the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Apr 2026
1.645 µm differential absorption lidar measurements of atmospheric methane using an Er:YAG laser
Abstract. A differential absorption lidar (DIAL) based on an Er:YAG laser was used to retrieve methane concentration profiles within the mixed layer along a near-horizontal line of sight (4° above the horizontal) from the École Polytechnique site, directed northward toward the western part of the city of Paris. The achieved precision remains below 1 % up to a range of 3.5 km for profiles with a spatio-temporal resolution of 470 m / 20 min. The measurements were compared with in situ observations from an ICOS site located 5 km from the lidar. The lidar successfully captured the late stage of the dispersion of a methane plume originating from a fire occurred at a waste-sorting facility in the city of Paris, in good agreement with the in situ measurements. Both random and systematic errors in the lidar measurements are dominated by uncertainties in the ON wavelength measurement.
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Status: open (until 18 Jun 2026)
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RC1: 'Comment on egusphere-2026-844', Jasper Stroud, 16 May 2026
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AC1: 'Reply on RC1', Dimitri Edouart, 27 May 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-844/egusphere-2026-844-AC1-supplement.pdf
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AC1: 'Reply on RC1', Dimitri Edouart, 27 May 2026
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RC2: 'Comment on egusphere-2026-844', Anonymous Referee #2, 04 Jun 2026
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Review for 1.654 micron differential absorption lidar measurements for atmospheric methane using an Er:YAG laser by Dimitri Edouart, Fabien Gilbert, and Claire Cenac
This paper presents initial methane mixing ratio profiles retrieved from a DIAL instrument. This paper is well written, complete, and can be published as is.
Between line 30 and 31, A new paragraph might be added that discusses the need spatial, temporal, and resolution of the methane measurements needed by the science community to help put this work in perspective.
Figure 2: Because you do not have temperature measurements, you would like to choose a temperature insensitive absorption line for the methane. Can you comment on the temperature sensitivity of the absorption lines chosen?
Figure 6: Using a swept window in both time and range may provide a better representation of the time series rather than the :pixelate” plot shown in figure 6 (left).
Line 259: Why are you comparing the CH4 mixing ratio measure at 2.1 km when the tower observation is 5 km away?
Line 261-262. Why is a line fit used to determine the differential absorption coefficient. You should be able to directly use the return signal and the DIAL equation to retrieve the mixing ratio. Perhaps a few sentences should be added to describe how you are completing the retrieval would be in order.
Line 364: “Figure 10 (right) …” Figure 10 is missing from my copy of the paper.
Citation: https://doi.org/10.5194/egusphere-2026-844-RC2
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The authors report on a DIAL system operating in the 1.645 um region to detect methane. The results are compared to a ICOS site and successfully measured increased concentrations of methane created by a fire. The work is well described and the results are impressive. I recommend publication of this manuscript with some minor changes.
Line 83.
“The wavelength measurement alternates between the two DFBs using a fiber switch (sSWT) toggling every 10 s, thereby tracking the slow drift of each seeder locked to the laser cavity.”
Line 180
“The Licel acquisition module integrates 2000 laser shots for both ON and OFF wavelengths in real time, resulting in ON and OFF profiles recorded with a temporal resolution of 4 s.”
Please clarify the above two statements. Is that 4s for both, or 2 sec each? Where did 10 sec go?
Figure 3. Switching the axis of the range is confusing
Line 259
“The uncertainty bars of the ICOS measurements represent the dispersion of the measurements within each hourly interval.”
Why not match the resolution of the lidar system? Is there a limitation, or is this a choice?
Line 267.
“The differences in methane concentration between the ICOS measurements (at 100 m) and the lidar measurements averaged over one hour up to a range of 3.5 km remain below 50 ppb. A fire occurred at a waste sorting facility located in the city of Paris on the evening of April 7..”
Roughly how far from the point sensor (at 100 m) is the center of mass of the lidar results?
Figure 6. Can you highlight the region in the left image used in the right image. The error bars and overlap is very difficult to see at the plum tail, maybe include zoomed in insert.