Preprints
https://doi.org/10.5194/egusphere-2025-3691
https://doi.org/10.5194/egusphere-2025-3691
14 Aug 2025
 | 14 Aug 2025
Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Total power calibration in FTIR emission spectroscopy for finite interferograms

Lukas Heizmann, Mathias Palm, Justus Notholt, and Matthias Buschmann

Abstract. The commonly used total power calibration procedure in FTIR (Fourier transform infrared) emission spectroscopy first described by Revercomb et al. (1988) is strictly speaking only valid for double-sided, infinite interferograms. Here we investigate the effect of interferogram truncation on calibrated mid-resolution emission spectra, describe the underlying theory, and quantify the errors in a case study. This quantification is important as the demands on precision in atmospheric measurements increase. While application of the Revercomb formula might lead to large errors in the unequal-sided case, we show that one can obtain the same spectral estimate as from equal-sided interferograms. This is achieved by modifying Revercomb's method incorporating a phase correction procedure using low-resolution phase spectra from the black body measurements. We include a case study simulating atmospheric and black body spectra with a line-by-line radiative transfer model. For a mid-resolution use case, we find that the effect of truncation on the spectral radiance measurement is generally well below 0.05 %. Only in spectral regions where strong absorption lines in the black body spectra are present can the errors be larger. However, those spectral regions are usually saturated in atmospheric spectra for observations from the ground and thus contain little information about the atmosphere except for the lowermost layer and can and usually should be excluded for most retrieval procedures.

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.

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Lukas Heizmann, Mathias Palm, Justus Notholt, and Matthias Buschmann

Status: open (until 19 Sep 2025)

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Lukas Heizmann, Mathias Palm, Justus Notholt, and Matthias Buschmann
Lukas Heizmann, Mathias Palm, Justus Notholt, and Matthias Buschmann

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Short summary
Fourier transform infrared emission spectroscopy measures the infrared radiation originating from the thermal emission of the atmosphere. The measured interferograms are transformed into spectra via Fourier transformation. The established method for calibration is strictly speaking only valid for infinitely long interferograms. We quantify the errors caused by this idealization in a case study and find that they are generally below 0.05 % in spectral regions that are of interest for retrievals.
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