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. Studies of stratospheric solar geoengineering have tendedto focus on modification of the sulfuric acid aerosol layer, and almost allclimate model experiments that mechanistically increase the sulfuric acidaerosol burden assume injection of SO2. A key finding from these modelstudies is that the radiative forcing would increase sublinearly withincreasing SO2 injection because most of the added sulfur increases themass of existing particles, resulting in shorter aerosol residence times andaerosols that are above the optimal size for scattering. Injection ofSO3 or H2SO4 from an aircraft in stratospheric flight isexpected to produce particles predominantly in the accumulation-mode sizerange following microphysical processing within an expanding plume, and suchinjection may result in a smaller average stratospheric particle size,allowing a given injection of sulfur to produce more radiative forcing. Wereport the first multi-model intercomparison to evaluate this approach,which we label AM-H2SO4 injection. A coordinated multi-modelexperiment designed to represent this SO3- or H2SO4-drivengeoengineering scenario was carried out with three interactive stratosphericaerosol microphysics models: the National Center for Atmospheric Research (NCAR) CommunityEarth System Model (CESM2) with the Whole Atmosphere Community ClimateModel (WACCM) atmospheric configuration, the Max-Planck Institute's middleatmosphere 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 ModelIntercomparison Project (GeoMIP). The intercomparison explores how theinjection of new accumulation-mode particles changes the large-scaleparticle size distribution and thus the overall radiative and dynamicalresponse to stratospheric sulfur injection. Each model used the sameinjection scenarios testing AM-H2SO4 and SO2 injections at5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity.All three models find that AM-H2SO4 injection increases theradiative efficacy, defined as the radiative forcing per unit of sulfurinjected, relative to SO2 injection. Increased radiative efficacy meansthat when compared to the use of SO2 to produce the same radiativeforcing, AM-H2SO4 emissions would reduce side effects of sulfuricacid aerosol geoengineering that are proportional to mass burden. The modelstudies were carried out with two different idealized geographicaldistributions of injection mass representing deployment scenarios withdifferent objectives, one designed to force mainly the midlatitudes byinjecting into two grid points at 30∘ N and 30∘ S, andthe other designed to maximize aerosol residence time by injecting uniformlyin the region between 30∘ S and 30∘ N. Analysis ofaerosol size distributions in the perturbed stratosphere of the models showsthat particle sizes evolve differently in response to concentrated versusdispersed injections depending on the form of the injected sulfur (SO2gas or AM-H2SO4 particulate) and suggests that prior model resultsfor concentrated injection of SO2 may be strongly dependent on modelresolution. Differences among models arise from differences in aerosolformulation and differences in model dynamics, factors whose interplaycannot be easily untangled by this intercomparison.

atmospheric chemistry and physics

An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; or accumulation-mode sulfuric acid aerosols