Review on the manuscript entitled “Cross validations of the Aeolus aerosol products and new developments with airborne high spectral resolution lidar measurements above the Tropical Atlantic during JATAC” by Trapon D. et al.

The manuscript presents the results of a validation study on the Aeolus Level 2A products using as reference collocated airborne lidar measurements from the NASA’s HALO instrument onboard the DC-8 aircraft that was deployed at the CPEX-CV campaign (Cabo Verde, summer 2022) in the framework of the Joint Aeolus Tropical Atlantic Campaign (JATAC). The study is focused on evaluating the performance of the Aeolus L2A products from the reprocessed Aeolus dataset with the latest available version (Baseline 16) that are provided in the highest available horizontal resolution (~18 km) from the Standard Correct Algorithm (SCA) and the Maximum Likelihood Estimation (MLE) algorithms. Moreover, ground-based lidar observations from the PollyXT lidar during the JATAC campaign have been used to facilitate the wavelength and the total to parallel conversions for the harmonization of the HALO products with Aeolus. The manuscript demonstrates the capability of Aeolus to retrieve profiles of the aerosol properties (L2A products) in higher horizontal resolution and provides valuable recommendations for the Aeolus data users.

Overall, the manuscript is well structured and well written even though I have the feeling that some parts could be further explained and/or discussed. The scientific significance makes the manuscript suitable for publication in AMT, after some minor revisions have been considered from the authors.

Specific comments

Line 15 “… Moreover the cross with independent…”: It is unclear what cross (validation?) refers to, please revise.

Lines 27 – 28 “…hence the will to focus on aerosol-free regions of the atmosphere with atmospheric signal by molecules from the Rayleigh channel.”: This part of the sentence is unclear, please rephrase.

Lines 28 – 29 “…attenuation of the overlying molecular atmosphere…”: Do the authors mean attenuation of the laser beam due to the overlying molecular atmosphere? Please clarify.

Line 38 “…higher horizontal resolution.”: Maybe provide here the resolution value for the sub-BRC level product?

Line 48 “. The limited geolocation”: I think that the word "limited" may give a negative sense, while such small geolocation/time offsets can provide a very good collocation as you mention below. Kindly consider to change the "limited" with another word (e.g. small).

Line 84 “…derived from the cross-talk products…”:  Do the authors mean “derived from the cross-talk corrected products”? Kindly revise accordingly.

Line 85 “. They …”: It is not so clear in which quantities do the authors refer to. To my understanding they are the β_part, β_mol, α_part.  I would suggest to rephrase or maybe add their shortcuts (e.g. β_part, α_part, etc).

Lines 96 – 97 “…signal accumulation of 600 consecutive laser pulses comprised in measurements up to 30, then averaged over ≈ 90 km horizontal to form an observation referred as BRC.”:  It may be unclear for a reader not so familiar with the concept of 30 measurements averaged to produce one BRC. I would kindly suggest to rephrase.

Line 98 “…with measurement level and…”: The measurement level (i.e. horizontal resolution ~ 18 km) is defined in Line 102 but I think it should be introduced here firstly, mentioning that the measurement level corresponds to 18 km hor. resolution.

Line 102 “…It was decided…”: It was decided from who? The authors of the current study, the MLE developers, the Aeolus DISC team? I would suggest to rephrase and add that the MLE-sub is an additional product next to the original MLE product (in BRC level) and both products are available.

Line 103 “… JATAC September 2022 settings…”: I don’t get what kind of JATAC 2022 settings the authors refer to? Kindly include more details.

Lines 107 – 109 “The L2A products …. for particles α_part.”: This sentence repeats what is mentioned in lines 103 – 104 (“…both SCA and MLEsub … referred as measurement level.”). But, in lines 107 – 109 the “then” word implies that you apply further processing to the SCA attn. backscatter and the MLE-sub products to align them to the measurement level, which is not correct as the SCA attn. backscatter and the MLE-sub are already provided in measurement level. Please revise accordingly.

Lines 115 – 122 “The main objectives… convection (Flamant et al., 2024).”:  These lines should be rephrased to better explain the JATAC objectives (main objective of Aeolus validation and additional scientific objectives), the lidar instrumentation the authors would like to focus on (maybe split into ground-based and airborne instrumentation, rather than include some of them in one sentence as is now). I think that the Safire Falcon-20 aircraft and the deployed doppler lidar for Aeolus validation should also be mentioned next to the DLR aircraft. Moreover, the discussion about HALO and DC-8 should be moved to the CPEX-CV section. Kindly check for more details on the deployed instruments and observational platforms as well as the JATAC objectives the abstract of Fehr et al.: The Joint Aeolus Tropical Atlantic Campaign 2021/2022 Overview– Atmospheric Science and Satellite Validation in the Tropics, EGU General Assembly 2023, EGU23-7249, https://doi.org/10.5194/egusphere-egu23-7249, 2023.

Line 129 “CPEX-AW campaign”:  Are there any references to be cited for CPEX-AW?

Line 132 – 133 “On-board instruments and dropsondes have then be operated to be cross analysed

with external aerosol retrievals”: I would suggest to remove the “then” word and rephrase as it erroneously implies that the instrumentation in CPEX has been collecting measurements with the goal to be cross analysed with external instrumentation, which is something that is performed in the context of the current study.

Line 140 – 141 “It is an active instrument differential absorption lidar (DIAL) and HSRL with multiple configurations including water vapor DIAL and HSRL, and methane DIAL and HSRL.”: Are there any publications where the HALO instrument is described and can be used as reference for more details from the readers?

Lines 145 – 146 “…, the spectral dependence of optical properties for desert dust being known to be less pronounced between 355 nm and 532 nm than between 355 nm and 1064 nm”:  Here the authors could add a comment that this is the reason why the HALO products at 532 nm are preferred over the products at 1064 nm for the conversion to 355 nm.

Lines 147 – 148 “Moreover HALO transmits linear polarization whereas Aeolus transmits circular polarization”: Here the authors could add a comment on if and how this difference is being considered for the HALO and Aeolus cross-comparisons

Lines 151 – 168: While eq. (1) contains terms that depend on the HALO measuring technique, such as the measured signal and the transmission term for the iodine filter, which is not introduced before and it is unclear for a reader not familiar with the HSRL technique how this filter and its specs affects the retrieval of the extinction, equations 2 – 4 are generic equations. I would suggest to include same level of detail for eq. 2 as for eq. 1 or include the equations for the calculation of the cross-polarized and co-polarized backscatter coefficients. Moreover, the total scattering ratio is introduced in eq. 4 and used in the analysis later, but it is not mentioned before as a parameter that is used in this study (e.g. in lines 148 – 150). As such, kindly revise the section where you describe which HALO parameters are used in the study.

Equation 1: Does this δ/δr means the partial derivative? The δ symbol is also used for the depolarization ratio. Please clarify and use the symbols that are commonly used for the partial derivative.

Lines 158 – 159 “  is filtered molecular scattering channel”: It is not clear if this is the measured signal. Kindly clarify in the text.

Line 181 “It is proposed to…”: From who is it proposed? I would suggest to revise and maybe use a more neutral phrase to start this sentence.

Line 191 “…optical properties are cloud-screened and are quality assured…”: Please clarify in the text which optical parameters (  and  or Angstrom exponents) and from which instrument (PollyXT or HALO).

Lines 216 “A mean value of 5.0⁻⁴ has then been derived and used as a constant for all scenes”: Please add the units and clarify in the text what this constant is (is it the molecular extinction above DC-8 up to 80 km?).

Lines 219 – 220 “Only the valid bins with positive values are considered for statistics (i.e., the invalid measurements coded as NaN in one product being ignored on the second dataset and vice-versa).”: Please clarify if you apply this flagging (i.e. only positive values) for all the products from Aeolus and HALO that are used in the cross-comparisons.

Lines 231 – 234 “The HALO 532 channel… 300 m vertical average.”: I find hard to easily understand the difference on the vertical resolution (i.e. bin length of 30 m) and the sampling interval. Does this sampling interval correspond to the ~0.5 seconds of sampling? Please revise this section and clarify what is the vertical resolution and the sampling interval. Moreover, if the vertical resolution is 30 m, then why do you interpolate to 15 m, please clarify. In line 233, please name, next to the extinction coefficient, which products that are used in the study are calculated after further vertical averaging of 300 m.

Lines 234 – 235 “The HALO data are sampled each ≈ 0.5 seconds, a 10 second average being applied to the backscatter coefficient along the direction of flight”: Does this mean that only the backscatter coeff. is calculated after an additional 10-second averaging is applied? What about the rest products that are used in the study. Please clarify and revise the text if needed.

Figure 3 “The NASA DC-8 flight track is superimposed in red color code and the 6 consecutive BRC level observations of Aeolus orbit file no. 23562 are displayed (a). They correspond to 30 Aeolus measurements given at sub-BRC level ≈ 18 km, and to 225 HALO profiles (b)”: I find these two sentences a bit confusing so I would kindly suggest to split the info (flight track and number of profiles) in two separate parts, one for HALO and one for Aeolus.

Line 259 – 260 “Figure 4a illustrates how the Aeolus MLEsub βpart up to 18 km altitude helps assessing the particle-free conditions above the DC-8 flying at ≈ 10.7 km altitude for the 16 September 2022 case”: Please add a brief discussion on the “how”, bridging with lines 251 – 254.

Lines 261 – 263 “PollyXT emitting linear polarization (Engelmann et al., 2016), the circular depolarization ratio at 355 nm is derived to recompute the parallel backscatter coefficient at 355 nm with equations similar to Eqs. (5) and (6)”: I find this sentence confusing, kindly re-phrase.

Line 265 “Figure 4b shows how the HALO 532 nm…”: Please add that the figure shows also the converted PollyXT 355 parallel backscatter.

Line 266 “The HALO parallel 355 nm profile agrees with Aeolus MLEsub (i.e., violet)”: Since the authors denote the color of the Aeolus MLEsub backscatter, consider adding the color info also for the HALO parallel 355 backscatter. Moreover, I would suggest to discuss also the underestimation of MLEsub at 3 km.

Line 273 – 275 “Similar observations can be made with case 09 September… (Appendix A4b)”: Although the case on 15 September is mentioned as one of the three cases, only the results for 9 and 16 September are discussed. Please add a dedicated discussion also for the results on 15 Sep.

Figure 4: (4b) The converted HALO parallel backscatter 355 nm does not agree with the parallel backscatter from PollyXT, but agrees with the total backscatter from PollyXT inside the dust layer heights (1-4 km). This difference is also visible at 2 km in Figure A4(b), but in Figure A5(b) the profiles are as expected (i.e. similar values for the parallel backscatter 355 from PollyXT and HALO). Could it be the depolarization ratio from the two systems that is used in the conversions, or the backscatter related Angstrom exponent? Maybe an additional subplot with the particle linear depolarization ratio profiles (355 nm for PollyXT; 532 nm for HALO) that are used in the conversions could add in the discussion on the observed differences? As such, I think that a dedicated part where the authors discuss these differences and with their potential source(s) should be added in section 5.1. Moreover, based on the presented results and the discussion, a concluding remark should be added on the evaluation of the wavelength and total to parallel conversions.

(4c) It seems to me that when comparing the extinction from HALO and PollyXT, a small vertical displacement is visible (e.g. check the peak at ~2.6 km for HALO instead the peak at ~3.5 km for PollyXT). Could the authors comment on this?

Line 299 “…for second-last measurement 34 is even more clear.”: I would suggest to specifically mention to which instrument/platform (Aeolus or HALO) belongs the mentioned measurement number (e.g. “for second-last Aeolus measurement 34”). Kindly harmonize throughout the whole section.

Line 314 “up to ≈ 3^-3 km^-1 sr^-1”: Do the authors mean 3·10^-3? If yes, kindly correct it and harmonize all similar fields (e.g. line 316) with the correct format.

Line 316 “…with values below ≈ 2 ^-3 km^-1 sr^-1…”: please clarify if the mentioned values are measured from Aeolus of HALO.

Line 323 “The HALO dominant aerosol type points to marine aerosols”: The third regime in Figure 6f contains also points classified as dusty mix, so why do the authors claim that the dominant type is marine aerosols? Could the authors support their claim (e.g. number of points classified as marine vs dusty mix) and include a dedicated part in the text?

Figure 6: A general suggestion for the HALO particle depolarization ratio that is illustrated in all the 2-D histograms and discussed in the subsections of 5.2 is to use the particle linear depolarization ratio that is directly retrieved from HALO, instead of the converted circular depolarization ratio, in the corresponding figures and in their discussion throughout the manuscript as the linear depolarization ratio can be easily linked with references to past studies for aerosol characterization (e.g. Freudenthaler et al., 2009). However, the authors may also keep (as an additional info) the indicative particle circular depolarization ratio values that are already mentioned in the discussion of the related figures.

Lines 339 – 340 “…deviations correspond to the first regime where aerosol loading is weak and falls below the classification threshold of 0.2 in HALO aerosol scattering ratio at 532 nm σ_aer,532”: I would suggest adding the color code that corresponds to the first regime too. Moreover, the threshold value of σ_aer,532 = 0.2contradicts with the definition of σ_aer,532 in Eq. 4 which cannot take values less than 1. Please revise Eq. 4 and clarify throughout the manuscript which one of the total/aerosol scattering ratio is used.

Lines 347 – 348 "Removing the contributions from low scattering below HALO σ_aer,532 of 1.1 has a significant impact on the slope (i.e., increased from 0.29 to 0.48)”: The authors apply this threshold only in the comparison for the SCA attenuated backscatter. It would be interesting to see the scores after applying the threshold also in the MLEsub comparisons.

Lines 372 “One could flag the cloudy regions applying a scattering ratio threshold of 5…”: Same as previous comment (Lines 347 – 348). It would be interesting to see if this total scattering ratio mask (1.1 < σ_aer,532<= 5) for low scattering and cloud contaminated regimes would improve the comparison between Aeolus and HALO also in the cases on 9 and 15 September. Could the authors support why do they apply this scattering ratio mask only for the case on 16 September, while clouds are also present in the cross sections of 9 and 15 September?

Line 275 “Marine aerosols are well characterized by Aeolus SCA (blue color code in Fig. 10f)”: Is Aeolus SCA a typo here? Do the authors actually refer to the characterization of marine aerosols by HALO? Please clarify and revise the sentence.

Lines 411 – 414 “We then do recommend … then appearing as the best compromise”: In the results section I didn’t see any discussion about the different range bin settings and how they impact the scores, as the regimes are identified by the depolarization ratio or the aerosol type products from HALO. I would suggest the authors to include relevant discussion about the impact of range bin settings in the “Results” section before concluding to the recommendations in the “Conclusions” section.

Technical corrections

Line 4 “… (i.e., backscatter, extinction, attenuated backscatter, and depolarization coefficients) …”: Suggested change to "backscatter, extinction, attenuated backscatter coefficients, and depolarization ratios"?

Line 29 “…decreases with altitude…”: Suggested change to " decreases with altitude decrease"?

Line 29 “…the smaller values…”: Suggested change to " with the smaller SNR values"?

Line 31 “forms an L2A”: “forms a L2A”

Line 49 “…of these flights with Aeolus…”: Suggested change to “between DC-8 flights and Aeolus”

Line 91 “. It makes use …”: Suggested change to “The MLE approach makes use”

Line 218 “… as SCA particulate attenuated backscatter…”: is something missing here?

Line 265 “before crossing Aeolus and HALO…”: do the authors mean before comparing (or cross analyzing) Aeolus and HALO?