An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO<sub>2</sub> or accumulation-mode sulfuric acid aerosols

Abstract

Abstract. Studies of stratospheric solar geoengineering have tended to focus on modification of the sulfuric acid aerosol layer, and almost all climate model experiments that mechanistically increase the sulfuric acid aerosol burden assume injection of SO2. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO2 injection because most of the added sulfur increases the mass of existing particles, resulting in shorter aerosol residence times and aerosols that are above the optimal size for scattering. Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce particles predominantly in the accumulation-mode size range following microphysical processing within an expanding plume, and such injection may result in a smaller average stratospheric particle size, allowing a given injection of sulfur to produce more radiative forcing. We report the first multi-model intercomparison to evaluate this approach, which we label AM-H2SO4 injection. A coordinated multi-model experiment designed to represent this SO3- or H2SO4-driven geoengineering scenario was carried out with three interactive stratospheric aerosol microphysics models: the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM2) with the Whole Atmosphere Community Climate Model (WACCM) atmospheric configuration, the Max-Planck Institute's middle atmosphere version of ECHAM5 with the HAM microphysical module (MAECHAM5-HAM) and ETH's SOlar Climate Ozone Links with AER microphysics (SOCOL-AER) coordinated as a test-bed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to stratospheric sulfur injection. Each model used the same injection scenarios testing AM-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 emissions would reduce side effects of sulfuric acid aerosol geoengineering that are proportional to mass burden. The model studies were carried out with two different idealized geographical distributions of injection mass representing deployment scenarios with different objectives, one designed to force mainly the midlatitudes by injecting into two grid points at 30∘ N and 30∘ S, and the other designed to maximize aerosol residence time by injecting uniformly in the region between 30∘ S and 30∘ N. Analysis of aerosol size distributions in the perturbed stratosphere of the models shows that particle sizes evolve differently in response to concentrated versus dispersed injections depending on the form of the injected sulfur (SO2 gas or AM-H2SO4 particulate) and suggests that prior model results for concentrated injection of SO2 may be strongly dependent on model resolution. Differences among models arise from differences in aerosol formulation and differences in model dynamics, factors whose interplay cannot be easily untangled by this intercomparison.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere

“…The concentration of P1, from the SO 3 + HONO 2 reaction, is distributed between 10 and 35 km, as shown in Figure S4 (see SI). An increased SO 3 represents a sensitivity exercise assuming the injection of large amounts of SO 2 , e.g., 5 and 25 Tg­(S) yr –1 to the stratosphere as proposed in some geoengineering scenarios . Increasing SO 3 in the stratosphere would increase the concentration of P1, suggesting that new partitioning of sulfur can occur because of the SO 3 + HONO 2 reaction.…”

“…An increased SO 3 represents a sensitivity exercise assuming the injection of large amounts of SO 2 , e.g., 5 and 25 Tg(S) yr −1 to the stratosphere as proposed in some geoengineering scenarios. 57 Increasing SO 3 in the stratosphere would increase the concentration of P1, suggesting that new partitioning of sulfur can occur because of the SO 3 + HONO 2 reaction. Intriguingly, experimental balloon-borne observations between 30 and 40 km detect a NO 3 SO 3 − ion.…”

Climate, variability, and climate sensitivity of “Middle Atmosphere” chemistry configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6))

“…This includes: stratospheric ozone loss (Solomon et al, 1986;Solomon, 1999), its recovery (Fang et al, 2019), and the potential limits of recovery due to future aircraft (J. Zhang et al, 2021) and wildfire emissions (Solomon et al, 2022); geoengineering intended to offset greenhouse gas-induced warming (National Academies of Sciences, Engineering, and Medicine, 2021; Kravitz et al, 2015;Tilmes et al, 2020;Visioni et al, 2021;Weisenstein et al, 2022) and its side effects (Visioni et al, 2020;Tilmes et al, 2021Tilmes et al, , 2022; sudden stratospheric warming impacts on upper atmosphere variability (Baldwin et al, 2021;Pedatella et al, 2021); space weather (Sinnhuber et al, 2012;Damiani et al, 2016;Sinnhuber et al, 2018;Meraner & Schmidt, 2018) and meteor (Plane, 2012) impacts on stratospheric ozone; and the acceleration of the Brewer-Dobson circulation (Abalos et al, 2019;Polvani et al, 2019;Chrysanthou et al, 2020;Abalos et al, 2021), its potential impacts on stratospheric (Butchart & Scaife, 2001;Maliniemi et al, 2021) and tropospheric ozone (Neu et al, 2014), and its implications for global volcanic aerosol transport (Aubry et al, 2021).…”

Particle number concentrations and size distributions in the stratosphere: Implications of nucleation mechanisms and particle microphysics

“…New particle formation (NPF) (or nucleation) affects not only the number abundance but also the size distributions of stratospheric particles (e.g., Brock et al, 1995;Lee et al, 2003). There is increasing evidence (Weisenstein et al, 2022, Laakso et al, 2022) that a careful treatment of microphysical processes is necessary for projecting realistic radiative forcing response to SAI.…”

“…Although some of the advances in our understanding of nucleation gained in the last two decades can be applied to stratospheric conditions, focused studies specifically examining the mechanisms of NPF under stratospheric conditions are quite limited. Indeed, the H2SO4-H2O binary homogenous nucleation (BHN) parameterization developed two decades ago by Vehkamäki et al (2002) (named BHN_V2002 thereafter) has been used in most of SAI modeling studies when nucleation mechanism is considered (e.g., Weisenstein et al, 2022, Laakso et al, 2022. To our knowledge, the performance of this widely used BHN_V2002 under stratospheric conditions has not been carefully examined, probably due to the lack of suitable in situ measurements of freshly nucleated particles in the stratosphere for constraining the scheme.…”