| 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.
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: