Microphysical simulations of sulfur burdens from stratospheric sulfur geoengineering

Recent microphysical studies suggest that geoengineering by continuous stratospheric injection of SO₂ gas may be limited by the growth of the aerosols. We study the efficacy of SO₂, H₂SO₄ and aerosol injections on aerosol mass and optical depth using a three-dimensional general circulation model with sulfur chemistry and sectional aerosol microphysics (WACCM/CARMA).We find increasing injection rates of SO₂ in a narrow band around the equator to have limited efficacy while broadening the injecting zone as well as injecting particles instead of SO₂ gas increases the sulfate burden for a given injection rate, in agreement with previous work. We find that injecting H₂SO₄ gas instead of SO₂ does not discernibly alter sulfate size or mass, in contrast with a previous study using a plume model with a microphysical model. However, the physics and chemistry in aircraft plumes, which are smaller than climate model grid cells, need to be more carefully considered.We also find significant perturbations to tropospheric aerosol for all injections studied, particularly in the upper troposphere and near the poles, where sulfate burden increases by up to 100 times. This enhanced burden could have implications for tropospheric radiative forcing and chemistry. These results highlight the need to mitigate greenhouse gas emissions rather than attempt to cool the planet through geoengineering, and to further study geoengineering before it can be seriously considered as a climate intervention option.