This manuscript presents the calibration algorithms used for the ACDL lidar onboard the DQ-1 satellite, with the analysis of the 532 nm nighttime polarization and high-spectral-resolution channels. The study is of significant interest to the lidar community, particularly as the launch of the EarthCARE ATLID lidar in May 2024 presents opportunities for comparative analyses between these two advanced lidar systems. This paper was originally submitted in May 2024, and my previous reviewer comments, along with the authors' responses, are available at: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-588/ under Anonymous Referee #1.

The authors have made substantial improvements to the manuscript in response to earlier feedback. I now have the following minor comments and grammatical suggestions to further refine the manuscript:

Minor:

Line 36: please be more specific on the high accuracy, temporal or spatial.

Line 83-86: please update the status accordingly, as EarthCARE is already in orbit. 

Grammar:

Line 41: "To enable ACDL quantitative measurement..." -> "To enable ACDL's quantitative measurement..."

Line 44: include -> including

Line 47: hereafter referred to as "the" HSRL channel

Line 71: ... detecting clouds and aerosols

Line 74: duplicated 'the'

Line 74: 'aerosol stratospheric scattering ratio' -> 'stratospheric aerosol scattering ratio'

Line 91: a verb is missing, possible correction "...with a portion (70%) of the signal passing through an iodine vapor absorption filter to block Mie scattering,..."

Line 124: add a space between 2km

Line 129: consisting -> consists

Line 131: Remove

Line 139: 'the' calibration range

Line 140: for 'the' calibration procedure

Line 143: by 'applying' sliding averages

Line 183: to be consistent: (polarization gain ratio) PGR

Line 188: verb is missing. '...are determined using the...' ?

Line 188: molecular normalization technique?

Line 304: ...underlying terrain on the signals...

Line 315: '..., an additional sliding average of 500 km in the direction of adjacent-track distances is applied."'

Line 337: The Eq.(26) are -> Eq.(26) is 

Line 401: as shown in Figure 10. 'as' -> 'is'

Line 419: It's not clear from the sentence "...estimations of the error terms contributing will be revised". Does the author mean "the estimations of the contributing error terms will be revised"?

Line 440: Standard -> standard

Line 465: assessed -> assess

The manuscript describes the nighttime calibration of the spaceborne HSR lidar ACDL/DQ-1. It is important to clarify how the signals are calibrated. There is some heritage of the CALIPSO mission, and something to learn for the new EarthCARE mission. At parts the authors present great details, but at others the authors go rather quick over some issues. In general, the paper is good and should be published after major revisions.

My major comments are:

1. Please show the calibration of the cross-polarized channel as well. I am not satisfied to state that it is the same except with the polarization gain ratio (PGR). You might provide in the supplement all the figures for the cross-polarized channel which you have provided for the parallel-polarized and HSRL channel in the manuscript.
2. The PGR is discussed in L264 onwards. However, important points are missing. One point, which is presumably hard to characterize on ground is the angle in the polarization plane between the emitter and the receiver. Further uncertainties might originate from impurities in the laser polarization. These uncertainties are not included in the systematic error assessment in Sect. 3.1. L271 onwards, you state that “based on the statistical analysis of the measured signals over a long time, it can be concluded that the gain of the ACDL is consistent with that of the ground at this stage.” Please proof it. Actually, the insertable depolarizer is included to check the PGR in flight. Please make use of it and proof that it is the same as on ground.
3. Later, you state that PGR has an uncertainty of 1% (L446). How did you determine it? The same question was asked by the Anonymous Referee #2 to your previous submission and you haven’t answered it in the re-submission.
4. It remains unclear who you combine the parallel and cross-polarized signal to get the total signal for the attenuated backscatter coefficient. Please explain and provide formulas and uncertainties.
5. You nicely characterize the iodine vapor cell in Fig 6. But how does the Fabry-Perot etalon perform? Please comment on it and add some more details about it.
6. How do you keep the laser frequency stable at the iodine vapor cell wavelength? How do you deal with spectral cross-talk?
7. You mention the denoised lidar signal (e.g., L136). How do you denoise your signals? Please elaborate on it. And how the denoising might affect the later described filtering.
8. I am a bit puzzled about your low altitude verification. Why do you use the 8 – 12 km height range? In Fig. 12, you mark the red boxes at 26 – 30 km height, because obviously between 8 and 12 km is the huge cirrus cloud. In other words, aerosol particles and clouds are often present in the 8 – 12 km range. So, I would not expect a scattering ratio of 1 in this height range. The 1.03 shown in Fig. 13 seem to be reasonable. However, you cannot conclude on your calibration, because the true value is maybe not 1.0 but a bit higher.
9. The behavior of the calibration constant in Fig. 14 misses any explanation except the SSA. Why does the southern Pacific Ocean behave differently than the same latitudes over the Atlantic and Indian Ocean?
10. One should separate between verification and validation. Validation always includes external observations (suborbital or from another satellite), whereas verification can be done with only one’s own data. In your case, you present verification and refer to Liu et al., 2024, for validation. Maybe some more comments on the validation presented in the other paper might be helpful at this point.
11. The results are shown for observations in 2022. Could you comment on the stability of the calibration constant over the years, e.g., by showing an example from 2024. Does the calibration constant changes with time?
12. The manuscript has been previously submitted as https://doi.org/10.5194/egusphere-2024-588 and was not accepted. Especially, the Anonymous Referee #2 provided valuable comments on the manuscript. Many of his/her comments were included in the current version to improve the manuscript, but some are still missing, e.g., the determination of the PGR or a clear discussion about the variations in the calibration constant (Fig. 14). As a reviewer, I feel disappointed that not all comments from a previous review process were carefully included in the manuscript. Please take again his/her comments and improve the manuscript accordingly. I will try to check it while reading the revised version.
13. Unfortunately, the data are not publicly available which hampers the validation support by suborbital measurements from various lidar groups. And as a consequence, it hampers the validation of ACDL with data from different geographic regions.

Specific comments

• I welcome, that the title just contains the abbreviation of the satellite’s name and the description is given in the first lines of the abstract. Sometimes, the journal requests to not use abbreviations in the title, but here I support the authors and prefer the way as they have done it.
• L44 I would not call it particle backscatter if you include both aerosol and molecules.
• The introduction consists of 3 blocks which are consistent within each block, but are loosely linked to each other. Block 1 (L31-55) describes the satellite, block 2 (L56-86) presents the history of lidar in space and block 3 (L89-99) outlines the optical setup of the receiver. I would move block 3 out of the introduction and to Section 2. And then, you can ponder how to arrange the introduction best. Maybe a general introduction, then the history (block 2) and then presenting your satellite (block 1)? However, it is just a suggestion, and not a recommendation.
• In Fig. 2 – Could you provide a date for the orbit? And please already state a bit earlier, that during summertime there is daylight north of 60°N.
• L130 onwards: You describe the 6 steps quite short here. Please mention, that you’ll describe them in greater detail later on.
• L203: How do you align ERA5 to your ACDL profiles? Interpolation? Nearest-neighbor?
• L222: “backscatter as the central Cabannes line” – unclear formulation, maybe something like “backscatter because only the central Cabannes line”
• L258-260: Unclear formulation. What do you mean with additional schema in Fig. 6? The F-P etalon? Please discuss the F-P etalon in greater detail (see major comment above).
• L280: Unclear formulation. Does it mean what the calibration module in Fig. 1 emits an additional laser beam? Please explain the module in greater detail.
• L305: C is not shown as a function of C~, but as a function of latitude.
• L315: What do you mean by the adjacent-track distances? Do you use it already to derive global calibration constants? Please show it. Or do you just use the 500 km sliding average? It is not clear from your text.
• L344: How do you select the different factors. Please give a reasoning for the selection. Ideally would be to show how they are selected. Based on which criteria?
• Fig. 8+9 Please show the cross polar channel as well (or in the supplement).
• L395 What do you mean by negative spikes?
• Fig. 10. What are the differences between the parallel channel and the HSRL channel and why? What is the difference in this figure compared to the one shown in your response to reviewer #2 from the previous submission? What happened to the high values above Antarctica?
• Maybe I missed it, but in eq 31 and 32, there appears an E(z_c). What does it stand for?
• From Section 3.2 onwards, you switch the naming of the channels. Previously, you speak about the parallel and the HSRL channel, now you state polarization channel and HSRL channel. Usually, polarization channel refers to the cross-polarized channel. I would suggest a different naming for the non-HSRL channel. What about Mie channel, elastic channel or still parallel-polarized. Also, the name “total polarization signal” is misleading and should be removed.
• In Fig. 11 it is hard to spot the yellow line. The orbits (c+d) are not necessary to understand the figure.
• Why are you doing such complicated calculations in eq. 37? If you define the total backscatter coefficient as the sum of molecular and aerosol part, the result is obvious from eq. 36.
• In the text and caption of Fig. 13 you swapped the naming of “dashed” and “solid” lines. Furthermore, the scales in Fig. 13 could be improved. In a) it would be sufficient to show the x-axis between 0 and 2. In b) you could zoom in the y-axis and provide some more minor ticks.
• L 513: Where were the suborbital validation measurements performed? With which instrumentation? Airborne or ground-based.
• L519: Dark noise is mentioned here for the first time.
• L528: [28] – is probably a reference which does not show up correctly. It does not happen if you prepare the manuscript in latex.
• The summary is rather short. I think you should emphasize at some point what is a legacy of CALIPSO and what you developed newly for ACDL.
• L545: You have not validated your results, just verified. See my previous comment.

Technical corrections

Most of my technical corrections were already spotted by the anonymous reviewer #1. In general the language might be improved in some parts.

L270: which is defined

L328: It is shown in the upper left corner.

L356: subscript “valid” should not be italic.

Fig. 9 There are no orange lines, but red lines.