Study on Local Power Plant Emissions Using Multi-Frequency Differential Absorption LIDAR and Real-Time Plume Tracking

Round 1

Reviewer 1 Report

The authors present the work performed on a new DIAL lidar systems and its implementation for CO₂ fluxes measurements using real time plume tracking.

This work is innovative and of very high scientific quality, in terms of lidar emitter, DIAL sensing, plume tracking as well as flux algorithm retrieval. It was also implemented in a real-case scenario.

Given the variety of the technical areas addressed here, I would suggest, in order to make the paper clearer, to add the following clarifications:

·        Why are the authors using 5 wavelengths to probe the CO2 line ? Are they using the lines to retrieve a line shape ? or are they benefitting from the fact that the wavelength at the top of the line could be absorbed too strongly in case of high CO2 fluxes ? Also, in Fig 1.B, there are only 8 black dots to represent the emitted frequencies, when the text indicates 10.

·        Concerning the beam output, page 3, the authors should detail if it is measured at full width at half maximum, or diameter @ 1/e². EDFA 3 output fiber, given the given beam expander factor of 10, should then have a core diameter of 410µm diameter, if this is the right value, the authors should comment on the output beam quality.

·        Page 4, the authors should give the expected commercial telescope transmission in the 1.5µm region.

·        Page 5, the authors reposition the line of sight direction every 45 to 60s. Is it sufficient for all wind conditions ? In the reference   https://doi.org/10.1051/epjconf/202023703016, the authors used a constant line of sight and tested a DIAL on a controlled CH4 gas leak and the plume appeared and disappeared with the same order of magnitude in terms of timescale, but the wind speed value was not reported.

·        Page 6, line 3, the authors should refer to the wind speed graph reported in annex.

·        Concerning the plume concentration retrievals (paragraph C), severals points should be in my opinion clarified . i) It is not clear how the plume area is retrieved ? From the lidar spike ? From the overall DIAL sensing ? Is a  cylindrical dimension plume supposed ? ii) Equation 4 should be explained. ∆DIAL CO₂ is not defined. Is the equation 4 only valid for gas plumes dimensions L_p < L_DIAL and using the dual bin approach ? In this case, should L_DIAL value be twice the range bin ?

·        In Figure 3, we can have the impression that some very low CO2 levels are measured, along the line of sight, after some CO2 peaks. Can the DIAL sensing be saturated due to a too strong absorption, leaving low laser energy levels at the plume output and thus inducing a bias in the atmospheric CO2 sensing after the peaks ? This seemed also the case in the reference cited above.

·        What is the sensitivity (measurement error) of the lidar in terms of ppm, when there is no plume ?

·        Page 9, beside the future use of a wind lidar, the authors could also mention that simultaneous wind and gas measurement have been performed on methane gas leaks (https://doi.org/10.1364/OE.394553) using a single coherent detection instrument, and that a coherent CO2 and wind lidar was also tested previously (https://doi.org/10.1364/AO.43.005092)

As a minor improvement, the authors may also cite, in the introduction first paragraph, the review paper from B. Orr, "Infrared LIDAR Applications in Atmospheric Monitoring". In Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.), (2007). https://doi.org/10.1002/9780470027318.a0711.pub2".

As a general remark the resolution of the figures should be enhanced. In Figure 3b, there are no vertical tick labels to indicate the levels in dB. Figure 3 a could also be improved changing the color map to better fit the dynamic of the signals displayed.

Author Response

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Reviewer 2 Report

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Round 2

Reviewer 2 Report

The authors have addreessed all the issues I raised, and the current manuscript looks good. Thus,I suggest publishing in present form.