Towards a New MAX-DOAS Measurement Site in the Po Valley: Aerosol Optical Depth and NO₂ Tropospheric VCDs

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General Comments: (overall quality)

The work is interesting because it integrates the synergy of ground-based (MAX-DOAS) and satellite observational data to retrieve NO2 tropospheric concentration and aerosol extinction profiles and their corresponding tropospheric columns. Although only one study targeting the Po Valley is presented, the obtained results demonstrate the ability of the DOAS Optimal Estimation of Atmospheric Profile (DEAP) model to estimate tropospheric NO2 concentration with better accuracy than the TM5-MP global chemical transport model used in Sentinel-5P TROPOMI processing data. The methodology and results are presented and discussed clearly, easy to read. The figures and tables used are focused on the theme of this manuscript. However, a few questions should be addressed, and I would appreciate further discussion in the manuscript.

Specific comments:

Introduction

1. Do you know of previous studies that integrate the synergy of MAX-DOAS ground-based and satellite observational data to retrieve NO2 tropospheric concentration and aerosol extinction profiles and their corresponding tropospheric columns? I recommend that you provide an overview of what has been done before in terms of integrating observational data from ground-based instruments (MAX-DOAS, Pandora, Sunphotometer) and on board Earth observation satellites.

Section 2.2.1 "The DEAP retrieval code and set-up"

1. Explain the rationale behind the VCD of the NO2 a-priori profile set-up to 8x10^15 molec/cm^2.

2. How did varying the VCD of the NO2 a-priori profile influence the results?

3. It would be beneficial to explain in few sentences the cloud filtering approaches. Also, you must include some key references for that.

4. Please elaborate on the decision to use the color index for cloud filtering.

5. Explain the rationale behind the threshold value for color index set-up to 1.2. How does this choice affect the model accuracy?

Conclusions

1. What were the main technical challenges in the DEAP model for estimating aerosol extinction profiles? How can it be adjusted to be used with other similar instruments MAX-DOAS and PANDORA?

Technical comments:

1. Split Fig. 13 in two figures to be more clear to reader. For example, the "red dots" from lower panel are not visible. The same recommendation for Fig. 15.

Author Response

Comment: The work is interesting because it integrates the synergy of ground-based (MAX-DOAS) and satellite observational data to retrieve NO2 tropospheric concentration and aerosol extinction profiles and their corresponding tropospheric columns. Although only one study targeting the Po Valley is presented, the obtained results demonstrate the ability of the DOAS Optimal Estimation of Atmospheric Profile (DEAP) model to estimate tropospheric NO2 concentration with better accuracy than the TM5-MP global chemical transport model used in Sentinel-5P TROPOMI processing data. The methodology and results are presented and discussed clearly, easy to read. The figures and tables used are focused on the theme of this manuscript. However, a few questions should be addressed, and I would appreciate further discussion in the manuscript.

Response: The authors gratefully acknowledge the reviewer for his/her useful comments that help us to improve our work. The replies are in blue below each comment. The updates in the manuscript are reported in blue.

Specific comments:

Introduction

Comment: 1. Do you know of previous studies that integrate the synergy of MAX-DOAS ground-based and satellite observational data to retrieve NO2 tropospheric concentration and aerosol extinction profiles and their corresponding tropospheric columns? I recommend that you provide an overview of what has been done before in terms of integrating observational data from ground-based instruments (MAX-DOAS, Pandora, Sunphotometer) and on board Earth observation satellites.

Response: In the revised version of the manuscript, we added the following sentences in the introduction:

“ The comparison of MAX-DOAS or Pandora and satellite results in several sites has been thoroughly investigated in previous works (Yombo Phaka et al., 2023, Chan et al. 2020,  Judd et al., 2020, Ialongo et al., 2020)”

And the following references

 

Yombo Phaka, R., Merlaud, A., Pinardi, G., Friedrich, M. M., Van Roozendael, M., Müller, J.-F., Stavrakou, T., De Smedt, I., Hendrick, F., Dimitropoulou, E., Bopili Mbotia Lepiba, R., Phuku Phuati, E., Djibi, B. L., Jacobs, L., Fayt, C., Mbungu Tsumbu, J.-P., and Mahieu, E.: Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of NO₂ and H₂CO at Kinshasa and comparisons with TROPOMI observations, Atmos. Meas. Tech., 16, 5029–5050, https://doi.org/10.5194/amt-16-5029-2023, 2023.

 

Chan, K. L., Wiegner, M., van Geffen, J., De Smedt, I., Alberti, C., Cheng, Z., Ye, S., and Wenig, M.: MAX-DOAS measurements of tropospheric NO₂ and HCHO in Munich and the comparison to OMI and TROPOMI satellite observations, Atmos. Meas. Tech., 13, 4499–4520, https://doi.org/10.5194/amt-13-4499-2020, 2020.

Judd, L. M., Al-Saadi, J. A., Szykman, J. J., Valin, L. C., Janz, S. J., Kowalewski, M. G., Eskes, H. J., Veefkind, J. P., Cede, A., Mueller, M., Gebetsberger, M., Swap, R., Pierce, R. B., Nowlan, C. R., Abad, G. G., Nehrir, A., and Williams, D.: Evaluating Sentinel-5P TROPOMI tropospheric NO₂ column densities with airborne and Pandora spectrometers near New York City and Long Island Sound, Atmos. Meas. Tech., 13, 6113–6140, https://doi.org/10.5194/amt-13-6113-2020, 2020.

Ialongo, I., Virta, H., Eskes, H., Hovila, J., and Douros, J.: Comparison of TROPOMI/Sentinel-5 Precursor NO₂observations with ground-based measurements in Helsinki, Atmos. Meas. Tech., 13, 205–218, https://doi.org/10.5194/amt-13-205-2020, 2020.

 

Section 2.2.1 "The DEAP retrieval code and set-up"

Comment: 1. Explain the rationale behind the VCD of the NO2 a-priori profile set-up to 8x10^15 molec/cm^2.

Response: We set the a-priori profile at this value because it is a value that is within the ones that are observed in the Po Valley as can also be seen from the TROPOMI data. As highlighted in the reply to Comment 2, a variation of +/-25% of the a-priori profile has an effect of only 1% on our retrievals. This means that the choice of this value has not a critical impact on the retrieved results.

To clarify this point in the text we add the sentence:

“This value is within the ones that can be observed in the Po Valley, as can also be seen from TROPOMI data.”

Comment: 2. How did varying the VCD of the NO2 a-priori profile influence the results?

Response: We made some test varying the a-priori NO2 profile of +/- 25% for the 1 March 2022. The results indicate that the variation of +/25% in the a-priori profiles lead to a variation of only about 1% in the retrieved values.

We added a sentence on this in the paper:

“A variation of +/- 25% in the NO2 a-priori profile lead to a variation of only about +/-1% in the retrieved VCDs.”

Comment: 3. It would be beneficial to explain in few sentences the cloud filtering approaches. Also, you must include some key references for that.

Response:

In the manuscript we added:

“The color index approach is widely used for flagging cloud-contaminated MAX-DOAS measurements (e.g. Wagner et al., 2014, 2016, Van Roozendael et al., 2024, Sarkissian et al., 1991, 1994; Enell et al., 1999).Other methods involve the effects of 202 clouds on e.g. radiance, O4 absorption and Ring effect (Wagner et al., 2014).”

 

And related references:

Enell, C.-F., Steen, Å., Wagner, T., Frieß, U., Pfeilsticker, K., Platt, U., and Fricke, K.-H.: Occurrence of polar stratospheric clouds at Kiruna, Ann. Geophys., 17, 1457–1462, doi:10.1007/s00585- 999-1457-7, 1999.

 

Sarkissian, A., Pommereau, J.-P., and Goutail, F.: Identification of polar stratospheric clouds from the ground by visible spectrome- try, Geophys. Res. Lett., 18, 779–782, 1991.

 

Sarkissian, A., Pommereau, J., Goutail, F., and Kyrö, E.: PSC and volcanic aerosol observations during EASOE by UV-visible ground-based spectrometry, Geophys. Res. Lett., 21, 1319–1322, 1994. 

 

Wagner, T., Beirle, S., Remmers, J., Shaiganfar, R., and Wang, Y.: Absolute calibration of the colour index and O4 absorption derived from Multi AXis (MAX-)DOAS measurements and their application to a standardised cloud classification algorithm, Atmos. Meas. Tech., 9, 4803–4823, 2016. Doi:10.5194/amt-9-4803-2016. URL https://amt.copernicus.org/articles/9/4803/2016/amt-9-4803-2016.html.

The following references were already in the manuscript : 

Van Roozendael, Michel, Francois Hendrick, Martina M. Friedrich, Caroline Fayt, Alkis Bais, Steffen Beirle, Tim Bösch, Monica Navarro Comas, Udo Friess, Dimitris Karagkiozidis, and et al. 2024. "Fiducial Reference Measurements for Air Quality Monitoring Using Ground-Based MAX-DOAS Instruments (FRM4DOAS)" Remote Sensing 16, no. 23: 4523.https://doi.org/10.3390/rs16234523 

 

Wagner, T., Apituley, A., Beirle, S., Dörner, S., Friess, U., Remmers, J., and Shaiganfar, R.: Cloud detection and classification based on MAX-DOAS observations, Atmos. Meas. Tech., 7, 1289–1320, https://doi.org/10.5194/amt-7-1289-2014, 2014.

Comment: 4. Please elaborate on the decision to use the color index for cloud filtering.

Response: The use of color index (CI) allows to filter the cloudy data without the use of external information but using DOAS and MAX-DOAS spectra only. Actually the particles scattering effect on CI makes this indicator ideal for investigating cloud contamination.

In the text we added the following sentences:

“The use of color index (CI) allows to filter the cloudy data without the use of external information but using measured spectra only. Actually the particles scattering effect on CI makes this indicator ideal for investigating cloud contamination ”.

 

Comment: 5. Explain the rationale behind the threshold value for color index set-up to 1.2. How does this choice affect the model accuracy?

Response: The threshold value of 1.2 has been chosen in order to filter out the heavily contaminated spectra without removing too much data from the dataset, as can be seen it is in the middle of the transition region of the bimodal distribution. Using a value of 1.1 will not affect significantly the model accuracy while lower threshold should results in using spectra affected by clouds. In the other direction, using 1.3 will just result in removing some valid spectra while higher values will remove too much clear sky spectra. The 1.2 value is, thus, an ideal compromise.

In the text we added the following sentences:

“The threshold value of 1.2 has been chosen in order to filter out the heavily contaminated spectra without removing too much data from the dataset.”

 

Conclusions

Comment: 1. What were the main technical challenges in the DEAP model for estimating aerosol extinction profiles? How can it be adjusted to be used with other similar instruments MAX-DOAS and PANDORA?

Response: The main technical challenges in estimating the aerosol parameters are linked to the parameters we set in the SCIATRAN code, such as the surface and single scattering albedo, the Henyey-Greenstein phase function and the asymmetry factor. As we wrote in the paper, we use fixed values and have no way of having information on them at the moment. The second point is the possible impact of NO2 interference into AOD retrievals. About the use of the DEAP code on other MAX-DOAS or PANDORA instruments, it can be easily adapted to other instruments since its inputs are SCDs and the only parameters that need to be changed are the ones related to measuring geometry as well as information on the used spectral ranges.

Following these considerations we added the following sentence in the Conclusions:

“The DEAP code can be easily adapted to other instruments, like PANDORA or other MAX-DOAS, if information on measuring geometries and used spectral range is provided together with SCDs.”

 

Technical comments:

Comment: 1. Split Fig. 13 in two figures to be more clear to reader. For example, the "red dots" from lower panel are not visible. The same recommendation for Fig. 15.

Response: Ok, done, “red dots” was a typo it is “black crosses”.

 

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents a new MAX-DOAS measurement site and the retrieval results using the DEAP code. Various methods are employed to evaluate the performance of the DEAP code comprehensively. The results indicate the instrument and DEAP code are promising candidates for further study over the Po Valley. The study is well-structured, methodologically sound, and provides valuable insights into the background pollution conditions of the Po Valley. However, there are still some minor issues in the text that need to be corrected. There are a lot of various abbreviations in the text, so it is recommended to add descriptions (e.g., make a table) to make it more readable. Besides, most of the figures should be presented in a better form. In some cases, the legends of some figures are too large.

 

Line 310, sentences need to be adjusted to be presented more completely in the text.

Figure 7, some x-axis labels are misaligned, they should be adjusted.

Figure 12, what is the meaning of '%' in the lower right legend? Please check.

Section 4, there are too many paragraphs in this section; it is recommended to condense them for clarity.

Line 583, it doesn’t read well, please check.

Line 591-593, please reorganize the statements.

Line 599-600, add this sentence to the previous paragraph.

 

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Comment: This manuscript presents a new MAX-DOAS measurement site and the retrieval results using the DEAP code. Various methods are employed to evaluate the performance of the DEAP code comprehensively. The results indicate the instrument and DEAP code are promising candidates for further study over the Po Valley. The study is well-structured, methodologically sound, and provides valuable insights into the background pollution conditions of the Po Valley. However, there are still some minor issues in the text that need to be corrected. There are a lot of various abbreviations in the text, so it is recommended to add descriptions (e.g., make a table) to make it more readable. Besides, most of the figures should be presented in a better form. In some cases, the legends of some figures are too large.

Response: We gratefully acknowledge the reviewer for his/her useful comments that help us to improve our work. The replies are in blue below each comment. The updates in the manuscript are reported in blue.

As suggested by the reviewer, we also add a list of acronyms in Appendix A.

 

Comment: Line 310, sentences need to be adjusted to be presented more completely in the text.

Response: Ok, done

 

Comment: Figure 7, some x-axis labels are misaligned, they should be adjusted.

Response: OK, done also in Figure 8

 

Comment: Figure 12, what is the meaning of '%' in the lower right legend? Please check.

Response: The meaning of this % is that this is the number of points in percentage that fall within, above or below the MAIAC EE, represented by the two external dotted lines. We added this explanation in the caption.

 

Comment: Section 4, there are too many paragraphs in this section; it is recommended to condense them for clarity.

Response: Ok, done

 

Comment: Line 583, it doesn’t read well, please check.

Response: We rewrote the statement as “The results we present here highlight the potential of the SPC observatory in providing a valuable insight into the background Po Valley’s conditions”.

 

Comment: Line 591-593, please reorganize the statements.

Response: We rewrote the statement as “ Thus, the SPC MAX-DOAS site proves to be a valid candidate for validating the Sentinel-5P TROPOMI and future Sentinel-4 and 5 missions under the background pollution conditions of the Po Valley”

 

Comment: Line 599-600, add this sentence to the previous paragraph.

Response: Ok, done

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents the retrieval of tropospheric NO₂ and aerosol extinction profiles using a MAX-DOAS instrument, deployed at the "Giorgio Fea" observatory in San Pietro Capofiume (SPC), Po Valley since October 2021. The retrieval process utilizes the newly developed DEAP code, which has been validated using synthetic and real differential Slant Column Densities (dSCDs) and compared with three established retrieval codes. Additionally, the results are inter-compared with Sentinel-5P TROPOMI for NO₂ Vertical Column Densities (VCDs) and MODIS-MAIAC for Aerosol Optical Depth (AOD). The findings indicate a bias of -0.6×10¹⁵ molec/cm² for NO₂ VCDs and 0.04 ± 0.08 for AOD, showing a good agreement with satellite data. The study shows SPC as a representative site for background pollution conditions in the Po Valley, making it a valuable location for satellite validation and atmospheric research.
Just a few suggestions to improve this manuscript:
1) Eq. (6): To help the reader to better understand the iterative method in the retrievals, I suggest adding 1-2 figures to display convergence process of cost function (chi-square value as a function of iteration number) and the fitting of the measurement vector y (at different elevation angles) within several (<20) iterations.
2) How to calculate the degrees of freedom (DOFs) or DFS and corresponding representations need to be briefly explained.
3) Line 234, 261: 9x1010 molec/cm3, please double check this multiplication sign.
4) Eq. (9-10): “TROPOSPHERIC ”-> “TROP”.

Author Response

Comment:This manuscript presents the retrieval of tropospheric NO₂ and aerosol extinction profiles using a MAX-DOAS instrument, deployed at the "Giorgio Fea" observatory in San Pietro Capofiume (SPC), Po Valley since October 2021. The retrieval process utilizes the newly developed DEAP code, which has been validated using synthetic and real differential Slant Column Densities (dSCDs) and compared with three established retrieval codes. Additionally, the results are inter-compared with Sentinel-5P TROPOMI for NO₂ Vertical Column Densities (VCDs) and MODIS-MAIAC for Aerosol Optical Depth (AOD). The findings indicate a bias of -0.6×10¹⁵ molec/cm² for NO₂ VCDs and 0.04 ± 0.08 for AOD, showing a good agreement with satellite data. The study shows SPC as a representative site for background pollution conditions in the Po Valley, making it a valuable location for satellite validation and atmospheric research.

 

Response: We gratefully acknowledge the reviewer for his/her comments, below each comments

 In blue the reviewer can find our answer. The updates in the manuscript are reported in blue.

Just a few suggestions to improve this manuscript:

Comment: 1) Eq. (6): To help the reader to better understand the iterative method in the retrievals, I suggest adding 1-2 figures to display convergence process of cost function (chi-square value as a function of iteration number) and the fitting of the measurement vector y (at different elevation angles) within several (<20) iterations.

Response: We understand the reviewer’s point of view, however we think that the convergence of the chi-square in iterative methods and the way the simulated SCDs change from the ones obtained with the initial guess to the final ones with respect to the measured ones are common and well understood features of this type of retrieval. Furthermore, the convergence process can be different from one scan to another so such a type of plots will just be representative of one single case. For this reason and the fact that we already have a lot of figures in the paper we prefer not to include such plots in the paper.

 

Comment: 2) How to calculate the degrees of freedom (DOFs) or DFS and corresponding representations need to be briefly explained.

Response: We have inserted “calculated as the trace of the AK matrix” before the first time we speak about DOFs

 

Comment: 3) Line 234, 261: 9x1010 molec/cm3, please double check this multiplication sign.

Response: Ok done

Comment: 4) Eq. (9-10): “TROPOSPHERIC ”-> “TROP”.

Response: Ok done