Background CO<sub>2</sub> levels and error analysis from ground-based  solar absorption IR measurements in central Mexico

Baylon, Jorge L.; Stremme, Wolfgang; Grutter, Michel; Hase, Frank; Blumenstock, Thomas

doi:https://doi.org/10.5194/amt-10-2425-2017

Articles | Volume 10, issue 7

https://doi.org/10.5194/amt-10-2425-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Twenty-five years of operations of the Network for the Detection...

https://doi.org/10.5194/amt-10-2425-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 10, issue 7

Research article

|

06 Jul 2017

Research article |

| 06 Jul 2017

Background CO₂ levels and error analysis from ground-based solar absorption IR measurements in central Mexico

Jorge L. Baylon, Wolfgang Stremme, Michel Grutter, Frank Hase, and Thomas Blumenstock

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Final revised paper (published on 06 Jul 2017)
Preprint (discussion started on 11 Jan 2017)

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

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RC1: 'Review of: Background CO2 levels and error analysis from ground-based solar absorption IR measurements in central Mexico Jorge L. Baylon, Wolfgang Stremme, Michel Grutter, Frank Hase, and Thomas Blumenstock', Anonymous Referee #1, 02 Feb 2017
RC2: 'Review of Baylon et al., “Background CO2 levels and error analysis from ground-based solar absorption IR measurements in central Mexico”', Paul O. Wennberg, 04 Feb 2017
AC1: 'Response to referees comments', Jorge L. Baylon, 15 Mar 2017

Peer-review completion

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

AR by Jorge L. Baylon on behalf of the Authors (24 Mar 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Apr 2017) by Hal Maring

RR by Paul O. Wennberg (08 May 2017)

Suggestions for revision or reasons for rejection

I am partially satisfied with the reply to my comments, but suggest a few further changes.

1. Abstract (line 8). 0.56 ppm in XCO2 is not a “small bias”. In fact this bias alone would consume the entire error budget of TCCON. In fact, site-to-site bias of 0.56 ppm would essentially render TCCON useless for most carbon cycle science. The precision requirements described (Rayner and O’Brien; Olsen and Randerson) refer to bias-free measurement precision and so are not relevant to the discussion here where a one hour average of the FTS data will have essentially negligibly small random precision. Given the uncertainty in the cause of the bias between the two beamsplitters, it isn’t clear how it could be ‘homoginized’ without extensive and on-going characterization. In summary, I think this study rather than confirming that KBr beamsplitter is acceptable offers significant evidence that this option should be approached cautiously.

2. I continue to object to the comparison between the diurnal changes in XCO2 shown here and in situ measurements from MLO. This makes absolutely no sense and the implication that this comparison gives confidence in the XCO2 diurnal signal is ill advised. What is the flux implied by the changes in XCO2? Answer: huge! I urge the authors to scrub this from the manuscript. Fine to show the diurnal figure, but I believe that the changes shown are (largely) just evidence for airmass bias. A plot of XCO2 vs airmass (which should certainly be included) would illustrate that the majority of the diurnal signal is associated with airmass rather than time of day (as would be expected from a flux).

3. What is the basis for the final sentence: “This data set confirms a small influence of anthropogenic emissions especially during the afternoon, when the regional boundary layer reaches the height of the station.”? A figure of XCO might offer some evidence for this, but that is not discussed or shown.

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ED: Publish subject to minor revisions (Editor review) (20 May 2017) by Hal Maring

AR by Jorge L. Baylon on behalf of the Authors (28 May 2017) Author's response Manuscript

ED: Publish as is (30 May 2017) by Hal Maring

AR by Jorge L. Baylon on behalf of the Authors (31 May 2017)

Short summary

In this investigation we analyze two common optical configurations to retrieve CO₂ total column amounts from solar absorption infrared spectra. The noise errors using either a KBr or CaF₂ beam splitter, a main component of a Fourier transform infrared spectrometer, are quantified in order to assess the relative precisions of the measurements. A 3-year XCO₂ time series at the high-altitude site of Altzomoni in central Mexico presents clear annual and diurnal cycles and a trend of +2.2 ppm yr^-1.

Background CO2 levels and error analysis from ground-based solar absorption IR measurements in central Mexico

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Background CO₂ levels and error analysis from ground-based solar absorption IR measurements in central Mexico