Review of the manuscript “High-resolution stratospheric volcanic SO2 injections in WACCM”, Axebrink et al

Dear Editor, dear Authors,

The manuscript “High-resolution stratospheric volcanic SO2 injections in WACCM” by Axebrink et al discusses large-scale modelling of the Sarychev eruption in 2009, with the aim of investigating the importance of input injection parameters and other model set-up aspects in the description of stratospheric volcanic eruptions, towards the estimation of radiative impacts of such events. The topic of the manuscript is important and of certain interest for the ACP readers. There is an active ongoing scientific debate, at the international scale, about how to represent volcanic plumes (stratospheric but also tropospheric), and their impacts, with numerical modelling. The consistency of confined plumes (volcanic emissions, wildfires, etc) observations and modelling is still to be achieved, to be honest. For this reason, this manuscript has the potential to be an important contribution to this debate. Unfortunately, I have fundamental concerns about the model set-up and cannot recommend this manuscript for publication as it is, see Specific Comments 20 and 23-29. Based on these comments, I rather recommend clarification or re-design of the experiments before I can fully evaluate this manuscript. For this reason, for the moment, I have not evaluated the Results section, and I’m waiting for such clarifications before going further in this review. In addition, I have found the manuscript severely lacking in text quality (i.e. different statements without justification) and the literature citation (knowledge?) is also to be strongly improved, see Specific Comments 1-22.

Please find Specific Comments in the following. Please address all these comments and I will be happy to review a further manuscript version, if the Authors decide to resubmit it to ACP.

I am sorry if I cannot be more positive this time but I strongly encourage the Authors to address my comments, improve the manuscript, re-design and re-run the simulations if needed, and then resubmit a new manuscript version.

Regards.

Specific Comments:    

1) L22-23: why a full stop between the two sentences?

2) L23-24: "Aerosol emissions...greenhouse gases", please add one or more references for this statement.

3) L23: "These effects result in a net cooling...", not always! See the case of black carbon aerosols, e.g.: https://www.nature.com/articles/s41467-020-20482-9 or https://acp.copernicus.org/articles/22/9299/2022/ and others

4) L25: "...natural sources", why only natural aerosol sources? What about anthropogenic aerosol sources?

5) L28-29: Please rephrase: the SO2 emissions do not have a direct impact on the radiative balance, the subsequently formed sulphate aerosols have (it is said right after).

6) L29: the SO2 actually converts to binary solution droplets of sulphuric acid + water (e.g. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015RG000511). Maybe this sentence can be rephrased accordingly.

7) L30: "...which can create years of...", please mention the very long lifetime of sulphate aerosols in the stratosphere

8) L33: "explosiveness" --> "explosivity"

9) L33: "the SO2 mass" --> "the mass of the injected SO2"

10) L35: please state clearly that the cooling of the Earth's climate system from volcanic eruption is *transient*

11) L39-40: we are not at all in a background stratospheric aerosol condition, and the radiative effects of moderate stratospheric eruptions extends well beyond the "beginning of the 2000s": please mention, at least, more recent eruptions such as Raikoke 2019 (e.g. https://acp.copernicus.org/articles/21/535/2021/) and the very special case of the Hunga eruption 2022 (https://www.nature.com/articles/s43247-022-00618-z)

12) L44-46: This sentence sounds like a repetition of what already said before and can be suppressed.

13) L44: "SPA, 2006" is rather "SPARC, 2006"? This looks like too generic as a reference and the Authors can easily find more specific references

14) Please mention representative cases, like this one (plus others, in case): https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JD035974

15) L48-49: please briefly state how satellite observations are used by modelers. Synergies studies can be cited, e.g.: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JD035974 and https://acp.copernicus.org/articles/16/6841/2016/ and others

16) L49-50: "Most SO2..." "most" or "all"? How SO2 can be measured with active observations? Also, the question of the vertical resolution of satellite observations (especially in a nadir geometry) is complicated and should be briefly discussed here (e.g.: passive sensors do not lack vertical measurements but rather have limited vertical sensitivity, etc)

17) L51-52: "Clarisse et al. (2014) showed that IASI can provide SO2 data with vertical resolution down to ∼2 km,", this sounds a bit overestimated for infrared observations at the nadir, please check

18) L58: "column" --> "total vertical column"

19) L65: "implemented" --> the Authors mean "used as input"? See also L83

20) L73-76: I honestly did not understand the difference between second and third data sets. Please clarify.

21) L79-81: this three-lines introduction can be suppressed as it is redundant

22) L86: SO2 (AIRS) and aerosol (CALIOP) vertical profiles do not "have the same height profile" but the Authors assume this is the case, which implies the fact that the Authors assume that the SO2 and aerosol plumes are collocated. This has to be stated and the chemical/microphysical implications of this assumption should also be briefly mentioned.

23) L124: "M16 is a single column (1D) emission dataset with a vertical resolution of 1 km." How can a "single column emission dataset" have a "vertical resolution of 1 km" <-- this means that the emissions are not based on a single column but on different vertical layers (at 1 km resolution).

24) L125: why M16's and S21-1D emissions are released at a different time interval (15-16/6) than S21-3D (19/6)? If it is now known that Sarychev emissions were mainly on 19/6, why making simulations of the "wrong" days?

25) L125-126: how SO2 is released during the 6-hours period? Is it a constant emission rate? Is there a peak at some time? Why only 12:00 to 18:00 for the two days and not before/after? This sound as an unphysical way of “erupting” for a volcano and should be fixed.

26) L130-131: how SO2 is released here as well (cfr previous comment)? Same for the third dataset (L133-135)

27) L132-133: this is very puzzling. Do the Authors mixed-up the vertical and horizontal definition of "single column"? What's the actual meaning of "1D" here?

28) L151-152: using CALIOP data as a comparison data set is not completely satisfactory in terms of independence with the simulations, as one the simulation was partially initialised with CALIOP information (S21-3D)

29) Fig. 1 caption: In panel b, this is a daily average or at a specific hour? In panels c and d are also for S21-3D simulation? Please mention this in the caption.