Atmospheres article number: 2022JD037666.

Visible, short-wave infrared (VSWIR) and longwave infrared (LWIR) reflectance spectra as well as longwave transmission spectra presented in that paper are included in this Zenodo repository.

The spectral data are stored in CSV file format in columns and the first columns are the wavelengths.

VSWIR reflectance: VSWIR was measured in the wavelength range between ~ 350 and 2500 nm using Spectral Evolution (SE), model RS-5400 portable spectroradiometer with a spectral resolution of 1.5 to 3.8 nm. Soil samples were placed in an aluminum holder and the reflectance spectra collected using a contact probe, equipped with an internal halogen light source. A reflectance spectrum for a spectralon white reference panel was obtained prior to each soil measurement for calibration. All soil reflectance spectra were automatically ratioed to that of the spectralon calibration target. All spectral measurements were converted to absolute reflectance using the correction for spectralon. To obtain adequate signal to noise ratio (SNR), 60 individual scans were averaged for each final output spectrum.

LWIR reflectance: LWIR reflectance was measured in the wavelength range between ~ 2.5 and 25 µm using a benchtop Nicolet 380 Fourier Transform Infrared (FTIR) spectrometer. We used a diffuse reflectance attachment that holds the sample and reference plate horizontally. Gold was used as a reference reflectance standard, because it is highly reflective at all LWIR wavelengths. Once the reference background is collected, the loaded sample holder is placed into the FTIR device to measure the sample reflectance, which is automatically ratioed to the reflectance of the gold plate. This removes the effects caused by the instrument and by atmospheric gases in the instrument path length so that features in the final spectrum are solely due to the sample. To improve the SNR, we set the numbers of scans averaged for the samples and gold reference to 100 and 200, 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.