Solid particles for stratospheric solar geoengineering: Climatic impacts and chemical uncertainties
Our project investigates the viability of solid particles such as alumina, calcite, and diamond as alternatives to sulfate aerosols for stratospheric aerosol injection (SAI) in solar radiation management (SRM). Through a combination of global climate modeling and experimental chemistry (AP-XPS, HI-ERDA), we aim to assess the potential benefits and uncertainties associated with solid particle injections. In the first year of this project, we focused on refining climate models, improving experimental setups, and conducting initial simulations and laboratory experiments. In our modelling activities, we mostly focused on long-term transient experiments to investigate the SAI impacts on climate. We found that solid particles induce less stratospheric heating and Arctic residual warming compared to sulfate aerosols, with diamond emerging as the most effective material for mitigating global warming. Alumina and calcite injections are also leading to smaller stratospheric heating and reduced radiative side effects relative to sulfur-based SAI, though uncertainties in heterogeneous chemistry persist. To address this uncertainty, our experimental chemistry efforts have focused on analyzing the uptake of stratospheric acids on calcite surfaces via XPS and HI-ERDA. We measured the uptake of HNO3 and HCl on calcite under near-stratospheric conditions, and determined their penetration into deeper layers below the surface and the chemical transformation of calcite into calcium chlorides and nitrates. We found that the uptake coefficient of HCl and HNO3 decreases with stratospheric exposure time. The reason for this is a layer that is increasingly enriched with nitrogen- and chlorine-containing reaction products (Ca(NO3)2 and CaCl2 hydrates), whose depth increases with exposure, as evidenced by the ERDA depth profiles. This, in turn, leads to an increasing protection of the underlying CaCO3 core. Over a typical stratospheric residence time of one year, uptake coefficients decrease, but a 5 Mt burden of CaCO₃ particles could still significantly reduce HNO₃ and HCl concentrations and convert half of the calcite mass to nitrates and chlorides. Consequently, the impact on ozone may be lower than previously estimated. In the second year of the project, we will start laboratory experiments with alumina particles, start combining our new lab data with the modelling activities, as well as launch a first multi-model intercomparison activity focusing on solid particle SAI.
For more information go here: https://www.simonsfoundation.org/event/solar-radiation-management-annual-meeting-2025/
