Dear Mr. Santaren,

thank you very much for considering the referee comments of round 1. I had another referee (#3) evaluate your answers to round 1 with a particular focus on the concerns raised by referee #2.

I accept your paper subject to minor revisions. Along the recommendations of referee #3 and those of referee #2, I would ask you to consider the following changes to the manuscript:

- Highlight at more prominent places in the manuscript (abstract, conclusion) that your OSSE assumes clear-sky conditions (cf. recommendation of referee #3) and that, in reality, systematic errors and error sources with different-than-assumed statistical properties (e.g. in NEE and boundary conditions) will impact the emission and uncertainty estimates.

- Punch out a concise conclusion section from the current (quite longsome) section 4 "Discussion and conclusion" (cf. recommendation of referee #3).

Best regards,
Andre Butz.

Referee #3 recommendations:
"
The assumption of clear sky conditions: In my opinion this assumption is acceptable in the context of these OSSEs as long as its implications are clear. The abstract mentions an UR of 50% without specifying that this is for a single clear sky satellite overpass pertaining to the integrated emission in a 6 hour time interval prior to satellite overpass. It should also be made clear that this results hold for a CO2M type instrument.

In the discussion section instrumental trade offs are discussed (combinations of precision / swath / spatial resolution) for which it should be very clear that they hold for clear sky overpasses, with the source in the centre of the swath. This information is provided in the discussion section, but I fear that it gets lost in the discussion. To avoid that (valid) discussions distract from the main conclusions, I strongly recommend to split the discussion & conclusion section into separate discussion and conclusion sections. This allows the authors the highlight what is new about this work compared with previous work, without going into detail about all the possible implications of assumptions that are made.
"