Dear authors,

Thank you for the careful revisions and detailed replies. The two reviewers had rated your manuscript between good and excellent and had asked for minor revisions only.
I share their assessment and have only a few small last points, before I can recommend your manuscript for publication. Please briefly reply to the following question, and correct the small points I have identified (no explicit response needed for those).

Question:
It is still not entirely clear to me how PBL height and wind speed from WRF were used in the mass balance calculations. Section 2.2.3 mentions that spatial means of PBL height and wind speed were calculated for each hour, but it is not very clearly stated, how these were used in the mass balance calculations described in Sect 2.2.4. Am I right that for each 5-minute averaging interval (say 11:45-11:50 UTC), mean PBL height and wind speed from the corresponding hour (e.g. hourly mean of 11-12 UTC, or maybe from the closest hour, i.e. 12 UTC in the above case) were used in the calculations? Or did you apply a time shift, similar to the one applied to the Mace Head background? If the travel time from east to west was X hours, why did you compute mean winds and PBL heights for the present hour h rather than for hour h-X/2, i.e. for the middle point of the air mass transit across the UK? Or maybe you did? Due to the potentially large diurnal variability of PBL heights, the difference between using hour h and using hour h-X/2 could be quite significant. Wouldn't it be useful to consider this in the uncertainty analysis?

Small points:

- Please update all figure references. Currently, the numbering does not account for the new figures

- The "author contribution" statement at the end of the manuscript needs to better explain the individual contributions

- Page 3, Line 34: "references gases" -> "reference gases". On the next line, you should probably refer to the World Calibration Center WCC-Empa rather than just to Empa.

- P4, L35: "finallty" -> "finally"

- P5, L20: Shouldn't it be "background concentrations" rather than "background emissions"?

- P5, L33: "as follow" -> "as follows"


Best regards
Dominik Brunner

Dear authors,

Thank you very much for the corrections and clarifications. I am fully satisfied with the responses and modifications. The manuscript is now ready for publication. Congratulations on this great piece of work.

Best regards
Dominik Brunner