An Aerosol-Climate Model Intercomparison of Stratospheric Solar Geoengineering by Injection of SO2 or Accumulation-Mode Sulfuric Acid Aerosols

Studies of stratospheric solar geoengineering have tended to focus on sulfuric acid aerosols, and almost all such climate model experiments assume that SO2 is injected to increase the sulfuric acid aerosol burden of the stratosphere. The radiative forcing increases sub-linearly 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. 15 Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce additional accumulation-mode particles (AM-H2SO4) after microphysical processing within an expanding plume, and such injection may reduce the average 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 co-ordinated multi-model experiment designed to represent this SO3or H2SO4-driven geoengineering scenario was carried out with three interactive stratospheric 20 aerosol-climate models: CESM2(WACCM), MAECHAM5-HAM, and SOCOL-AER coordinated as a testbed 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 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 25 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. We use two different injection patterns, one designed to force mainly the midlatitudes by injecting into two grid points at 30° N and 30° S and the other designed to force more uniformly by injecting in a belt along the equator between 30° S and 30° N. By 30 forcing each case with both SO2 and AM-H2SO4 we find opposite impacts on radiative efficacy for the two injection Debra Weisenstein 11/10/2021 9:25 AM Deleted: d.n Aerosol-Climate Model . [1] David Keith 10/30/2021 7:55 PM Deleted: Model s 70 Debra Weisenstein 10/13/2021 9:33 PM Deleted: Analyses.of stratospheric solar . [2] David Keith 10/30/2021 7:56 PM Deleted: enhancement Debra Weisenstein 10/13/2021 9:35 PM Deleted: on sulfate aerosol have David Keith 10/30/2021 7:56 PM Deleted: base on scenarios that Debra Weisenstein 10/13/2021 9:56 PM Deleted: d 75 David Keith 10/30/2021 7:56 PM Deleted: injection of Debra Weisenstein 11/16/2021 5:02 PM Deleted: sulf.ulfuric acid aerosolate . [3] David Keith 10/30/2021 7:56 PM Deleted: for this purpose. However, scattering . [4] Debra Weisenstein 10/13/2021 9:41 PM Deleted: Yet continuous injection of SO2 may produce overly large aerosols. njection of SO3 or 80 . [5] David Keith 10/30/2021 8:01 PM Deleted: allow . educe the average particle size, . [6] Debra Weisenstein 11/16/2021 5:02 PM Deleted: ate.to produce more radiative forcingve . [7] David Keith 10/30/2021 8:02 PM Deleted: and compare the effects of such an . his . [8] Debra Weisenstein 11/19/2021 11:59 AM Deleted: c.uld reduce some . ide-effects of . [9]