Performance Evaluation of Spaceborne Integrated Path Differential Absorption Lidar for Carbon Dioxide Detection at 1572 nm

Round 1

Reviewer 1 Report

The manuscript, titled “Performance Evaluation of Spaceborne Integrated Path Differential Absorption Lidar for Carbon Dioxide Detection at 1572nm”, presents an assessment of the IPDA lidar performance from space based platform. The lidar operates in the 1.6-μm wavelength and it is a planned mission by China. Although similar CO2 IPDA lidar instruments have been presented in many publications through different research groups, this manuscript provides an insight to China’s efforts addressing CO2 active sensing from space, which is an important goal for science community addressing the carbon cycle and climate change on Earth. The manuscript is suitable for publication, but the authors are encouraged to address the following issues;
1. Further discussion about the IPDA lidar hardware is required. The manuscript lack a brief description of the instrument transmitter and receiver. Instrument block diagram would aid communicating the work objectives to the readers. In particular, expanding instrument parameters, listed in Table 1, to accommodate all variables presented in theoretical analysis. For example, what detector is used including detector gain, feedback resistance were not listed, although presented in equation 6. Laser transmitter properties, such as pulse-width and line-width were not presented.
2. Limiting the detection bandwidth to 1 MHz, as indicated in Table 1, reduces the noise but could include other systematic errors in IPDA. Please comment on that.
3. A brief discussion of the instrument ground and airborne testing and validations would increase the value of the presented work. How the validation results compare to the presented results.
4. Line 52: Ultimately, differential absorption lidar (DIAL) is the required active remote sensing technique for global CO2 measurements, but it suffers some limitations due to low SNR that will limit accuracy. Please introduce DIAL as a mean to measure CO2 and its limitations. These limitations can be overcome by using the integrated path differential absorption lidar, which enhances measurement uncertainties due to high SNR.
5. Line 58: ASCENDS was adopted by NASA. Langley Research Center contributed to that work but reference [13] cites Goddard Space Flight Center. Please change the sentence to read “In 2007, the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS) project was adopt by NASA [13].” And replace reference [13] to [S. Kawa, et al., “Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS): Final Report of the ASCENDS Ad Hoc Science Definition Team,” NASA/TP–2018-219034 (2018)] for citing the whole ASCENDS program final report.
6. Line 58: Please change the sentence to “NASA Goddard Space Flight Center conducted the first airborne CO2 IPDA lidar experiment in 2008 [Abshire, et al., Tellus Series B-Chemical and Physical 619 Meteorology, 2010. 62(5): p. 770-783 and Atmospheric Measurement Techniques, 2018. 11(4): p. 2001-2025].
7. Line 60: Reference 16 cite the same NASA Goddard Space Flight Center work in references [13] and [14]. References [13] to [16] present only 1.6-μm instruments. Please change reference 16 to [Refaat, et al., Applied Optics, 55(15), 4232-4246 (2016)] to cite the pulsed 2-μm IPDA lidar at Langley Research Center.
8. Line 82: Please include that NASA Langley Research Center conducted space based IPDA lidar simulations and error assessments for the 2-μm system and cite reference [24].
9. Lines 82 to 84: Out of subject. Please consider removing the sentence “Kiemle et al. simulated the pseudo methane column concentration to assess the 82 error distribution of MERLIN based on the designed hardware parameters in 2014 [20], and the 83 results showed the monthly averaged precision of methane was 1.2%, 1.7% and 2.1% over land, water 84 and snow ice.” Since it presents methane not carbon dioxide, the main subject of the paper.
10. Lines 209 to 211: The statement “variations in surface albedo are not negligible especially considering the large laser spot (70~100m), which should be simulated as a source of systematic errors.” Need to be discussed further. What is the overlap volume between the transmitted on and offline pulses and how that error will be assessed.
11. Line 156: According to reference 18, the denominator of equation 6 is missing other significant noise sources, mostly related the to the trans-impedance amplifier that follows the detector. Please cite the reference before equation 6, update the denominator and discuss.
12. Lines 201 to 202: The statement “During the operation of satellite, the instability of laser frequency and energy and the non-monochromaticity of laser also cause a certain degree of uncertainty in the measurement of Δ??????2.” Presents systematic error problems due to laser transmitter. How these error will be addressed during instrument operation. Please discuss.
Other minor issues;
Line 40: Please include the word “global” to the sentence to read “…main method for global CO2 detection…”.
Line 51: Please include the words “and nighttime” to the sentence to read “…data missing at high latitudes and nighttime.”
Line 63: Please change to […could be less than 0.8 ppm…].
Line 103: please change “absorption peak” to “CO2 absorption peak”.
Line 113: Based on reference 18, η is not the overall optical transmittance, but is the optical efficiency of the receiver system including the overlap function.
Line 114: please change “…are the signal optical depth…” to “…are the optical depth…”.
Line 118: Based on reference 18, ΔtL is the transmitted laser pulse width not the pulse width of laser echo. Please cite reference 18 before equation 2.
Line 121: Air density and pressure are related. Please change “…air density and pressure…” to “…air temperature and pressure..”.
Line 127: Please change “at the certain pressure” to “as a function of pressure”.
Line 132: Please change “columnar concentration” to “weighted average column concentration”.
Line 134: Please change “weight function” to “weighting function”, and all over the document.
Line 136: Please change “absorption cross-section” to “differential absorption cross-section”.
Line 145: Please change “relative” to “weighted average”.
Line 159: In equation 6, T is the temperature of the feedback resistor, RF, of the trans-impedance amplifier not the detector temperature.
Line 160: Pback is the background radiation not the background noise.
Line 162: Isolar is the background solar irradiance not the solar background noise.
Lines 172 to 174: please remove “and after averaging a certain number of lidar echo signals, the SNR of speckle noise is much larger than other noises, so the” from sentence to read “According to G.Ehert et al., study, using direct detection the influence of speckle noise for lidar signals could be neglected after averaging ????ℎ????? times [18].”
Line 186: In Table 1, the last parameter is for the detector noise-equivalent-power not dark cuurent.
Line 189: Please define each term in equation 11.
Line 265: The spectral range of figure 2 is too wide. Please consider focusing x-axis from 6359 to 6363 cm-1, for example, for better presentation.
Line 279: Please define “TOD”.

Author Response

Dear Reviewers and Editor:

We would like to thank all the reviewers for the time spent to evaluate our work and for the useful and constructive comments they gave which help to improve our work. We also acknowledge the editor. Here we submit a revised version of manuscript which includes all proposed changes during the discussion that we think are necessary to be made in the revised manuscript. Thanks again for your contribution.

Note that our answers are in red in the attachment. The actual changes made in the manuscript is in italics.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors are developing a spaceborne CO2 IPDA lidar for global column-averaged CO2 (XCO2) measurements. This CO2 lidar, together with aerosol and cloud lidar, or the Aerosol & Carbon Detection Lidar(ACDL), is planned to be launched in mid-2021. Active approach using liar is the future of greenhouse gas remote sensing with some major advantages over current passive approaches, including global, full seasons and day/night coverage, high measurement sensitivity for independent retrievals of a prior and high accuracy and precision to meet scientific goals. Authors present their results of the Observing System Simulation Experiments (OSSE) study for ACDL in this paper. The scope of the study fits publication goals of this journal and is very important to the atmospheric CO2 observations for carbon cycle sciences.

Authors had an appropriate methodology to study the global performance of XCO2 measurements from ACDL based on the notional design of the instruments. This study is systematic and covers most key parts of OSSE for a future spaceborne mission. However, there are two major issues in this study which are directly relevant to results and conclusions. First issue is about the comparisons between XCO2 ‘truth’ values from European model CAMS and XCO2 retrievals from CO2 IPDA lidar absorption measurements – these XCO2 values used difference vertical averaging kernels and can’t be directly compared to each other for estimating retrieval uncertainty. Instead, the CO2 vertical profiles from the model should be used to re-compute XCO2 with CO2 IPDA lidar averaging kernels or weighting functions as ‘truth’ values for comparison with retrievals. Second, the global pseudo data (either transmittance or optical depth) calculated using HITRAN and model meteorological data (vertical profiles of temperature, pressure and water vapor, and winds) with sensor noises should be also based on CO2 profiles from model with vertical gradients of CO2, rather than vertically uniform CO2 used in this study, and seasonal variability for a realistic global evaluation of the lidar XCO2 measurements. The results and conclusions could be significantly different after model CO2 profiles are used in the experiments. Therefore, I recommend a major revision of experiments and  this manuscript without comments on other smaller issues in this study but will be happy to review revision based on realistic experiments and adequate comparisons.

Author Response

Dear Reviewers and Editor:
We would like to thank all the reviewers for the time spent to evaluate our work and for the useful and constructive comments they gave which help to improve our work. We also acknowledge the editor. Here we submit a revised version of manuscript which includes all proposed changes during the discussion that we think are necessary to be made in the revised manuscript. Thanks again for your contribution.
Note that our answers are in red in the attachment. The radiative transfer model has been updated as the reviewer’s suggestion and the main difference of the pseudo measurement results are shown as follows.

Please see the attachment.

Author Response File: Author Response.docx