the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jun 2025
Assessing Non-Ideal Instrumental Effects in High-Resolution FTIR Spectroscopy: Instrument Performance Characterization
Abstract. This study investigates the impact of non-ideal instrumental effects on the performance of high-resolution Fourier Transform Infrared (FTIR) spectrometers, with a focus on the Bruker FTS 120M. Key non-idealities, including retroreflector misalignments, baseline drift, and spectral channeling, were systematically analyzed using advanced diagnostic tools such as ALIGN60 and LINEFIT. The nominal configuration exhibited significant anomalies, notably modulation efficiency (ME) deviations of up to +10.9 %, phase error (PE) variability of 2.11 × 10−2 radians, and spectral channeling frequencies such as a persistent 2.9044 cm−1, along with emerging frequencies around 0.24 cm−1 attributed to retroreflector wear and CaF2 beamsplitter degradation. A pronounced anomaly at 40.672 cm−1, likely induced by environmental factors such as external vibrations or mechanical instability, was also identified. Implementation of a modified configuration successfully mitigated these issues, reducing PE variability to 0.042 × 10−2 radians, aligning ME within the NDACC-acceptable threshold of 1.1, and achieving substantial improvements in the instrument line shape (ILS), including sharper peaks, narrower full-width at half maximum (FWHM), and reduced side-lobe asymmetry. Analysis of HBr transmission spectra revealed improved fitting of the P(6) line, characterized by lower residuals and enhanced spectral quality. Simulated Haidinger fringes near zero path difference (ZPD) highlighted alignment degradation patterns, underscoring the necessity for precise optical adjustments. Temporal trends showed a 14 % increase in ILS peak height and significant RMSE reductions in the modified configuration. Overall, this study provides a robust framework for diagnosing and correcting instrumental artifacts, ensuring the accuracy, reproducibility, and long-term stability of FTIR measurements essential for atmospheric trace gas retrievals.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(7003 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(7003 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
This study investigates non-ideal instrumental effects that may degrade the performance of a high-resolution infrared spectrometer. Using standard gas cell measurements, it identifies key sources of modulation loss and spectral artifacts. A simple atmospheric example is included to demonstrate how these instrumental effects can influence scientific measurements. The results improve understanding of instrumental stability and support more accurate long-term use of the spectrometer.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-2232', Anonymous Referee #1, 11 Aug 2025
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AC2: 'Reply on RC1', Gezahegn Sufa Daba, 05 Dec 2025
Dear Referee,
We are grateful for your careful reading of our manuscript and for your valuable comments and suggestions. We have addressed each point in the revised version and provide a full point-by-point response, attached here for your review. We hope that the revisions improve the manuscript and adequately resolve the issues raised.
We greatly appreciate the time and expertise you have dedicated to reviewing our work and helping us strengthen the manuscript.
With kind regards,
Gezahegn
(on behalf of my co-author)
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AC2: 'Reply on RC1', Gezahegn Sufa Daba, 05 Dec 2025
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RC2: 'Comment on egusphere-2025-2232', Chris Boone, 17 Oct 2025
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AC1: 'Reply on RC2', Gezahegn Sufa Daba, 05 Dec 2025
Dear Referee,
We would like to express our sincere gratitude for your insightful and constructive comments on our manuscript. Your thoughtful feedback has been invaluable in improving the clarity, rigor, and overall scientific quality of the work. In response to your observations, we have thoroughly revised the manuscript and prepared a comprehensive, point-by-point response addressing each comment in detail. Both the revised manuscript and the response document are included in the submission.
We greatly appreciate the time and expertise you have dedicated to reviewing our work and helping us strengthen the manuscript.
With kind regards,
Gezahegn
(on behalf of all co-authors)
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AC1: 'Reply on RC2', Gezahegn Sufa Daba, 05 Dec 2025
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-2232', Anonymous Referee #1, 11 Aug 2025
The paper systematically investigates non-ideal instrumental effects (e.g., retroreflector misalignments, baseline drift, spectral channeling) in the Bruker FTS 120M spectrometer, employing diagnostic tools (ALIGN60, LINEFIT) for characterization and mitigation. This paper well highlights the importance of routine performance evaluations and maintenance schedules for long-term operational stability of FTIR measurements essential for atmospheric trace gas retrievals. While the research is technically sound and addresses a critical gap in NDACC/TCCON protocols, several issues require clarification and revision to enhance clarity, consistency, and scientific rigor.
- Figure 5 labels the x-axis in Hz (frequency domain), while Figure 15 uses cm⁻¹ (wavenumber domain) without justification. This inconsistency requires clarification or standardization.
- Figure 12 (temporal evolution of FTIR transmission spectra) is not cited or discussed in the main text, leaving its relevance unclear.
- The manuscript alternates between "Figure" (e.g., Section 4) and "Fig." (e.g., Section 2.2.1) for figure references. A uniform style should be adopted.
- The methodology for computing the ILS averaging kernel in Figure 16 is not described, including whether it derives from LINEFIT retrievals or ALIGN60 simulations.
- Channeling frequencies listed in Table 3 (e.g., 2.9750 cm⁻¹) are not explicitly mapped to the FFT peaks in Figure 15, creating ambiguity in their correlation.
- Residuals in Figure 14 appear identical for "Nominal" (absolute values) and "Modified" (percentage), undermining direct comparison. Figure 13 similarly lacks clarity in residual scaling.
- The cited RMS range (3.7×10⁻³ to 5.04×10⁻³) does not match the values displayed in Figure 14’s residual plots, necessitating verification.
- While ILS improvements are shown, there is minimal validation of how these translate to more accurate retrievals of key atmospheric trace gases (e.g., CO, C₂H₆). Concrete examples linking ILS metrics to retrieval errors would strengthen relevance.
- The paper lacks a detailed flowchart or step-by-step workflow for ALIGN60 and LINEFIT procedures. A visual schema would enhance reproducibility, especially for non-specialists.
Citation: https://doi.org/10.5194/egusphere-2025-2232-RC1 -
AC2: 'Reply on RC1', Gezahegn Sufa Daba, 05 Dec 2025
Dear Referee,
We are grateful for your careful reading of our manuscript and for your valuable comments and suggestions. We have addressed each point in the revised version and provide a full point-by-point response, attached here for your review. We hope that the revisions improve the manuscript and adequately resolve the issues raised.
We greatly appreciate the time and expertise you have dedicated to reviewing our work and helping us strengthen the manuscript.
With kind regards,
Gezahegn
(on behalf of my co-author)
-
RC2: 'Comment on egusphere-2025-2232', Chris Boone, 17 Oct 2025
-
AC1: 'Reply on RC2', Gezahegn Sufa Daba, 05 Dec 2025
Dear Referee,
We would like to express our sincere gratitude for your insightful and constructive comments on our manuscript. Your thoughtful feedback has been invaluable in improving the clarity, rigor, and overall scientific quality of the work. In response to your observations, we have thoroughly revised the manuscript and prepared a comprehensive, point-by-point response addressing each comment in detail. Both the revised manuscript and the response document are included in the submission.
We greatly appreciate the time and expertise you have dedicated to reviewing our work and helping us strengthen the manuscript.
With kind regards,
Gezahegn
(on behalf of all co-authors)
-
AC1: 'Reply on RC2', Gezahegn Sufa Daba, 05 Dec 2025
Peer review completion
Journal article(s) based on this preprint
This study investigates non-ideal instrumental effects that may degrade the performance of a high-resolution infrared spectrometer. Using standard gas cell measurements, it identifies key sources of modulation loss and spectral artifacts. A simple atmospheric example is included to demonstrate how these instrumental effects can influence scientific measurements. The results improve understanding of instrumental stability and support more accurate long-term use of the spectrometer.
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Gezahegn Sufa Daba
Gizaw Mengistu Tsidu
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(7003 KB) - Metadata XML
The paper systematically investigates non-ideal instrumental effects (e.g., retroreflector misalignments, baseline drift, spectral channeling) in the Bruker FTS 120M spectrometer, employing diagnostic tools (ALIGN60, LINEFIT) for characterization and mitigation. This paper well highlights the importance of routine performance evaluations and maintenance schedules for long-term operational stability of FTIR measurements essential for atmospheric trace gas retrievals. While the research is technically sound and addresses a critical gap in NDACC/TCCON protocols, several issues require clarification and revision to enhance clarity, consistency, and scientific rigor.