An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO2 or accumulation-mode sulfuric acid aerosols
Authors: Weisenstein Debra K. , Visioni Daniele , Franke Henning , Niemeier Ulrike , Vattioni Sandro , Chiodo Gabriel , Peter Thomas , Keith David W. 
Published: March 2022

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.