Evaluation of a regional air quality model using satellite column NO<sub>2</sub>: treatment of observation errors and model boundary conditions and emissions

Pope, R. J.; Chipperfield, M. P.; Savage, N. H.; Ordóñez, C.; Neal, L. S.; Lee, L. A.; Dhomse, S. S.; Richards, N. A. D.; Keslake, T. D.

doi:https://doi.org/10.5194/acp-15-5611-2015

Articles | Volume 15, issue 10

https://doi.org/10.5194/acp-15-5611-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-15-5611-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 10

Research article

|

21 May 2015

Research article |

| 21 May 2015

Evaluation of a regional air quality model using satellite column NO₂: treatment of observation errors and model boundary conditions and emissions

R. J. Pope, M. P. Chipperfield, N. H. Savage, C. Ordóñez, L. S. Neal, L. A. Lee, S. S. Dhomse, N. A. D. Richards, and T. D. Keslake

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Final revised paper (published on 21 May 2015)
Preprint (discussion started on 26 Aug 2014)

Interactive discussion

Status: closed

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

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RC C7310: 'Interactive comment on “Evaluation of a regional air quality model using satellite column NO2: treatment of observation errors and model boundary conditions and emissions” by R. J. Pope et al.', Anonymous Referee #1, 22 Sep 2014
RC C9817: 'Review ‘Evaluation of a regional air quality model using satellite column NO2: treatment of observation errors and model boundary conditions and emissions’ by R. J. Pope et al.', Anonymous Referee #2, 05 Dec 2014
AC C11007: 'Reviewer 1 Response', Richard Pope, 09 Jan 2015
AC C11020: 'Reviewer 2 Response', Richard Pope, 09 Jan 2015

Peer-review completion

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

AR by Richard Pope on behalf of the Authors (22 Jan 2015)

ED: Referee Nomination & Report Request started (24 Jan 2015) by Chul Han Song

RR by Anonymous Referee #2 (09 Feb 2015)

Suggestions for revision or reasons for rejection

Re-review ‘Evaluation of a regional air quality model using satellite column NO2: treatment of
observation errors and model boundary conditions and emissions’ by R. J. Pope et al.

Many issues raised in my previous review have been addressed in this version of the manuscript by Pope and co-workers. Unfortunately, the section on averaging kernels and the averaged retrieval error have only become more confusing. These parts need to be clarified, or the paper will propagate wrong information in the scientific community.

P6, lines 19-20: kernels do not modify vertical profiles. The proper definition of the averaging kernel for DOAS retrievals is that they represent the relationship between the vertical distribution of the tracer and the retrieved trace column. If kernels are applied on the modeled profile, then this ensures that the resulting modeled column can be compared directly to the satellite-retrieved column. This may all seem a matter of semantics, but it is essential to express oneself clearly in this matter to avoid confusion to proliferate in the literature.

P8, L12-24: the added discussion on why kernels are lower over London than over Dartmoor does not clarify things. I agree with the authors that the kernel differences are probably not driven by differences viewing geometries, albedo, or clouds, and indeed, differences in NO2 loading indeed appear to drive the differences in the kernels. But referring to optical thickess here makes no sense – all the averaging kernels in the DOMINO v2 retrievals have been determined assuming NO2 absorption is sufficiently small so that the optically thin limit may be used. I urge the authors to remove L19-21 (“Eskes and Boersma … of the tracer.”). The mathematical expression of the averaging kernel is the altitude dependent air mass factors divided by the air mass factor. It is the latter that depends on the local NO2 amount (because the AMF depends on the assumed NO2 profile; the altitude dependent AMFs, scattering weights, etc. are always calculated within the optically thin limit). Since (TM4) NO2 amounts are higher over London than over Dartmoor, differences in NO2 loading can indeed explain the differences in kernels. Also in line 22, the reference to optically thin and strong absorbers should be dropped.

P18: the solar diffuser issue was relevant for the GOME-sensor, but OMI does not have that problem. Should be removed.

The main error in the manuscript is with Eq. (6). In my previous review, I remarked that the contribution of the systematic error in the slant column to the (systematic) error in the stratospheric slant column could be discarded in Eq. (6), in line with the discussion in Belmonte-Rivas et al. (2014). This does not mean however, that the stratospheric *uncertainty * can be discarded from Eq. (4). Only the systematic component in the stratospheric can be neglected (0.03 Xtotal/AMFtrop)^2 term), but the random contribution (given by 0.25 x 1015/AMFtrop) should remain, but as the middle term in Eq. (4), and not squared together with the AMF uncertainty term as in the current Eq. (6). The authors should really revise this issue. Failure to do so would propagate an important conceptual error in the peer-reviewed literature, and this should be avoided at all costs.

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RR by Anonymous Referee #1 (01 Mar 2015)

Suggestions for revision or reasons for rejection

The manuscript is improved after the revision. Authors evaluated the performances of AQUM, using the OMI NO2 columns and conducting several sensitivity tests. Also, authors suggested an algorithm to reduce the retrieval errors via averaging satellite NO2 column data and using AKs. The revised manuscript is valuable and acceptable for the final publication in ACP with following revisions.
i) As shown in Fig. 7a and 8b, the biases between the AQUM and OMI-derived NO2 columns were influenced by the uncertainty in the anthropogenic point source emissions. Probably, there is also large uncertainty in the area source of NOx emissions. I think that the uncertainty in the NOx emissions used in the current study was not clearly/fully discussed for the evaluation of the AQUM in the revised manuscript. Therefore, authors should provide further discussion (or analysis) regarding the uncertainty in the manuscript.
ii) Authors presented Fig. 9c and d from the sensitivity runs utilizing GEMS LBC and 0.02 of γN2O5. I wonder the results from the sensitivity run combing MACC LBC with 0.02 of γN2O5. Which one produces better results, compared with the OMI-retrieved NO2 columns? (Also, I just wonder how the satellite and in-situ measurements were treated in the GEMS and MACC reanalysis data.)
iii) Authors missed the previous question about ‘2-3. Aerosol surface area (A) in the Schwartz formula (Eq. 11)’. As mentioned in your manuscript, aerosol surface areas can be changed by hygroscopic growth of aerosols, depending on aerosol types and relative humidity. Also, there are other atmospheric processes such as coagulation and condensation/evaporation, related to the aerosol surface areas. I wonder how those processes are treated in the AQUM model.

Referee Report: PDF

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ED: Reconsider after major revisions (02 Mar 2015) by Chul Han Song

AR by Richard Pope on behalf of the Authors (18 Mar 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Mar 2015) by Chul Han Song

RR by Anonymous Referee #1 (08 Apr 2015)

RR by Anonymous Referee #2 (14 Apr 2015)

ED: Reconsider after minor revisions (Editor review) (16 Apr 2015) by Chul Han Song

AR by Richard Pope on behalf of the Authors (22 Apr 2015) Author's response Manuscript

ED: Publish as is (27 Apr 2015) by Chul Han Song

AR by Richard Pope on behalf of the Authors (27 Apr 2015)

Evaluation of a regional air quality model using satellite column NO2: treatment of observation errors and model boundary conditions and emissions

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Evaluation of a regional air quality model using satellite column NO₂: treatment of observation errors and model boundary conditions and emissions