The CHRONOS mission: capability for sub-hourly synoptic observations of carbon monoxide and methane to quantify emissions and transport of air pollution

Edwards, David P.; Worden, Helen M.; Neil, Doreen; Francis, Gene; Valle, Tim; Arellano Jr., Avelino F.

doi:https://doi.org/10.5194/amt-11-1061-2018

Articles | Volume 11, issue 2

https://doi.org/10.5194/amt-11-1061-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-11-1061-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 2

Research article

|

23 Feb 2018

Research article |

| 23 Feb 2018

The CHRONOS mission: capability for sub-hourly synoptic observations of carbon monoxide and methane to quantify emissions and transport of air pollution

David P. Edwards, Helen M. Worden, Doreen Neil, Gene Francis, Tim Valle, and Avelino F. Arellano Jr.

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Final revised paper (published on 23 Feb 2018)
Preprint (discussion started on 28 Jun 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Comment', Anonymous Referee #1, 27 Jul 2017
- AC1: 'Author response: See Supplement pdf', D. P. Edwards, 28 Oct 2017
RC2: 'Anonymous referee comments', Anonymous Referee #2, 18 Sep 2017
- AC2: 'Author response: See Supplement pdf', D. P. Edwards, 28 Oct 2017

Peer-review completion

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

AR by D. P. Edwards on behalf of the Authors (28 Oct 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Nov 2017) by Andre Butz

RR by Anonymous Referee #3 (27 Nov 2017)

Suggestions for revision or reasons for rejection

The paper describes a new space mission and discusses its capabilities. As such the paper is clearly within the scope of AMT. As far as I can judge, references to existing work are adequate. I have just a number of issues which, I think, need consideration or further discussion.

l 59-62: The numbers presented here are estimates only but the text sounds as if they represent the absolute truth. A weaker wording is necessary here, e.g., "are estimated to decrease average life..."

l 81: Is CHRONOS an acronym? If yes, please define; if not, why all capital letters?

Figure 3: I assume that the CHRONOS spatial resolution has been derived from the instrument characteristics. However, the retrieval scheme is optimal estimation, and if the horizontal resolution of the a priori information is limited, this would also degrade the horizontal resolution of the result (even if L2 analysis is made pixel by pixel and no formal a priori horizontal covariances are considered).

Figure 4 is misleading because here the A-signal and the D-signal seem to be spectral radiances while the text says these are band-integrated radiances. The information that the analysis is based on band-integrated spectral radiances comes quite late (line 370) and the reader might already be confused before reaching this line. For the authors and some readers from the gas correlation filter radiometry community this is certainly clear but AMT articles are written for a wider readership. Please remind the reader earlier in the paper that the information is contained in the band-integrated radiances.

l 378: It is not quite clear to me how the D/A approach solves the aerosol or cloud problem. If my understanding is correct, the D/A approach helps to remove the influence of the background radiation in a way that the total column between the background source and TOA is retrieved correctly. However, in the case of opaque clouds or aerosol layers, we get information about the partial column between the aerosol or cloud layer and TOA and not about the total column between surface and TOA. As far as I can judge, the D/A approach does not remove the artefact arising from the misinterpretation of this partial column as total column. Or let me put it differently: The authors say that "to first order" their approach reduces related uncertainties. "To first order" is often used as a synonym for "In linear approximation" but radiative transfer in the case of opaque cloud or aerosol layers is extremely non-linear. The capability of the D/A approach to solve related problems remains unclear. This issue seems to me to deserve some more thorough discussion.

l 447 Add blank before "As".

l 589, 597, 643: In Rodgers (2000) the Bayesian (or Shannonian) method is no longer called "optimal estimation" but "maximum a posteriori". To my knowledge, Clive Rodgers has decided no longer to use the term "optimal estimation" because there are many possible optimality criteria.

Sect 5: The retrieval diagnostics strongly depend on how the constraint in the optimal estimation retrieval is set up. How large are the a priori errors? Are the numbers used ad hoc choices or can they be justified? Are there a priori covariances in the altitude domain or is the a priori covariance matrix used diagonal? Is the abundance or the log of it retrieved? The text provides, as far as I have seen, no information on these issues although the results critically depend on them. Only in the legend and the caption of Fig 14. I found at least some information for CH4; I have not found such information for CO. Without such information, the results are not traceable. The results should be discussed in the context of these choices.

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ED: Reconsider after major revisions (27 Nov 2017) by Andre Butz

AR by D. P. Edwards on behalf of the Authors (21 Dec 2017) Author's response Manuscript

ED: Publish subject to technical corrections (03 Jan 2018) by Andre Butz

AR by D. P. Edwards on behalf of the Authors (09 Jan 2018) Author's response Manuscript

Short summary

The CHRONOS space mission would provide observations for emissions and transport studies of the highly uncertain air pollutants carbon monoxide and methane, with sub-hourly revisit at fine horizontal spatial resolution across North America. CHRONOS uses an imaging gas filter correlation radiometer hosted in geostationary orbit. CHRONOS' capability for monitoring evolving, or unanticipated, air pollution sources would find societal applications for air quality management and forecasting.