the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Sep 2025
Uncertainties of SAI efficiency and impacts depending on the complexity of the aerosol microphysical model
Abstract. Significant differences exist between Earth System Models in simulating the efficiency of stratospheric aerosol injection (SAI) experiments, particularly in terms of aerosol burden, radiative forcing, and impacts, such as tropical lower stratospheric heating and changes in ozone. However, the primary reasons for these differences have not been identified. Previous studies have proposed that these differences can be attributed to the use of different aerosol microphysical schemes, model resolution, or other physical parameterizations. Here, we compare two sets of SAI experiments using the same modeling framework of the Community Earth System Model, differing only in their aerosol microphysical schemes: the modal aerosol model (MAM4) and the sectional aerosol model (CARMA). We analyze scenarios varying in injection location (point vs. regional), amount (5 vs. 25 TgS/yr), and material (sulfur dioxide (SO2) gas vs. accumulation-mode sulfuric acid (AM-H2SO4) aerosol). Our results suggest that the SAI radiative efficiency may be substantially overestimated when using the modal aerosol model, particularly at higher injection rates, with implications for other impacts. While both sets of models confirm that AM-H2SO4 injections are more effective than SO2 injections in reducing net top-of-the-atmosphere radiative forcing, MAM4 yields significantly larger aerosol burdens and weaker size-dependent sedimentation, particularly at 25 TgS/yr. In contrast, CARMA produces a smaller aerosol burden, with more mass shifted into larger particles and a declining radiative efficiency at increased injection rates. These findings suggest that more sophisticated sectional models may be necessary to accurately assess the efficacy, side effects, and climate impacts of SAI.
Competing interests: Simone Tilmes is an editor for ACP.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4274', Anonymous Referee #1, 08 Oct 2025
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RC2: 'Comment on egusphere-2025-4274', Anonymous Referee #2, 22 Nov 2025
There’s some very good work in this paper. There have been other studies looking at modal vs sectional microphysical models, and the models show different answers, so it’s worth seeing what another model shows.
Most of my general comments stem from the fact that I was hoping to see direct comparison with other models, like the simulations from Laakso et al. It’s a very important question to look at how different models differ in their aerosol microphysical uncertainties. The questions answered in the present work seem to be somewhat niche by comparison. I like the last science question posed in Section 1 (lines 64-65), but I don’t really feel like it was answered.
For instance, I struggled with the novelty of looking at point vs regional injections (see, for example, English et al., 2012 or Niemeier et al., 2013). Figure 1 illustrates this point quite well, in that the aerosol burden increases where the aerosols are injected for both MAM and CARMA, which one could have hypothesized prior to doing any simulations. CARMA shows a systematic low bias (or MAM shows a systematic high bias – it’s hard to say which), but you didn’t need to spread out the injection to learn that. I think the purpose of looking at this particular aspect needs to be better justified, especially with regard to what fundamental uncertainties this study is aiming to solve.
Studying the microphysics of accumulation mode aerosols are fine from a purely scientific standpoint, although given the highly questionable feasibility of this method, I would hope for a better tie-in. That is, how can we use accumulation mode injection to learn about microphysics and stratospheric processes more generally? (See my point above about the science questions.) If the only justification is that it’s a proposed type of aerosol, then I put it on par with studying diamond aerosol injection – interesting but ultimately just a modeling exercise. Lines 180-189 provide a good illustration of what I’m talking about. These lines basically said that some people proposed an idea, and that’s it. You could have rephrased this to be more scientifically interesting – in the previous section, you found that the nucleation stage is critical, and AM-H2SO4 allows you to isolate nucleation from coagulation growth, allowing you to further narrow which processes contribute to uncertainties.
All of that said, the study does add to the knowledge base in general, and the study is done well. I don’t see any faults in the analysis.
Specific comments:
Line 23: I don’t understand what this means. Just write it out please.
Line 60: Why did you use a fixed QBO?
Lines 92ff: The experimental setup seems strange. Why do you need 30-year simulations with fixed SSTs? I suppose there’s nothing wrong with doing extra, but it seems like overkill.
Lines 147-156: Is there anything particularly surprising in these results? Surely this parallels results that others have found.
Lines 175-178: I wanted to see more about this. This is the really interesting stuff.
Line 190: Well, yes, because you put more injection in the tropics, so of course you’re going to see more aerosol there.
Lines 206-208: This is written as though it’s surprising, but essentially these models are doing what they’re designed to do.
Lines 222-223: I found this sentence really frustrating. It’s written like a throwaway, but this is exactly the sort of thing that needs to be investigated further.
Citation: https://doi.org/10.5194/egusphere-2025-4274-RC2
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Summary
This study systematically compares stratospheric aerosol intervention (SAI) simulations using two aerosol microphysical schemes – MAM4 (modal) and CARMA (sectional) – implemented within the same CESM2-WACCM6 framework, with all other model components identical. The experiments vary in injection material (SO2 vs. accumulation-mode H2SO4 aerosol), injection pattern (regional vs. point), and injection amount (5 vs. 25 TgS/yr). They show that the choice of microphysics alone can cause up to a twofold difference in simulated aerosol burden, and can even reverse the relative effectiveness of injection strategies (e.g., for SO₂, regional > point in MAM4 but the opposite in CARMA). These discrepancies arise because CARMA resolves a broader size distribution, leading to more nucleation of small particles and greater growth into coarse sizes, which enhances sedimentation and reduces total burden. They further show that these differences propagate to radiative forcing efficiency, stratospheric heating, and ozone responses, and that model divergence increases at higher injection rates.
Overall, this work provides a rigorous and timely evaluation of how aerosol microphysics shape SAI outcomes. As current SAI simulations (e.g., ARISE, GLENS) often rely on MAM schemes, this paper clearly points out that a better aerosol scheme might be needed when designing future community simulations. I recommend publication after minor revisions, as outlined below.
Comments
Technical corrections: