The authors extend the existing GEOS-Chem Classic modelling framework to double the spatial resolution relative to previous implementations, yielding a regional simulation with a resolution of 0.125°×0.15625° (roughly 12×12 km²). They show that the effects of some processes manifest differently in this new configuration, most notably a strengthening in vertical transport.

The central advance is incremental, but useful. Being able to perform higher-resolution regional simulations with GEOS-Chem provides a new and relevant capability for the user community. The announcement of new data archives is particularly welcome, and the authors provide some excellent insights into the new results which can be developed. However, I have one concern which I would argue is not well addressed by the current manuscript but which is important to interpreting its results. I have also listed some minor errors.

My major concern is the question of whether the results the authors are seeing are specifically because of better resolving the meteorology, or just that an increase in resolution allows for gradients to be better preserved. This concern is raised in part due to the question below regarding "native" resolution, and I recognize that this may be beyond the scope that the authors intended for a technical note. However, the abstract specifically argues that the different results are due to better resolution of transport variables (line 21), but do not provide evidence of this. I would recommend that the authors consider running a simulation in which their input meteorology is degraded to 25 km resolution, but their simulation grid remains at 12 km. This would allow them to directly assess how much of the benefit comes from the meteorology being better resolved compared to from the ability to better resolve emissions and chemical gradients.

My more minor issue concerns the way that the grids are described. The authors state that GC-Classic can operate on native GEOS grid resolutions (line 63), but this is not true. To my knowledge, GC-Classic cannot directly ingest cubed-sphere data and it does not use a cubed sphere grid. As it stands, the paper risks misleading the reader by implying that there has been no regridding (i.e. information loss) between the GEOS and GC-Classic grids, which is not true. Based on section 2 (specifically lines 91-92) it appears that the data has already been regridded from C720 to 0.125°×0.15625°; such an operation will have introduced spurious resolution at the poles, and will have greatly degraded the resolution at the equator where the C720 grid is actually finer. Furthermore the misalignment of grid edges likely means that, even where the C720 and 0.125°×0.15625° grids are roughly the same size, the data will have been "smoothed out" by the regridding process. This is inevitable in a cubed sphere to rectilinear regrid, and does not detract from the idea that the use of a higher resolution rectilinear grid should still yield benefit. However, the manuscript currently flips between describing the GC-Classic simulation as being at 0.125°×0.15625° (presumably correct), "native" (likely incorrect unless the architecture has changed to allow C720 simulation), and 12-km (definitely incorrect as even C720 has some variation in grid cell area - albeit much less than at 0.125°×0.15625). I would recommend that the authors revise the manuscript to precisely describe exactly what grid resolution is being used where (e.g. are the pre-calculated archives, listed as "12 km", actually at C720 or at 0.125°×0.15625?), and to be clear about where lossy regridding is or is not being applied. I would also request that claims such as "[t]he 12-km simulation has no spatial averaging of winds relative to the parent GEOS ESM" (lines 166-167) be removed, as this is not true unless GC-Classic is now running on a cubed sphere at C720.

Finally, my understanding is that GEOS-Chem is usually referred to as "GEOS-Chem X.Y.Z" rather than "GEOS-Chem version X.Y.Z" (line 79), although I would defer to the senior authors on this manuscript in that regard given their leadership in the community.

The manuscript “Technical note: 12-km resolution capability for the global GEOS-Chem model of atmospheric composition” by Xiaolin Wang et al. presents the new option to apply a 12 km nest in GEOS-Chem simulations using the global wind archive from NASA GEOS_FP meteorological data assimilation system at 12 km resolution. 12 km nested simulations are evaluated against simulations at 25 km resolution and observations.

This 12 km resolution nested capability of GEOS-Chem is a great advance and very helpful for the atmospheric chemistry and composition modelling community. I think the manuscript would better fit the scope of GMD than ACP. I would suggest transferring it and the review to GMD. Independent of the journal, some points should be addressed before publication.

Major comments

The manuscript is lacking of conclusions. This is kind of surprising because the first chapters of the manuscript are well structured. A small summary is provided at the end of chapter 5, but only the last sentence of the summary is related to chapter 5 itself. I would suggest to move this summary into a proper conclusion chapter, which also should include an outlook.

Related to the missing conclusions: The manuscript describes the model development well, but lacks in highlighting the scientific impact of these developments.

Minor comments

In general:

Please check for a uniform presentation, i.e. “0.125°×0.15625°” (line 16) and “0.125° × 0.15625°” (line 35) or “≈12 × 12 km²” (line 35), “” and “≈ 25-km” (line 73).

The date of latest access is missing for most of the links.

Line 32-33: For my understanding, does that mean that the chemistry and meteorology are not coupled, i.e. the chemistry has no feedback on the meteorology?

Line 38: It might be good to include already here, why the simulations are possible from March 2021 and not from an earlier date.

Line 38: What does onward mean? Until when are the NASA GEOS-FP data available?

Line 99 and 100:  Is ‘online’ meant by “on the fly”?

Line 115: ‘7 times’ instead of “7×”.

Line 116: See remark to line 115.

Line 225 and following: It would be helpful to include one or two sentences about the basics of analytical inversion of satellite data for readers, who are not familiar with this topic.

Figure 2:

Out of curiosity: Can you explain, what causes the stratospheric negative delta ²²²Rn difference between 20°N and 30°N in June?

Do you know, what causes the enhanced ⁷Be at 12 km resolution compared to 25 km resolution in the troposphere between 20°N and 30°N in June?

It would be great to see for reference how vertical winds for the two different resolutions look like.