Review of “Technical note: A comparative study of chemistry schemes for volcanic sulfur dioxide in Lagrangian transport simulations: a case study of the 2019 Raikoke eruption” by Mingzhao Liu et. al.

The authors enhance the MPTRAC model by introducing and evaluating two chemistry schemes for simulating volcanic SO2. The explicit scheme models first-order and pseudo-first-order SO2 loss processes, while the implicit scheme, tailored for SO2 transport in the UT/LS, employs a reduced chemical mechanism. The Raikoke eruption serves as a case study. The implicit scheme provides a more accurate representation of SO2 lifetime, albeit at a higher computational cost. The paper is well-written, with clear figures supporting the text and conclusions. I believe the paper is publishable, pending the authors' response to my query regarding the implicit chemical mechanism. Specifically, I'm unclear about its derivation and validation. Are there any prior studies that have employed this mechanism?

Minor questions/comments:

1st sentence. Not clear what is driven by wind and velocity fields, models or aerosols? Please rephrase.

Line 83: reference on TROPOMI is missing

1st sentence in Sec. 2.2.2 does not correlate with what is shown in Fig. 1.

Line 149: please see my major question above.

Table 1: what is prod?

Line 184: please expand ‘ESA’?

Line 208: Please include evolution of SO2 plume evolution computed using explicit and corrected explicit schemes in Fig 3 and 4.

Line 209: maybe replace “movement” by ‘transport’?

Line 212: ‘... the model results begin to lose…’. Please use different wording.

Line 240: Please rephrase the 1st sentence (situation the SO2 plumes?)

Line 258: ‘transport’ simulations?

Line 267: Please specify which gases CAMS assimilates. I do not think that is assimilates OH and H2O2.

Line 271: Please remove Fig 8 or add analysis of what is shown there.

Fig 9. Not clear on MPTRAC figures. How they obtained? Is it SO2 free run? Is it implicit, explicit or explicit with correction chemistry?

Line 340: Please include this correction test in the manuscript. It will strengthen the publication.

Sec 4: Summary and conclusions can be shortened and avoid repetitions.

Overview 

The paper presents a study of using three approaches to simulate chemical loss of the volcanic SO2 with the Lagrangian MPTRAC model for the 2019 Raikoke eruption.  The authors use a) an interactive chemistry scheme of 31 gas phase an photolysis reaction, b) prescribed OH and H2O2 monthly mean fields calculated by the CLAMS model and c) a variant of b scaled to match the results of a).  Retrievals of the volcanic SO2 from TropOMI are used as reference to estimate the decay of the emitted SO2, and to provide the source term. The OH fields are quantitatively comparted against the OH fields from the CAMS reanalysis. Scheme a) performs best and c) can mostly replicate a) at a much-reduced computational cost. 

General remarks

The paper is well written and informative. But, It remains a technical note in nature as it mainly compares the computational cost and relatively briefly the scientific results of the comparison of the chemistry schemes, without going into much detail about specific aspects and issues of the simulation. For example a longer discussion comparing the impact of the reaction with H2O2 associated to wet removal and the loss by OH gas phase chemistry could have been interesting. Other aspects of the volcanic SO2 modelling such as mixing and transport could have been of interest too. Finally, the resulting stratospheric Sulphate is not discussed at all. 

     

Specific remarks

 

L24 Please add also some disadvantages of Lagrangian dispersion model here. 

L44 Please clarify second-order, do you mean bimolecular reactions 

L49 Please clarify why a mixing scheme is required for the chemistry.

L53 You talk about mixing only for the first group. Is it not important for the other types? 

L130 Please clarify the differences between MPTRAC and CLAMS (here or in the introduction) 

L148 “embarrassingly” – I am not sure if this is the right word in this context. 

L 193 Perhaps that detail on the unit conversion does not need to be mentioned here.

L 219 Please comment on the change in the SO2 decay rate derived from S5P from about 3.7-

15.7. (Fig 6) 

L229 Please explain in more detail here or in the model description if and how cloud uptake was considered.  It is only mentioned in the description for the explicit scheme. Is wet removal modelled in all scheme with the same method? 

L259 Please discuss whether this correction could be used also for other cases, or if it is specific to the Raikoke case.

L275 Please comment on the lack of the CAMS "OH plumes" over Japan and the Pacific in the MPTRAC fields

L 329 Please clarify if you took this into account and try to quantify the impact of this process on the SO2 removal and lifetime.