Towards a New MAX-DOAS Measurement Site in the Po Valley: NO₂ Total VCDs

Round 1

Reviewer 1 Report

This paper presents the installation of a new commercial instrument at a remote station in the Po valley, and the 2 preliminary campaigns where the instrument was operated in parallel to other existing instruments (MAXDOAS in Bologna and Pandora in Rome). The topic is interesting, but there is a lack of presentation of the new site (what is the typical NO2 content? is it subject to typical wind patterns, ...) and the "preliminary" analysis done so far (only focusing on zenith-sky measurements and retrieving NO2 total VCD) lack explanations (nothing is said about the used AMF!), previous references and error analysis. The results are quantitatively compared to the others ground-based instruments and to satellite data from OMI and TROPOMI, but only very small explanation is given on the found biases (between 10 and 20% at least) and there is no error analysis/quantification.
The work should be improved in several aspects before publication.

In the introduction, the context is set up, but there is not enough references to past similar activities and how things are done differently here to retrieve total NO2 VCD during the whole day from zenith-sky measurements (eg mention SAOZ measurements at twilight (eg Pommeraau and Goutail 1988) and/or efforts to retrieved tropospheric VCD from zenith-sky, as Chen et al 2009 and Tack et al. 2015, add references about OMI and TROPOMI validation, ...)
Generally, references are missing (eg in section 2.5, lines 294-295 stating that MAXDOAS and Pandora are used for satellite validation, some references should be included as examples!)
In Section 2 Material and Methods, only the slant columns SCD are presented and nothing is said about the AMF, which are known source of biases (eg Wagner et al .2007, Hendrick 2006). AMF is only mentioned at the end of page 17 (be careful, line numbers are missing after Table 3 in page 17!).
Moreover, the choices for the QDOAS analysis presented in Table 1 and 2 are not justified by any literature or any explanations, while they include non "standard"/non so commonly included absorbers (eg NO3 and OClO) - see e.g. Roscoe et al., 2010. Also, the references fro the trace gases cross-sections are not included in the References list. Note also the misspell "Van Daele" --> "Vandaele" and the non coherence of the temperatures (eg NO2 298K and revference "from Van Daele et 294K"), the repetitions (for Glyoxal, Glyoxal Glioxal Volkamer) or the inconsistencies (HCHO in the tables, CH20 in the text, which is also a not very common notation!). Please revisit tables 1 and 2 in detail!
In section 2.3 O4 is mentioned and is role to filter for clouds (lines 213-231), but some references to previous work should be given. At least Wagner et al 2004
line 322-323: The selection of the OMI cloud free pixels (only those with the inverse of CF greater then 0.75) is not conventional (often CF<20%), with no explanations or references. please comment on this choice.

The errors should be quantified (the SCD error, the estimation of the reference content, the AMF), and put in relation to the found biases between the instruments (eg the estimation of content in the reference spectra is often much larger than the bias presented in fig 5 and 6).

Figures 10,11, 13: explain why TROPOMI VCD are a "bar" - do you show the values of all the pixels within the selected radius? What is then used for table 3 and 4? the comparison to the mean value of the satellite (or to the closest pixel?)? and what is the valuie after plus and minus? the spread? please clarify.

Figure 7 and 13: please check the legend --> "TRO TROPOMI"

Figure 14: please mention which site is Bologna and which SPC, and please clarify in the caption that the satellite pixels are here only represented as a point in the pixel center (the pixels are not presented with their full extension here!).
 

references:
Chen, D., Zhou, B., Beirle, S., Chen, L. M., and Wagner, T.: Tropospheric NO2 column densities deduced from zenith-sky DOAS measurements in Shanghai, China, and their application to satellite validation, Atmos. Chem. Phys., 9, 3641–3662, https://doi.org/10.5194/acp-9-3641-2009, 2009.

Pommereau, J. P., and F. Goutail (1988), O3 and NO2 ground-based measurements by visible spectrometry during arctic winter and spring 1988, Geosphys. Res. Lett., 15, 891–894, doi:10.1029/GL015i008p00891.

Tack, F., Hendrick, F., Goutail, F., Fayt, C., Merlaud, A., Pinardi, G., Hermans, C., Pommereau, J.-P., and Van Roozendael, M.: Tropospheric nitrogen dioxide column retrieval from ground-based zenith–sky DOAS observations, Atmos. Meas. Tech., 8, 2417–2435, https://doi.org/10.5194/amt-8-2417-2015, 2015.
 
Roscoe, H. K., Van Roozendael, M., Fayt, C., du Piesanie, A., Abuhassan, N., Adams, C., Akrami, M., Cede, A., Chong, J., Clémer, K., Friess, U., Gil Ojeda, M., Goutail, F., Graves, R., Griesfeller, A., Grossmann, K., Hemerijckx, G., Hendrick, F., Herman, J., Hermans, C., Irie, H., Johnston, P. V., Kanaya, Y., Kreher, K., Leigh, R., Merlaud, A., Mount, G. H., Navarro, M., Oetjen, H., Pazmino, A., Perez-Camacho, M., Peters, E., Pinardi, G., Puentedura, O., Richter, A., Schönhardt, A., Shaiganfar, R., Spinei, E., Strong, K., Takashima, H., Vlemmix, T., Vrekoussis, M., Wagner, T., Wittrock, F., Yela, M., Yilmaz, S., Boersma, F., Hains, J., Kroon, M., Piters, A., and Kim, Y. J.: Intercomparison of slant column measurements of NO2 and O4 by MAX-DOAS and zenith-sky UV and visible spectrometers, Atmos. Meas. Tech., 3, 1629–1646, https://doi.org/10.5194/amt-3-1629-2010, 2010.
 
Wagner, T., Burrows, J. P., Deutschmann, T., Dix, B., von Friedeburg, C., Frieß, U., Hendrick, F., Heue, K.-P., Irie, H., Iwabuchi, H., Kanaya, Y., Keller, J., McLinden, C. A., Oetjen, H., Palazzi, E., Petritoli, A., Platt, U., Postylyakov, O., Pukite, J., Richter, A., van Roozendael, M., Rozanov, A., Rozanov, V., Sinreich, R., Sanghavi, S., and Wittrock, F.: Comparison of box-air-mass-factors and radiances for Multiple-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) geometries calculated from different UV/visible radiative transfer models, Atmos. Chem. Phys., 7, 1809–1833, https://doi.org/10.5194/acp-7-1809-2007, 2007.

Wagner, T., Dix, B., Friedeburg, C. v., Frieß, U., Sanghavi, S., Sinreich, R., and Platt, U.: MAX-DOAS O4 measurements: A new technique to derive information on atmospheric aerosols – Principles and information content, J. Geophys. Res., 109, D22205, https://doi.org/10.1029/2004JD004904, 2004.
 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

1. Sumary

This paper presents two one-month-long inter-comparison campaigns in summer of 2021 conducted in Italy and Rome, respectively. The total NO2 VCDs of 3 types of ground-based remote sensing instruments and 2 satellite products (OMI and TROPOMI) were compared and instrument performances were assessed, including two MAX-DOAS instruments (SkySpec-2D and TROPOGAS) and Pandora. In general, it was found that the results of SkySpec-2D were within the other two ground-based instruments, and both of the two satellite products underestimated the total NO2 VCDs during the two campaigns.The paper also gives the first results of SkySpec-2D measurement at its final location (SPC, Italy) in rough six months, and repeated comparison with TROPOMI. An overestimation of satellite product on total NO2 VCDs was found during winter period.

  MAX-DOAS is a powerful technique for air quality observation, research, satellite products validation and improvement. It is of great importance that Italy has taken steps to start  MAX-DOAS operation in Po Vally and expand the FRM4DOAS validation network.

 

  This study fits well with the scope of of the journal. The manuscript is reasonably well written and efforts were made to discuss the results. I recommend publishing the paper after addressing the major concerns. 

2.Major concerns:

2.1 Though it makes sense to conduct comparisons between different instruments, it is of crucial importance to figure out the main objective of doing so. Take this manuscript for example, the reviewer agreed with the authors on the necessity of two inter-comparisons in summer of 2021, to quantitively assess the intrument performance of SkySpec-2D, assuring its ability for air quality observation, research and satellite products validation etc..

But, it seems unclear what the purpose is to compare the SkySpec-2 at SPC with the TROPOGAS  during October 2021 at Bologna, along with the repective comparison with TROPOMI, including line 404 to 419 and Figure 12-14).

These two measurements were conducted with different instrument at different sites ,which are dozens of kilometers away from each other and maybe effected by different emission sources.The reviewer consider it unnecessary to include this part in the manuscript.

2.2 The authors have made efforts in discussing the possible reasons for discrepencies found among different ground-based instruments and also between the instruments and satellite products during the two inter-comparison campaigns.  This is quite important because this will be the elite part of the paper, to explore why these comparison results are happening and thus improve our knowledgement in science,giving instructions to make improvements.

The reviewer think more work needs to be done in the discussion section of the manuscript.

It is found by many researches that the satellite product underestimate the total NO2 VCDs compared with ground-based measurements, in different areas of world,with different emission levels. Because of more severe (aerosol) pollution in winter period, the underestimation of the satellite product are often reported too.While an overestimation of TROPOMI (compared with SkySpec-2D) during cold season at SPC was presented in this manuscript, this phenomenon deserves a thorough investigation.

 Many reseaches have been done to explain the discrepencies lied between ground-based measurements and satellite products.Apart from the impact of SZA on AMF, showed in the manuscript, more possible reasons should be investigated, like the aerosol shielding effect, gradient smoothing effect, vertical profiles used by TROPOMI product and so on.

 

2.3 The reviewer suggest a more detailed analysis of the 6-month measurement at SPC, like the diurnal and seasonal variation patterns of total NO2 VCDs. Anyhow, this is part of the purpose of setting up this MAX-DOAS measurement at SPC.

 

3.Specific comments

1)Line 350-351:rewrite the sentence.Show the results for SkySpec-2D first and then the TROPOGAS, following the way of description for comparison results of TROPOMI.

2)More compasion and ananlysis results should be included in the Conclusion section.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript mainly introduces the comparative observation of SkySpec-2D with TROPOGAS and Pandora, and attempts to add the SkySpec-2D instrument set up in Po Valley to the FRM4DOAS network to validate the satellite data. However, the manuscript is more like an application report, and rarely sees methodological and scientific content now. The organization of the current paper and some major issues must be carefully considered.

 

1.       Some quantitative conclusions from comparative observations should appear in the abstract.

2.       The necessity for SkySpec-2D set up in Po Valley to join the FRM4DOAS network was not clearly explained in the Introduction. Does it matter just because there is no FRM4DOAS site in Po Valley?

3.       In section 2.1.1, Can one spectrometer observe the spectrum in the 300-550 nm band?

4.       How are the settings of SCIATRAN during calculating AMF? i.e. surface albedo, aerosol optical properties, temperature-pressure profiles, aerosol and trace gases profiles.

5.       How to ensure the accuracy of AMF simulations if encountering high-altitude polluted air masses?

6.       Why choose a fixed reference spectrum instead of a scan reference spectrum? Have you compared the differences between them?

7.     What are the standards for cloud filtering?

8.       The ground-based data within ±60 minutes of the satellite overpass is selected during the validation between MAX-DOAS and TROPOMI/OMI. if this time is shortened, such as ±30 minutes, will the validation be better?

9.       The settings of SkySpec-2D and TROPOGAS instruments are not clearly explained, such as how to set the elevation sequence? What is the azimuth of the observation?

10.    SkySpec-2D and TROPOGAS are both scattered light observation mode, while Pandora is a direct light observation mode.

- Since Pandora is a direct sunlight observation mode, there is a situation in which the azimuth angle is inconsistent with that of the MAX-DOAS observation. Is such a comparison meaningful?

- The retrieval settings of Pandora cannot be controlled by itself, so how much error will the retrieval settings (such as the selection of absorption cross sections) introduce to the results?

11. In section 2.5, Why not compare the tropospheric NO2 VCD between satellite and MAX-DOAS?

12. As shown in Figure 7, under the same observation conditions and the same retrieval settings, what is the main reason for the large difference in the results of TROPOGAS SkySpec-2D VCDs?

13. Has the effect of water vapor absorption been taken into account when retrieving NO2? How is the humidity at the observation site?

14. The two comparison observations were very short, and they were basically carried out on sunny or clean days. How to evaluate the comparative results of each instrument in different seasons and under different pollution conditions? Will there still be high consistency?

15. Can the vertical profiles retrieved from SkySpec-2D and TROPOGAS also be compared? At the same time, the retrieved vertical profiles of aerosols and trace gases being input into the RTM model also can improve the accuracy of AMF calculations.

16. The section of “Materials and methods” is too long. Can you put some unimportant information in the supplyment?

17. The structure of the manuscript also needs to be reworked so that it does not look like an application report.

Comments for author File: Comments.pdf

Author Response

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Round 2

Reviewer 2 Report

The logic of the manuscript is clear. Both  of the intercomparisons were properly analyzed and presented.Looking forward to further study of NO2 columns and vertical profile characteristics derived from this new MAX-DOAS measurement in Italy.

Author Response

Thank you very much for your comment. We are really hopeful to improve soon the quality of our NO2 retrieval in the Po Valley.

Reviewer 3 Report

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Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf