General Comment

This paper, titled "A fully coupled solid particle microphysics scheme for stratospheric aerosol injections within the aerosol-chemistry-climate-model SOCOL-AERv2," presents an innovative and significant advancement in the field of geoengineering. The authors have successfully integrated a solid particle microphysics scheme into the SOCOL-AERv2 model, enabling a comprehensive assessment of stratospheric aerosol injection (SAI) using solid particles such as alumina and calcite. This approach addresses several limitations associated with traditional sulfur-based SAI methods, including ozone depletion and stratospheric heating.

The study is well-structured and meticulously detailed, offering a thorough explanation of the new model's components and its capabilities. The methodology is robust, with clearly described simulation scenarios and a comparative analysis between solid particle injections and sulfur-based aerosols. The results are compelling, demonstrating the potential advantages of using solid particles for SAI, such as higher radiative forcing efficiency and reduced adverse environmental impacts.

However, while the paper is comprehensive, it would be useful to include more information on the validation of the model and comparison with observational data. I believe that by adding information on how well the model can reproduce the observed data, the reliability of both the model and the paper will be enhanced, so please consider this aspect.

Overall, this paper makes a substantial contribution to the field of climate engineering, providing valuable insights and a new tool for evaluating the feasibility and impacts of SAI with solid particles. The SOCOL-AERv2 model with solid particle microphysics represents a significant step forward in exploring alternative materials for geoengineering, and the findings of this study will undoubtedly stimulate further research and discussion in this critical area.

The manuscript by Vattioni et al. investigates the injection of solid particles, in particular alumina and calcite particles for stratospheric aerosol injection (SAI). They also implement a solid particle microphysics scheme on SOCOL-AER chemistry climate model, which is then coupled with the existing sulfur cycle, the heterogeneous chemistry scheme and radiative transfer model. This allows a relatively thorough and comprehensive review of solid particle injections. Alternative injection materials are an important issue for SAI and research into them is very welcome and needed. Despite the expected limitations and uncertainties in modelling the subject, this study is a major step forward in the field. Overall, this is an interesting, well written and really good study. It is therefore difficult to find anything to criticise or disagree with. Therefore, I have only few comments for authors to consider and recommend that this manuscript be published.

The only thing I would have liked to know a bit more about was stratospheric heating. As the authors state, this is one of the main interests of using alternative material instead of sulfur-based injections. If there are some restrictions on why stratospheric heating cannot be shown, it would be nice to see values for absorbed radiation. It is also interesting that based on Supplementary Figure S5 the absorption cross section for calcite is larger than for H2SO4 or alumina for the visible light spectrum. I am unable to estimate whether this leads to significantly greater absorption of shortwave radiation, but it would be interesting to see.

The separate presentation of shortwave and longwave radiative forcing (e.g. in a supplement) could also be a good addition.

Few comments on specific lines:

P2 L21-23: This may give the impression that these listed uncertainties are results that have been investigated and shown in this study.

P3 L56: Not only are they inefficient at backscattering solar radiation, but they can also absorb thermal radiation, which is not good.

P3 L62 Perhaps there could be some discussion in the conclusions as to whether solid aerosols offered a solution to these limitations based on this study. Of course, it is quite difficult to estimate e.g whether there are lower inter-model uncertainties in the case of solid materials than for sulfur injections based on this study, but there were some rather large differences between Weinstein et al. 2015 and this study that I found interesting.

P3 L66: Just a comment: larger absorption leads to a reduction in (global) precipitation, which is especially important for larger injections (see e.g. Laakso, A et al: Earth Syst. Dynam., 15, 405-427, https://doi.org/10.5194/esd-15-405-2024, 2024). Personally, this is one of the main reasons why I think solid materials could be an interesting alternative to sulfur.

P6 L169: Emissions of SW radiation?

P7 L201: Is the lack of aerosols in the largest bins mainly due to lack of agglomeration? "Large number"-mers are quite large if the monomer radius is 200nm and I assume their lifetime in the atmosphere is not long.

P12 L315-323: It may just be me, but I had some problems understanding how the alumina particles are presented, but I have no suggestion as to how this could be made clearer. It may be that I was confused by the monomers and agglomerates and thought at first that they were something else than different bins in the same 10-bin distribution. It was also only later that I realised that the injections were indeed always made in the first bin, regardless of the size of the particles. However, unless I am completely wrong, there are now 10 bins for Alumina (partially coated particles), H2SO4 (partially coated), Alumina (fully coated), H2SO4 (fully coated)? A figure would always be helpful, but I am not sure if it is that important here. Maybe for a supplement at most.

P13-14 L357-361. When I read this, I wondered where the contact angle came from. Later in the results section it is found that it is predefined. It could be mentioned here.

P17 L445 CasO_4 -> CaSO_4

P19 L480-490 Since the model has been updated, it is not surprising that the burdens are different (lower) than in Weisenstein et al. (2015). However, if I have calculated and understood the units correctly (there is a good chance that I haven't), the sulfur burdens in your study are slightly higher than in Weisenstein et al. (2015). But this is just interesting (if true) and I don't mean that you should open up differences in every single process between models to explain it. But if you had an idea for a reason, you could mention it.

P19 L504-506 It is also interesting to note that in this study, the injection of 80nm particles resulted in a much larger radiative forcing than in Weisenstein et al 2015. Do you know why this is? The burden was larger in Weisenstein et al., but on the other hand, in this study the mass fraction is largest in 16mers, whereas in Weisenstein et al. it was 64mers (with 4 Tg/yr). This is probably the answer. If so, this is another nice example of how important aerosol microphysics is, even when simulating solid particles.

P21 L526. These fractions of depleted sulfate burdens might also be worth mentioning in calcite simulations, although they can also be seen in Fig. 9. It is also interesting that the fraction of depleted H2SO4 mass is larger in alumina than in calcite for 80 nm injections, but vice versa for e.g. 320 nm injections.

P26 L610 TOC already used in P25 L560, so could be opened there

P31 L681 “…on polar stratospheric clouds (PSC) is unclear..” <--please add “(PSC)”