Signature of tropical fires in the diurnal cycle of tropospheric CO as seen from Metop-A/IASI

Thonat, T.; Crevoisier, C.; Scott, N. A.; Chédin, A.; Armante, R.; Crépeau, L.

doi:https://doi.org/10.5194/acp-15-13041-2015

Articles | Volume 15, issue 22

https://doi.org/10.5194/acp-15-13041-2015

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

https://doi.org/10.5194/acp-15-13041-2015

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

Articles | Volume 15, issue 22

Research article

|

25 Nov 2015

Research article |

| 25 Nov 2015

Signature of tropical fires in the diurnal cycle of tropospheric CO as seen from Metop-A/IASI

T. Thonat, C. Crevoisier, N. A. Scott, A. Chédin, R. Armante, and L. Crépeau

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Final revised paper (published on 25 Nov 2015)
Supplement to the final revised paper
Preprint (discussion started on 17 Oct 2014)

Interactive discussion

Status: closed

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

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RC C9147: 'Review Thonat et al.', Anonymous Referee #1, 14 Nov 2014
- AC C13068: 'Response to referee #1', Thibaud Thonat, 29 Apr 2015
RC C9491: 'referee comment', Anonymous Referee #2, 24 Nov 2014
- AC C13053: 'Response to referee #2', Thibaud Thonat, 29 Apr 2015

Peer-review completion

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

AR by Thibaud Thonat on behalf of the Authors (09 Jun 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Jul 2015) by Ilse Aben

RR by Anonymous Referee #1 (22 Jul 2015)

RR by Anonymous Referee #2 (28 Aug 2015)

Suggestions for revision or reasons for rejection

I appreciate the efforts of the authors to take the comments of the two referees into account. Overall the paper is improved and the relation between the CO day-night excess and the diurnal cycle of fires better supported.

I remain, however, somewhat skeptical with the interpretations due to the possible influence of the changes in vertical sensitivity between the day and night measurements. They authors address this better and their results suggest a minor impact but I am not totally satisfied by the way the thermal contrast impact is handled and I urge the authors to modify this (see main comment below). If after this the conclusions do not change, and provided that the other comments below also can be dealt with (in particular I feel that the authors should be more careful in their statements considering the remaining uncertainties –main comment2-), I would be in favor of accepting the paper for publication in ACP.

MAIN COMMENT.

1) In the revised document and in reply to my first comments, the authors have extended the discussion on thermal contrast. However, I don’t think that using the day-night difference of thermal contrast as they do (Figures 10 and S1 and discussion in section 4.1.) is a suitable way to look into the results. Just to take an example, when thermal contrast is negative during the night (which seems from Figure 10b to be frequently the case) this TC difference will be very large but I don’t see why this should then correlate with high CO. This should be modified and the entire discussion around this revised.
To clarify, what I suggest is that the authors plot in Figure 10 the CO day-night excess as function of the day thermal contrast only, as that one is likely to bring the largest bias on the CO difference. Ideally they should also add in color scale the night thermal contrast, which on some occasions could also have an impact (see also the point on negative TC in the minor comments below). The entire discussion in section 4.1. should then be revised according to the new results. Figure S1 should also be changed.

2) I appreciate the efforts to make the new simulations with larger CO concentration in the PBL. I tend to agree that the amplitude of the “sensitivity impact” is too small in comparison to the signal actually observed. Yet, 500ptv in the pBL seems to be a pretty conservative number if surface vmr can be as high (as suggested page 11 line 22) as 27 ppmv. Can the author rule out such large values (this would be 50 times larger than the polluted profile considered) and if not, can they be so sure that what they see in the diurnal signal is not due to enhanced sensitivity to these large values during the day?
To be clear, my general impression is that the attribution of the CO excess during the day to the diurnal cycle of fire is much better supported now but still not entirely proven. I would therefore suggest that the authors be more careful in their statements. For example:
o Page 6, line 3: I don’t think that you really “highlight” the transport. You suggest it but there is no evidence. Such sentences should be rephrased.
o Section 3.2.2 (line 48). I would not say firmly that day and night measurements don’t agree with fire activity so much worse than the day-night difference. The day-night excess agree “better” (narrower distribution around the maximum) seems more appropriate to me.
o Page 7, line 6. “this shows…”; Here again I think it is more suggested than shown.
o Page 7, line 20: Something as “.. and we have assumed that we can then relate these signal to the nearby fires” …” would in my opinion be more appropriate than “ and can be interpreted in relation…”
o More caution should be put in the last paragraph before section 3.3.
o Again, more caution needed page 10, last paragraph before section 4.2. I would prefer something starting with: “Although a possible impact of changing vertical sensitivity between day and night on the CO excess cannot be totally ruled out, the results presented above strongly suggest that…”.
o In the conclusion section, line 17. “Unlike…”. sounds somewhat exaggerated.

MINOR COMMENTS

o On negative thermal contrast (page 4, line 33): The authors have changed their initial sentence to the following “A negative thermal contrast, on the contrary, decreases the sensitivity to CO in the lower layers of the troposphere”. This is not in-line with recent results on pollution monitoring with IASI (e.g. Boynard et al., 2014), which show temperature inversions to be favourable for boundary layer sensitivity. This is also not what is said (and shown in Figures 2 and 3) In Thonat et al., 2012. Please check and rephrase.
o I am puzzled now by the channel at 2501.75 cm-1 to retrieve surface temperature. The reflected solar radiation should be impacting in that region. Is this taken into account?
o In fact Figure 8 shows a key result and is indeed convincing. Just to be sure: have the authors checked that they would not get a similar type of correlation with the averaged day measurements only?

TECHNICAL CORRECTIONS

o Page 2, line 26: dependent
o Page 2, line 33, indispensable doesn’t sound good. Suggestion to change too “a necessary”
o Page 2, line 38: Measurement Of Pollution In The Troposphere (all with capital letters)
o Page 2, sentences starting line 35 and 46 give the same idea; this could bet expressed better in a single sentence.
o Page 6, line 42: “from November…”
o Page 15, ref Engelstaedter: “emiSsions”
o Page 17, Ref Spivakovsky should come after Seiler.
o Conclusion section, line36. I would remove here “unlike CO2 retrievals” as this is not done in this paper and does not refer to the same situations.

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ED: Reconsider after major revisions (06 Sep 2015) by Ilse Aben

AR by Thibaud Thonat on behalf of the Authors (18 Oct 2015) Author's response Manuscript

ED: Publish as is (11 Nov 2015) by Ilse Aben

AR by Thibaud Thonat on behalf of the Authors (13 Nov 2015)

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Short summary

Based on 5 years of CO columns derived from the IASI sounder, we show that tropospheric CO displays a diurnal signal with a seasonality and intensity that agrees well with the evolution of fires. We suggest that at night, after the passing of IASI at 21:30, a large amount of CO emissions from the smoldering phase is trapped in the boundary layer before being uplifted the next morning by natural and pyroconvection up to the free troposphere.