Please address the comments in the attached file. I would be grateful if you could provide two versions of the revised paper, one with and one without tracking.

The authors should address the following comments:

Line numbers are from the manuscript in the response from the authors.

L. 75: uncertainty -> uncertainties.

L. 121: Perhaps introduce the acronym for “CHIMERE”?

L. 130-131: Perhaps consider removing the brackets and introducing a comma, thus: “…is weak, when…”.

L. 177: Suggest you do not start a sentence with an acronym. Thus, you could use “The OSSEs…”.

L. 181: Re-reading this, perhaps it would be better to have “In OSSEs, twin…”.

L. 195-198: Re-reading this, the sentence looks clumsy. Consider rewording thus: “…state-of-the-art set-up which solves the NEE…level of evaluation. This set-up includes a variational atmospheric inversion system and the ICOS23 network containing…years.”

L. 212: this -> its.

L. 217: “…developments of mesoscale…”.

L. 222: “…and of future…”.

L. 229: Do you need “state”?

L. 246: “their typical estimate” -> “typical estimates”.

L. 267: I suggest “…typically distances less than 40km…”.

L. 273: measurements -> measurement.

L. 277: I suggest “…the use of this assumption…”.

L. 297-298: I suggest “…is relatively large compared to the typical…sites, due to atmospheric diffusion.”.

L. 323: To break the very long sentence I suggest: “…The operator H: …”.

L. 326: I suggest “The operator H is the combination…”.

L. 343: averages -> averaged.

L. 353, 378: I suggest you remove “below”.

L. 359: Do you mean: “…, as done in Hungershoefer et al. (2010)…”?

L. 377: I suggest you use “e.g.” instead of “i.e.”.

L. 390-391: “…by the ECMWF analyses.”.

L. 415: I suggest you omit “respectively”.

L. 442: What two types of correlations? In space and time?

L. 445: I suggest you omit “to”.

L. 448: proportionally -> proportional.

L. 448-449: Clarify the statement in brackets – I do not understand what you mean.

L. 451: Would this be better?: “…summer, providing typical values for…errors of…”.

L. 473: constrain -> constraint.

L. 498: seems-> is; I suggest: “may confirm” -> “suggests”.

L. 503: Not clear on the reference to Broquet. Is this result discussed in this paper, or are the hourly averages provided by Broquet? Please clarify.

L. 509: dimensions -> dimension.

L. 640 Do you mean “western European” countries and “eastern European” countries? If so, please clarify.

L. 657: “national scale”.

L. 659: Omit “the”.

L. 664: chose -> choose.

L. 685: “…and much larger…”.

L. 704, 714: “…the whole of Europe…”.

L. 743: I suggest you remove “even”.

L. 762-763: Reword this sentence – I do not understand it.

L. 785: I suggest “decreases” -> “reductions”; perhaps: critical -> important?

L. 811: Not clear whether the 64-67% refers to the relatively constant increase, or to values that are smaller by this amount. Please reword.

L. 819: As this is the synthesis section, I suggest you indicate that you consider here northern summer and northern winter.

L. 839: I suggest: “…promising for provision of accurate monitoring of the NEE…”.

L. 851: Remove “such”.

L. 872: “…, respectively”.

L. 880: “…than those in the…”.

L. 882: What is this target? Please indicate.

L. 893: reaching it -> to reach it.

L. 897: I suggest: depending -> which depends.

L. 900: “…western European countries…”.

L. 909: I suggest “…European network…”; it -> its.

L. 915: “…the increase in the size of the network…”.

L. 916: Perhaps “accuracy” is better than “precision”?

L. 951-952: “…results in very slight increases…”.

L. 952: differences in what?

L. 954: alter -> changes; “…only for specific areas…”.

L. 976-977: I suggest: “…Broquet et al. (2013), but only at the European scale.”.

L. 1403: In Fig. 5, the crosses are difficult to see. Consider increasing their size.