Risks and benefits of stratospheric solid particle injection for climate intervention

Recent studies have suggested that injection of solid particles such as alumina (Al₂O₃) and calcite (CaCO₃) instead of SO₂ for stratospheric aerosol intervention could reduce some of the adverse side effects of SAI such as ozone depletion, stratospheric heating, and changes in diffuse radiation. However, the expected improvements from alteration of stratospheric chemistry are subject to large uncertainties. We constrain some of these uncertainties by experimental work on calcite particles using elastic recoil detection analysis (ERDA) and in-situ experiments using X-ray photoelectron spectroscopy (XPS). Subsequently, we use a global aerosol-chemistry climate model that, for the first time, interactively couples microphysical and chemical processes of solid calcite and alumina particles as well as liquid sulfuric acid aerosols with model radiation and transport. Notably, SAI by solid particles only leads to more effective radiative forcing per aerosol burden compared to sulfuric acid aerosols, not per injected mass. However, reduced stratospheric warming remains a major advantage of solid particles. Furthermore, different assumptions on the heterogeneous chemistry of solid particles, based on the available experimental data, result in drastically different impacts on stratospheric composition, in particular, ozone. For alumina particles, which are thought to not undergo chemical aging during their stratospheric residence time we present a sensitivity analysis for heterogenous reactions to quantify uncertainty. For the alkaline calcite particles, which are thought to undergo chemical aging in the stratosphere via reaction with acids (e.g., HCl, HNO₃, H₂SO₄) we find even larger uncertainties due to unknown reaction pathways and highly uncertain rates under stratospheric conditions. The large uncertainty in predicted stratospheric ozone changes can only be reduced via substantial additional laboratory experiments under stratospheric conditions, i.e., partial pressures of relevant gases (e.g., HCl, HNO₃, H₂SO₄), temperatures < 220 K, relative humidity < 1% and realistic UV irradiance. From the present perspective, sulfur-based SAI has significantly lower uncertainty than injection of solid particles, which have significantly reduced stratospheric heating but very large uncertainties in impacts on stratospheric composition.