Characterization of OMI tropospheric NO<sub>2</sub> over the Baltic Sea region

Ialongo, I.; Hakkarainen, J.; Hyttinen, N.; Jalkanen, J.-P.; Johansson, L.; Boersma, K. F.; Krotkov, N.; Tamminen, J.

doi:https://doi.org/10.5194/acp-14-7795-2014

Articles | Volume 14, issue 15

https://doi.org/10.5194/acp-14-7795-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-14-7795-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 15

Research article

|

05 Aug 2014

Research article |

| 05 Aug 2014

Characterization of OMI tropospheric NO₂ over the Baltic Sea region

I. Ialongo, J. Hakkarainen, N. Hyttinen, J.-P. Jalkanen, L. Johansson, K. F. Boersma, N. Krotkov, and J. Tamminen

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Final revised paper (published on 05 Aug 2014)
Supplement to the final revised paper
Preprint (discussion started on 23 Jan 2014)

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Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC C191: 'Review of Ialongo et al.', Anonymous Referee #2, 22 Feb 2014
- AC C2068: 'Answer to referee n.2', Iolanda Ialongo, 05 May 2014
RC C732: 'review', Anonymous Referee #1, 25 Mar 2014
- AC C2069: 'Answer to referee n.1', Iolanda Ialongo, 05 May 2014
- AC C2111: 'Answer to referee n.1 - last page', Iolanda Ialongo, 06 May 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Iolanda Ialongo on behalf of the Authors (05 May 2014) Author's response

ED: Referee Nomination & Report Request started (06 May 2014) by Andreas Richter

RR by Anonymous Referee #1 (15 May 2014)

Suggestions for revision or reasons for rejection

As pointed out in my review, the original manuscript lacks of scientific significance and would have required major revisions.
The revised version of the manuscript sticks to some cosmetic corrections, but does not provide significant improvements.
I do not consider the revised mansucript acceptable for ACP.

Main concerns:

1. The authors missed the opportunity to extend their case studies to the Baltic sea region - according to the manuscript title.
In particular I do not understand why the authors refuse to estimate emissions and lifetimes for other hot-spots like St. Petersburg and Stockholm.
The selection of only one city is not significant for judging on whether the method is applicable to such weak sources. Helsinki might just be a lucky strike.

2. An appropriate discussion of errors is still missing. The authors just added some numbers, but it's not clear if they determined these numbers by themselves (if so: how?) or refer to Beirle et al. (if so: are these errors valid for Helsinki as well?)

3. I am still not convinced by the presented ship track. The authors state that it is a weak and noisy signal, but what then is the message?
In the reply to the review, the authors presented maps of the ship track with geometric AMFs. It can clearly be seen that the columns south of Gotland are higher for w>5m/s, while I would expect the opposite (as ship emissions are independent on wind speed and higher w leeds to broadening of the plume).
In order to investigate how far the NO2 pattern could be caused by ship emissions (at least the order of magnitude), the authors have to relate the STEAM emissions to an expected increase in NO2 column by assuming a NOx lifetime or by asking a CTM.

Further comments:

Concerning 1b) (potential outflow from Central Europe)
The authors argue that the NOx lifetime is only some hours, so NOx from Central Europe can not reach the Baltic Sea. But obviously (Fig. 5), it does!
The authors should be aware that the lifetime of background NOx might be considerably longer (higher altitude, transport in reservoir species like PAN etc.).

OMI pixel size: The statement that the OMI pixel size is 13x24 km2 is misleading and only valid for nadir. Still, this statement is made in the revised manuscript three times.

Fig. 2: I agree that the binning of wind data in steps of 30deg is more informative. Consequently, the authors should perform the sorting of OMI observations accordingly. This might offer the possibility to have two independent fits: one for 0-30 and one for 30-60, which would be very helpful in order to judge on the representitativeness and significance of the Helsinki fit results.

Fig. 3: The zoomed-in colorscale makes it hard to "read" the signal. In particular for easterly winds, the map looks very patchy, and any information about spatial patterns below 1.8e15 is lost.

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RR by Anonymous Referee #2 (19 May 2014)

ED: Reconsider after major revisions (21 May 2014) by Andreas Richter

Dear Iolanda Ialongo,

I have now received comments from the two reviewers. As you will see, one of them is quite satisfied with your revisions while the second one recommends to reject the manuscript.

After reading through your changes and the arguments of the reviewer, I agree with the second reviewer that there are still some important points to be improved before the paper can be accepted for publication in ACP.

In particular, I think that

* addition of St. Petersburg to the life time / emission study would strengthen the paper very much

* the error discussion needs to be clarified as requested by the second reviewer and also suggested by the first reviewer in an offline comment

* the shipping emissions need some more discussions.

In particular with respect to the shipping signals in the Baltic Sea which are a novel and important aspect for satellite data analysis there are some open points that need to be addressed:

1) It is worrying that the largest values are found at the large wind speeds - this is either the result of some sampling artefact or indication of a problem as the opposite would be expected.

2) The shipping lane in STEAM and in OMI data do not match well in their spatial pattern in Fig. 6. I assume that this is the reason for using different boxes in STEAM and OMI data as shown in the same figure but not discussed in the text. The choice of different regions as well as the high values found close to Gotland need to be discussed in the text.

3) One of the arguments to support the interpretation of the signals as shipping NOx is the comparison of the temporal evolution with the STEAM inventory. However, there are several problems with that: First, the change in OMI NO2 is (in relative units) larger by a factor of 2 than the change in ship emissions. If anything, I would expect the opposite to be the case. Second, as can be seen in Fig. 1, OMI NO2 was large over the shipping area only in 2007 and 2008, which makes the link to the economic crisis less clear.

4) The figures with geometric AMF shown in the response to the reviewers are odd - the values are larger than those using the tropospheric AMF which doesn't make sense. Please check.

5) The absolute values of the shipping signal appear quite large to me. In Fig. 1, the values for 2007 and 2008 are as large as in the centre of Stockholm. This is surprising and I think you should follow the suggestion of the reviewer to check if the magnitude of NO2 columns observed is consistent with what would be expected from the emissions.

Please also consider the other points made by the two reviewers and provide a point by point discussion together with your revised manuscript.

With best regards,

Andreas Richter

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AR by Iolanda Ialongo on behalf of the Authors (09 Jun 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (10 Jun 2014) by Andreas Richter

RR by Anonymous Referee #1 (26 Jun 2014)

ED: Reconsider after minor revisions (Editor review) (26 Jun 2014) by Andreas Richter

AR by Iolanda Ialongo on behalf of the Authors (27 Jun 2014) Author's response Manuscript

ED: Publish as is (30 Jun 2014) by Andreas Richter

AR by Iolanda Ialongo on behalf of the Authors (30 Jun 2014)

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Characterization of OMI tropospheric NO2 over the Baltic Sea region

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Interactive discussion

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Characterization of OMI tropospheric NO₂ over the Baltic Sea region