review of manuscript egusphere-2026-1802

Title: Assessing site-to-site greenhouse gas measurement consistency in the UK and Ireland atmospheric monitoring network using whole and synthetic air reference materials

Author(s): Emmal Safi et al.

General comments:

This study evaluates the consistency of greenhouse gas measurements (CO2, CH4, and N2O) by comparing traditional whole air reference materials traceable to the WMO-NOAA calibration scales with synthetic SI-traceable reference materials. The study demonstrates that whole air standards are generally better suited for high-precision atmospheric monitoring, while synthetic standards still face challenges related to uncertainty, matrix effects, and long-term stability that need to be addressed before they can fully replace whole air standards in atmospheric monitoring networks.

The manuscript makes a valuable contribution to ongoing discussions regarding future calibration and reference material requirements as atmospheric greenhouse gas monitoring networks continue to expand.

The article is clearly structured, well written, and falls well within the scope of Atmospheric Measurement Techniques. In my opinion, only minor revisions are required before the manuscript can be accepted for publication. My specific comments and suggestions are provided below.

Title

Consider adding "CO2, CH4, and N2O" to the title. Currently, the trace gases investigated are not mentioned until line 30 of the abstract.

Abstract and Introduction

Lines 34–35:

It is difficult to understand why small atmospheric variability would make it more difficult to meet the compatibility goals. Please clarify.

Line 41:

Please clarify whether "matrix effects" refer to effects within the cylinder itself or during instrument analysis. I assume the latter is more relevant. If so, this issue may largely depend on instrument-specific responses and therefore extend beyond the control of the reference material producer.

Line 41:

"...mole fraction dependent instrument responses." Instrument responses are, of course, expected to be mole-fraction dependent. Please rephrase to clarify the intended meaning.

Line 60:

Consider adding a reference to the WMO/IG3IS report #319, Integrated Global Greenhouse Gas Information System Good Practice Guidance for Estimating National-scale Greenhouse Gas Emissions using Atmospheric Observations, https://library.wmo.int/idurl/4/69714

Line 66:

The reference to "WMO, 2022" appears to be missing from the bibliography. It may need to read "WMO, 2024". More importantly, I suggest updating the citation to the most recent WMO GGM report (GAW Report No. 317; WMO, 2025) [I cannot provide the url since the WMO e-library (https://library.wmo.int/) is currently down].

In addition, it would be useful to elaborate on the GAW definition and rationale behind network compatibility goals. The footnote to Table 1 in WMO (2025) states: "Scientifically desirable level of network compatibility for measurements of well-mixed background air. These represent the maximum bias that can generally be tolerated in measurements of well-mixed background air used in global models to infer regional fluxes."

Lines 68–69:

Consider citing Worthy et al. (2023) (https://doi.org/10.5194/amt-16-5909-2023), which provides an assessment of the level of agreement currently achievable among leading greenhouse gas measurement laboratories.

Line 83:

Consider revising to: "In addition, several independently maintained calibration scales ..."

Line 83:

Consider revising to: "For example, the Scripps Institution of Oceanography ..."

Lines 104–114:

This paragraph could better emphasize the importance of independent calibration scales for identifying flaws or inconsistencies in existing scales and for maintaining continuity when scales are revised. Relevant references include:

https://doi.org/10.1029/2005JD006035 , https://doi.org/10.5194/amt-12-517-2019 , https://doi.org/10.5194/amt-14-3015-2021

Methods

Line 135:

Consider citing: https://doi.org/10.1175/BAMS-D-19-0364.1

Line 137:

WAO and RGL are described both as ICOS sites and DECC sites (Lines 130–131). Are separate measurement and data-processing systems operating in parallel? Please clarify.

Line 138:

According to https://meta.icos-cp.eu/labeling/ , the Mace Head laboratory is listed as approved. Please update accordingly.

Line 155:

Is the reference to CO relevant here?

Line 166:

Likewise, is a reference to CO required here?

Line 170:

The statement:

"The same sensitivity to water vapour interference exists for OA-ICOS as for CRDS" is not entirely correct. While the dilution effect is similar for both techniques, spectroscopic water-vapour effects differ between OA-ICOS and CRDS.

Lines 170 ff.:

For TAC, where a Nafion dryer is used, is an additional H2O correction applied to account for residual water vapour?

Line 193:

Consider citing: http://www.atmos-meas-tech.net/6/1153/2013/

Lines 203 ff.:

When introducing whole air mixtures, please consider referring to Section 3.2.1 and providing there a short introductory description, similar to the approach used for synthetic mixtures in Lines 280–281.

Lines 206–208:

Consider citing: https://doi.org/10.18160/V2NS-9WXH (also available via: https://www.icos-cal.eu/static/images/docs/FCL-QC_Report_2024_v_1.0.pdf )

Lines 211 ff.:

This section clarifies that RGL operates a single CRDS analyser, while the data are processed according to two different protocols. This interpretation should perhaps be clarified earlier (see comment on Line 137).

Lines 225–226:

You may wish to cite: https://doi.org/10.5194/acp-11-2381-2011 which quantified network N2O biases using inverse modelling.

Line 284:

The cited reference is missing a publication year and does not appear in the bibliography.

Lines 290–293:

Please provide the mole fractions (and associated uncertainties) of N2, O2, and Ar.

Line 309:

The statement:

"The GC-ECDs used within the network collected data at 10 min intervals..." could be misinterpreted. Please clarify that these measurements represent discrete sample injections (of xx mL) every 10 minutes rather than 10-minute averages.

Line 312:

For the FTIR measurements at WAO, are the reported values one-minute averages?

Results and Discussion

Figure 2:

Consider adding compatibility goals, similar to Figure 3. The same suggestion applies to Figure 4. A shaded grey band could be an effective way to illustrate the target range.

Table 2:

Please consider whether this table is necessary. The values are difficult to interpret and receive limited discussion in the manuscript.

Lines 390–391:

Consider reordering the sentence:

"...it is composed of the propagation uncertainty and the uncertainty of the analytical instrumentation used to assign the values (see Tables 1 and 2)..." since propagation uncertainties are reported in Table 2, while assignment uncertainties are reported in Table 1.

Table 5:

Why is the CH4 standard error largest for NPL-3064? Most other values appear to scale approximately with mole fraction, with lower mole fractions corresponding to lower standard errors in both the D66 and NPL subsets.

Line 402:

The WMO compatibility goal of 0.1 µmol mol-1 for CO2 applies only to the Northern Hemisphere.

Line 426:

Consider adding: "...can obscure atmospheric signals and jeopardize their use in global models for inferring regional fluxes."

Line 455:

Do the authors have any hypothesis regarding the large observed offset? Could isotope effects associated with the preparation of synthetic reference materials contribute?

Line 471:

Please provide a cross-reference to Section 4.4.2.

Section 4.4.1:

A discussion of isotopic effects may be worthwhile, particularly given that such effects could differ among measurement techniques depending on their spectral sensitivity (e.g., instruments with sensitivity to 12CO2 only).

Conclusions and References

Line 525:

Please define "CIPM.

Line 526:

Please clarify NOAA's role in CIPM-related activities. My understanding is that NOAA can participate despite not being a National Metrology Institute because it has been designated by WMO as the Central Calibration Laboratory of the GAW programme. If correct, this could be stated explicitly.

Editorial comments

There are several in-text citations that should be reformatted to conform to journal style. For example:

"AUTHOR et al. (YEAR) provide details..."

Overall, I find this to be a valuable and timely contribution. The manuscript is scientifically sound, clearly written, and highly relevant to the atmospheric greenhouse gas measurement community. The revisions suggested above are primarily intended to improve clarity, provide additional context, and strengthen the discussion. I therefore recommend minor revisions.

comment attached as pdf