Higher efficacy of SO ₂ and accumulation mode-H ₂ SO ₄ stratospheric aerosol injection: insights from CESM2 and GEOS-Chem with advanced particle microphysics (APM)

Abstract Stratospheric aerosol injection (SAI) has been proposed as a strategy to mitigate the risks and damages of global warming. However, its radiative efficacy–defined here as globally averaged radiative forcing per unit of sulfur injection rate (SIR)–remains highly uncertain in model simulations due to complex particle size evolution and large variability in aerosol representation. In this study, we employ a state-of-the-art, size-resolved (sectional) advanced particle microphysics (APM) module within two global models (CESM2 and GEOS-Chem) to investigate the evolution and efficacy of stratospheric sulfate aerosols from both SO 2 and accumulation mode sulfuric acid (AM-H 2 SO 4 ) injection. Our comparison of SAI radiative efficacy based on various global models including the two APM-based (14 models in total for SO 2 and 9 models for AM-H 2 SO 4 ) shows a large spread, with lower and higher end values differing by a factor of ∼2.5–3. Our APM-based results for SO 2 SAI efficacy, including its diminishing returns with increasing SIR, fall at the upper end of the inter-model spread. For AM-H 2 SO 4 SAI, the APM-based models show significantly higher efficacy–by ∼50% to 200%–at lower SIR (⩽5 Tg(S) yr −1 ), followed by a steeper decrease in efficacy as SIR increases. Notably, the APM-based global model simulations show that AM-H 2 SO 4 injections consistently exhibit higher efficacy than SO 2 injections, yielding ∼55%–75% greater radiative forcing per Tg(S) yr −1 due to more favorable particle size distributions. Global sulfate burden increase, effective particle sizes, and particle size distributions based on different models are compared and possible reasons leading to different SAI efficacy are discussed. The new SAI efficacy findings, if confirmed in further studies and model intercomparisons, could have important implications for climate intervention strategies, cost-benefit analyses, and risk assessments.