the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2024
Robustness of atmospheric trace gas retrievals obtained from low spectral resolution Fourier-transform infrared absorption spectra
Abstract. For atmospheric trace gas columns retrievals obtained from ground based Fourier-transform interferometer spectra we study the sensitivity of the retrieval processing chain to changes in the number of points in the recorded interferograms. Shortening an interferogram will alter the leakage pattern in the associated spectrum and we demonstrate that the removal of a relatively small number of points from the interferogram edges creates a beat pattern in the difference of the associated spectra obtained from the original and shortened interferogram. For low-resolution interferometers the beat pattern in the spectra may exceed the noise level and the effect on atmospheric gas column retrievals may be large. Sensitivity of the retrieval algorithm to the length of the underlying interferogram can be reduced by applying a non-trivial apodization such as Norton-Beer. A case study shows the effect on formaldehyde retrievals obtained from low-resolution spectra in Sodankylä and Kolkata.
- Preprint
(856 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-2764', Anonymous Referee #1, 17 Oct 2024
In their paper "Robustness of atmospheric trace gas retrievals obtained from low spectral resolution Fourier-transform infrared absorption spectra" Langerock et al. investigate the effects of removing sample points in recorded interferograms from Fourier-Transform infrared spectrometers, which are widely used in ground-based remote sensing. Thus the topic fits the scope of AMT and the results of the paper are of importance to the processing chains of ground-based remote sensing networks, such as NDACC, COCCON and TCCON.
However, I feel several important details need to be addressed:
- It is not clear why the interferogram should be truncated at all and if for some reason it is, why can't the correct number of points be used to determine the actual length of the interferogram. Clearly it is possible to use interferograms recorded at different maxOPD. How is this different?
- A discussion on the impact of the proposed apodization on the information loss in the retrieval is missing. Amato et al. 1998 only show that the retrieval is unaffected by the choice of the apodization function if a weak function is used. "In practice, heavy apodization moves the truncation point in the interferogram closer to the origin, and therefore it decreases the maximum path delay L, which inevitably causes a loss of information in the spectral range." (Amato98)
What happens if the apodization is strong enough to suppress the information under the noise limit at the end of the interferogram? Doesn't the difference in Fig 6a between (BX, d=0) and (NBS, d=0) show that there is an impact of the apodization without the issue of missing points?- Section 2 line 38 - I assume instead of time domain the spatial domain is meant? As in spectrum is spatial frequency and interferogram is derived from movement in space? So you are talking about DFT( DFT(I)-DFT(I_bandpass)) ? This part needs clarification/rephrasing. The time domain analogy is used several times and should be changed to avoid confusion.
- line 50 - and Fig 3 - I don't understand the ramp function. Is the goal to reduce the effect of the missing points by setting the long side of the interferogram gradually to zero? If so, shouldn't it be then 1 at the start and going down to zero. And you claim in line 50, that applying the ramp removes the impact of the point removal. Fig 3b shows otherwise, the beat is still at 2 %. In general the description of the actual steps done could be beneficial for understanding.
- Fig 6 and paragraph line 108 - It is reasonable to assume, that a stronger Norton-Beer apodization mitigates the effects, but at the same time you would loose information. Fig. 6a in particular shows that the retrieved H2CO columns are very different, but a discussion on the causes is missing. Is it due to the described beat pattern on the spectrum or due to loss of information due to a strongly apodized interferogram? I would argue, that especially in a low resolution scenario, the information contained in the tail end of the interferogram contains much of the information used in the retrieval, esp. information on the spectral lineshape. This effect would be smaller with high res interferograms/spectra. This is connected to the second comment above.
A minor detail:
line 14: TCCON uses the term dry-air mole fraction (DMF) instead of vertically integrated concentrations, as it better represents the approach of dividing by the O2 column.
Citation: https://doi.org/10.5194/egusphere-2024-2764-RC1 -
RC2: 'Comment on egusphere-2024-2764', Anonymous Referee #2, 30 Oct 2024
In this study, Langerock et al. examine the impact of varying the number of data points in interferograms recorded by FTIR instruments, which are commonly used for atmospheric composition measurements in networks like NDACC, TCCON, and low-resolution instruments. They present a case study analyzing the effects of low-resolution spectra in Sodankylä and Kolkata. The findings are pertinent to ground-based remote sensing networks and also to studies using low-resolution data from aircraft, making the paper relevant to AMT’s scope. Below are some comments that I suggest need to be addressed.
The manuscript lacks a clear overall motivation for conducting this work. What prompted the authors to undertake this analysis? I recommend providing a more detailed explanation of the study's significance in the introduction, along with a clear and in depth description of current practices in FTIR observations (both high and low resolution). Specifically, do all instruments perform the DFT individually, or do they rely on standard routines or software? What are these softwares actually doing? what are these routines typically designed to accomplish?
I suggest including more details on the standard procedures typically used by low-resolution instruments; for instance, how the network COCCON and others handle the DFT. If feasible, I recommend presenting results based on these common practices and providing an overarching set of recommendations.
The examples and applications presented here rely on solar absorption, which is valuable; however, I also recommend assessing the results using control-based cell spectra. Do you observe consistent findings, and what impact might the instrument line shape have?
Abstract: Opening Sentence (the first sentence is quite broad) and not clear. Do you mean something like this: “This study examines the sensitivity of atmospheric trace gas column retrievals from ground-based Fourier-transform interferometer spectra to variations in the number of points in recorded interferograms”.
The abstract is currently brief, leaving room for further improvement. I strongly recommend adding a motivating statement to highlight the importance of this work, along with quantitative findings on the formaldehyde results for a more comprehensive summary.
The introduction mentions the FTIR resolutions used by NDACC, TCCON, and low-resolution instruments, but does not address the significance of these differences. Is there a notable impact of using different resolutions? Can low-resolution instruments achieve the same gas retrievals as high-resolution ones?
In the second paragraph of the introduction, instead of starting directly with the removal of points, I suggest first explaining what determines the number of points in an interferogram. This can be followed by a description of both the removal of points and the process of zero-filling, along with guidance on when each is recommended.
In Figure 1 the noise level in the HR-FTIR appears higher than in the low-resolution instrument in terms of intensity [a.u.]. Could you consider including the signal-to-noise ratio for each spectrum?
In the introduction, you present the difference between spectra measured with and without the removal of points from the interferogram tails, yet the motivation behind this step is not explained. Why is it necessary to remove points from the interferogram? How are the original spectra generated, and is there a standard procedure for this? Additionally, is specific software used for these calculations, or is it a common practice to generate spectra directly from interferograms? Clarifying these points would offer valuable context for the reader.
Figure 6. The magnitude and shape of the diurnal variation using BX differ noticeably; is there an explanation for this? I also suggest adding the diurnal variation of HCHO from the HR-FTIR for comparison and the quantitative comparison/results.
Figure 6. I do not see green points on either plots a or b.
P4, L66, suggest to change “clean site” with remote
P6, L99. Is Sodankyla an official NDACC station? I could not find the data online.
Section 4 lacks details about the standard procedures typically employed for these measurements. For instance, what method does COCCON use for the DFT? I recommend including this information and, if possible, presenting results using the same retrievals as COCCON to provide a comprehensive and meaningful contribution to the community.
Is Figure 7 based on observations conducted in Sodankylä? It's intriguing that the magnitude depicted in this figure does not align with the values from either Sodankylä or Kolkata shown in Figure 6. For instance, the maximum values for Vertex 70 in Figure 7 are approximately 10, whereas the maximum values for Sodankylä and Kolkata in Figure 6 are around 3 and 40, respectively.
In my opinion, conclusion can be expanded with quantitative findings and overall effect in other gases.
Citation: https://doi.org/10.5194/egusphere-2024-2764-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 130 | 25 | 10 | 165 | 1 | 3 |
- HTML: 130
- PDF: 25
- XML: 10
- Total: 165
- BibTeX: 1
- EndNote: 3
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1