Importance of microphysical settings for climate forcing by stratospheric SO2 injections as modeled by SOCOL-AERv2

• Published in: Geoscientific Model Development 2024-05, Vol.17 (10), p.4181-4197
• Main Authors: Vattioni, Sandro, Stenke, Andrea, Luo, Beiping, Chiodo, Gabriel, Sukhodolov, Timofei, Wunderlin, Elia, Thomas, Peter
• Format: Article
• Language: English
• Subjects: Aerosol particles; Aerosol properties; Aerosols; Atmosphere; Atmospheric chemistry; Boundary conditions; Chemistry; Climate; Climate models; Climate prediction; Coagulation; Condensation; Configurations; Equilibrium; Gases; General circulation models; Geoengineering; Global aerosols; Greenhouse gases; Injection; Intercomparison; Nucleation; Ozone; Ozone depletion; Ozone layer; Ozonosphere; Particle size; Polar caps; Radiation; Radiative forcing; Representations; Side effects; Size distribution; Solar radiation; Stratosphere; Sulfur; Sulfur dioxide; Sulfuric acid; Sulphur; Sulphuric acid; Supersaturation; Ultrafines; Volcanic eruption effects; Volcanic eruptions
• ISSN: 1991-962X; 1991-959X; 1991-962X; 1991-9603
• DOI: 10.5194/gmd-17-4181-2024

Solar radiation modification by a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol–chemistry–climate models are often used to investigate the potential cooling efficiencies and associated side effects of hypothesized strat-SRM scenarios. A recent model intercomparison study for composition–climate models with interactive stratospheric aerosol suggests that the modeled climate response to a particular assumed injection strategy depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation) alongside host model resolution and transport. Compared to short-duration volcanic SO2 emissions, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the time steps and sequencing of microphysical processes in the sectional aerosol–chemistry–climate model SOCOL-AERv2 (40 mass bins) affects model-predicted climate and ozone layer impacts considering strat-SRM by SO2 injections of 5 and 25 Tg(S) yr−1 at 20 km altitude between 30° S and 30° N. The model experiments consider the year 2040 to be the boundary conditions for ozone-depleting substances and greenhouse gases (GHGs). We focus on the length of the microphysical time step and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the scenarios injecting 25 Tg(S) yr−1 of SO2 with a default microphysical time step of 6 min, changing the call sequence from the default “condensation first” to “nucleation first” leads to a massive increase in the number densities of particles in the nucleation mode (R1 µm). As expected, the influence of the call sequence becomes negligible when the microphysical time step is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing by SO2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg