Remote Sensing of Atmospheric Methane with IR OPO Lidar System
Round 1
Reviewer 1 Report
1.Article line 85: "is converted by molecules and aerosol particles" converted is not the right verb here. Interacts, absorbed, scattered, or attenuated?
2.There is any interfering absorption by other gases in the investigated sounding range of the methane (3.30-3.50um)?
3. The description of experimental studies does not fully reflect the metrological conditions for lidar measurements.
4. Article line 195: In Figure 9b I assume the red dot is the time the CH4 concentration measurements along the 800-m surface atmospheric path were made (there is no label on the x-axis of Figure 6b). Assuming that is correct then we only have one example that the lidar agrees with the IAO SB RAS. What would be much more interesting is to see what happens between 3.2 and 4.2 in figure 9b. Does the lidar track the CH4 increase from 2.1 to 2.55 ppm?
Author Response
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Reviewer 2 Report
Specific points to be clarified by authors
Authors have mentioned the estimation of absorption coefficients in the abstract but there is no section in the manuscript that describes this aspect. Page 1: section ‘abstract’, line no 14 - delete the word “with” Page 1: section ‘abstract’, line no 14 - “Backscattered signal in the 3.3 -3.50 ..... .sounding path in the atmosphere ” replace with “Lidar backscattered signals in the spectral band 3.30-3.50 mm has been measured and analyzed along the horizontal path in the atmosphere”. Page 1: section ‘abstract’, line no 18 - Mention the measured concentration of methane also. Page 1: line no 19 - delete ‘IR region’, ‘atmosphere’ & include ‘DIAL’, ‘IR lidar’ Page 1: line no 29 – replace ‘of’ with ‘using’ Page 3: line 116 – write expansion of PDC Page 4: section 3. IR OPO lidar system – sub section 3.6 should come first. Here, authors should describe the full system scheme and configuration with more details. Page 4, provide information on raw beam divergence and wavelength tuning interval. How wavelengths are measured and calibrated? Page 4, line no 132, correct ‘Mirros’ Page 5, line no 142 – merge this line with line no 141. Page 5 – What is the FOV of telescope or receiver optics system ? Explain, how is it optimized? Explain the behavior of the lidar return signal measured during day and night, bring out the influence of noise, if any. What is the bandwidth of interference filter used in the experiments, if any? Bring out the details of the focusing lens used in front of detector to focus the laser radiation Mention the set values of voltage gain and band width of detector used in the experiments in the Table 4. Page 6, Mention the duration of sample collection period for single pulse in the section 3.5 Page 6, line no 167 – contents presented in this section are not sufficient, elaborate the contents methodologically. Page 6, line no 170 - replace ‘with’ with ‘which’ Page 7, how authors encircle the spot of laser pulse in figure 5. Is it hitting any hard objects, please explain in the manuscript. Page 7, Figure 6 is not clear, provide good quality image with visible labelling, Page 7, how methane lines were calibrated? Page 8, what are x & y axis labels in figure 7? Plot the both curves in the single figure for best comparison and understanding. Page 8, Not able to distinguish online and offline wavelengths in figure 8. Page 8, what is the actual spatial resolution used in figure 9 (a). Is it 100 m or less? Explain the data processing method followed in the manuscript. The comparison of path averaged methane concentration versus time measured using lidar and results of TOR measurements recorded at same time would be more appropriate for matching. Hence, try to modify/improve the figure 9 accordingly. Page 9, line no 199 – Methane concentration ~ 2.085 ppm, is it path averaged value or measured at particular distance, please clarify. Page 9, line no 206 - processing of backscattered signals are not discussed in the manuscript, please include in the revised manuscript. Page 9, line no 208 – delete ‘and control’ Please rewrite the conclusion with suitable modifications. Page 9, line no 216 – who is Y.S ? Check it once again. Bring out the importance of scattering in the selection of wavelength pair associated with DIAL signal measurements. How the system is calibrated for DIAL measurements? A measurement plot of laser energy vs tuning wavelength range is required with a sensitive energy sensor. This gives energy stability over the wavelength range. Authors have not discussed anything about the values of absorption coefficient/absorption cross section of methane used in the manuscript. Is it experimentally measured value or from spectral database? If it is experimentally measured, please bring out the curve showing the signal intensity versus wavelengths.Author Response
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Reviewer 3 Report
Figure 6 and Figure 8: For better reading, the size of the letters needs to be increased. If not possible otherwise, the figures may be enlarged.
Lines 47-49: The long sentence may be cut to two shorter.
Line 100: Please replace "of" by "if".
Line 149: "full" may be deleted.
Line 168: Please replace "in" with "for".
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Reviewer 4 Report
This is a novel and interesting paper concerning recent range-resolved lidar measurements of methane in the atmosphere. The paper is important because it measures range-resolved gas clouds of methane in the atmosphere, and most previous research has been conducted for path averaged measurements. However, it is scientifically weak in that it only uses an existing gas calibration technique used at the TOR station as a calibration point and does not use atmospheric spectroscopy or laser absorption spectroscopy of known gas samples or analysis with known methane spectra experimentally to provide calibration. As such, the spectroscopy needs to be strengthened to help in this area. Overall, it can be published after some changes, and should be improved to be more scientifically sound and to reach a larger scientific audience. Suggested changes: line 64 expand on the analysis of the spectral databases and lab measurements.....either as a reference or the results....show the absorption overlap of the linewidth of the laser and the methane absorption lines; line 119....what was the spectral linewidth of the laser and how was it measured....did it change with tuning ?; line 185....photo of on/off range resolved returns....these should be placed on the same plot or have axis units to be able to show if there is a differential signal; Line 191....it is common to use a lab reference gas cell to help calibrate the lidar signals and the calculated methane concentration.....did you use one, or why was it not used....what was the spectral recording for a laboratory gas cell filled with methane so it can be used for calibration and comparison with the raw lidar data in Fig. 8....this is important because the linewidth of the OPO laser can vary and this would affect the deduced methane concentration ; Line 222....there are several earlier papers on OPO lidar detection of atmospheric methane done in the 1980s and 1990s which you might want to reference for completeness.....most were path averaged DIAL and yours is range-resolved which is important.
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Round 2
Reviewer 2 Report
The quality of the revised paper improved relatively better compared to the earlier one. Still, it needs improvement in quality of the manuscript. Authors are encouraged to bring out/include their replies to the reviewer comments in the manuscript at the appropriate places. I appreciate that English is not the main language of the authors; however an editorial review of the text would help to make the paper more readable. Also there are a number of typos errors in the manuscript, e.g line 118, ‘detain’. Authors did not clarify few points raised in my earlier comments, which are very important to claim the publication.
Specific points
Page 3, under section 2, line no 91, 99, 105, 109, 114 – all equations are not printed clearly. Authors should have checked it properly before submitting to the journal. Line no 123, delete ‘in’ and ‘in the IR spectral range’ Sentences given in line no 124-128 are not giving proper meaning. Replace the sentences as given ‘A part of laser radiation reflected by CaF2 plate is used to measure the transmitted pulse energy in the experimental set up. A visible laser at 532 nm is used to align the invisible IR radiation to maintain trans-receiver axes. Further, the wavelength of the laser radiation is measured using spectrometer S100 as shown in fig 1’. Line no 134, delete ‘range’ Table 1, line no 139, What is instrumental function width ? is it spectral (linewidth) of laser? If yes, please mention as spectral width of laser. Authors have not mentioned the raw beam divergence, please mention it. Line no 140, the correct the figure number Line no 148, Rewrite the sentence as given ‘A Cassegrain telescope developed at the IAO SB RAS was used as a receiver optics to collect the backscattered signal from the atmosphere’ Table 3, please check the value of ‘telescope aperture’ What is the FOV of receiver telescope? Authors mention that the focus of the telescope is fixed at¥. How do they ensure the focusing of the backscattered radiation onto a detector? Line no 153-154, do not give proper meaning, please correct it. Line no 158-159, authors mentioned that they have used three different detectors as mentioned in table 4. Please bring out, why do you need to use three different detectors?. The use of three numbers of same PD36-03 photodiodes would have been a best choice, please comment. Table 4, please mention the bandwidth. Line no 162, sentence is not meaningful Line no 163, replace the word ‘controlled’ with ‘measured’ Table 5, correct the word ‘Gsmaple/s’ Line no 180, replace the word ‘modernized’ with ‘used’ Line no 195, In general, Lambert-Beers law is used to derive the attenuation coefficient of solutions/gases. Please comment. Line no 206, correct the sentence. Line no 212-214, split the sentence and make it more meaningful. Line no 240, what are x & y axis labels in figure 7? Please mention in manuscript. Plotting methane online and offline signal on the same graph is more meaningful for best comparison and understanding. It is essential to show. Line no 254-255, replace the sentence as given ‘The background concentration of methane was calculated using the equation shown in Equ. (2). The path averaged methane concentration of about 2.085 ppm over the path length of 800 m was retrieved'. Line no 259, please clarify the sentence ‘concentration recovery correspond to 8-13 % relative to the averaged value’.
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Reviewer 4 Report
This paper has been somewhat corrected as suggested, but some aspects of the scientific soundness have not been completely corrected in the area of laser spectroscopic details. It seems that the authors are almost ignoring that one of the most important spectroscopic parameters for laser sensing of a molecule is the laser linewidth and how it overlaps the molecular absorption linewidth. In particular, the absorption linewidth of methane in the atmosphere is 0.1 cm-1 , and the laser linewidth of 1 cm-1 (indicated in their answer letter to the reviewer) or now in the revised paper linewidth of 5 cm-1). Also, their answer letter said that they measured the laser linewidth with a spectrometer, although usually a Fabry-Perot is needed to measure a laser linewidth.
In the tracked changes revised paper, line 88 says that "expand on the analysis of the" . Is this something that was going to be done.
Table 1 should include the laser linewidth values.
From a technical viewpoint, the paper is technically sound since the DIAL measurements are now compared to calibration absorption cell measurements of methane.
The paper can be published as is since it is technically correct and shows a new DIAL sensing of atmospheric methane. However, I suggest the authors add some of the above aspects regarding the laser linewidth and absorption spectrum of methane because this will convince the laser remote sensing community of the soundness of the technique.
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Round 3
Reviewer 2 Report
The current form of the manuscript is suitable for publication in the journal ‘Atmosphere’. The quality of the revised paper improved significantly. Acceptance of the paper is subject to implementation of the following suggested corrections/modifications in the manuscript. Authors are encouraged to bring out/include their replies to the reviewer comments in the manuscript at the appropriate places.
Specific suggestions
Page 3, under section 2, line no 91, 99, 105, 109, 114 – all equations are not printed clearly in the revised version. Please ensure that the correctness of the equations in the manuscript while converting word format to pdf format. Line no 121, delete the word ‘for’ Line no 122, delete the sentence ‘the IR spectral range’ Line no 132, delete the word ‘range’ given in ‘In this work, spectral range radiation of the laser …’ Line no 141, authors mentioned the mirror collimator is used to broaden the beam from 4 mm to 40 mm (magnification is 10X ??), which decreased the beam divergence to 100 microrad. In table 1, line no 137, authors have mentioned raw beam divergence as 2 mrad, please verify and correct it. Line no 148, the sentence ‘the focus of the telescope is fixed at infinity (¥)’ and see table 3, ‘effective focal length ~ 1457 mm’. Please check and correct it properly. Please include FOV of the telescope in table 3. Line 160, table 4, please remove the details of the detector PD43-03 since it is not used in the current manuscript. Please include the bandwidth of the detector PD36-03 in the table 4. Line no 203, table 6. Please rewrite as given ‘Table 6. The comparison of absorption coefficients in the spectral region of 3.346 – 3.479 micron’ Line no 206-210, consider rewriting the sentences as given ‘Experiments were carried out to identify the absorption lines of methane in the spectral region from 3.346 micron to 3.4790 micron. The absorption coefficients were calculated using these experimental data. Further, these lines have been validated by analyzing the HITRAN database. Based on our analysis, laser probing wavelengths at 3.4157 micron and 3.4177 micron were chosen as online and offline wavelength for methane measurements in the atmosphere.’
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Author Response
We thank the reviewer for carefully reading the manuscript and for the comments made. We hope we were able to improve the article!
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