Uncertainties of SAI efficiency and impacts depending on the complexity of the aerosol microphysical model

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 Tg S yr⁻¹), 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, including stratospheric ozone. 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 Tg S yr⁻¹. 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.