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Discussion papers
https://doi.org/10.5194/acp-2018-1070
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1070
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Nov 2018

Research article | 15 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Exploring accumulation-mode-H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol-chemistry-climate model

Sandro Vattioni1,2, Debra Weisenstein2, David Keith2, Aryeh Feinberg1, Thomas Peter1, and Andrea Stenke1 Sandro Vattioni et al.
  • 1Institute of Atmospheric and Climate Science, ETH Zürich, Zurich, 8002, Switzerland
  • 2Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA-02138, USA

Abstract. Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the global 3D-aerosol-chemistry-climate model, SOCOL-AER, to investigate 21 different SSG scenarios, each with 1.83MtSyr−1 injected either in the form of accumulation-mode-H2SO4 droplets (AM-H2SO4), gas-phase SO2, or as combinations of both. For most scenarios, the sulfur was continuously emitted at 50hPa (≈20km) altitude in the tropics and subtropics, zonally and latitudinally symmetric about the equator (ranging from ±3.75° to ±30°). In the SO2 emission scenarios, continuous production of tiny nucleation mode particles results in increased coagulation, which together with condensation produces larger coarse mode particles. These larger particles are less effective for backscattering solar radiation and sedimentation out of the stratosphere is faster. On average, AM-H2SO4 injection increases stratospheric aerosol residence times by 32% and stratospheric aerosol burdens 37–41% when comparing to SO2 injection. The modelled all-sky (clear-sky) short-wave radiative forcing for AM-H2SO4 injection scenarios is up to 17–70% (44%–57%) larger than is the case for SO2. Aerosol burdens have a surprisingly week dependence on the latitudinal spread of emissions with emission in the stratospheric surf zone (>15°N–15°S) decreasing burdens by only about 10%. This is because the faster removal through stratosphere-to-troposphere transport via tropopause folds found when injection is spread farther from the equator is roughly balanced by a decrease in coagulation. Increasing injection altitude is also surprisingly ineffective because the increase in burden is compensated by an increase in large aerosols due to increased condensation. Increasing the local SO2 flux in the injection region by pulse or point emissions reduces the total global annual nucleation. Coagulation is also reduced due to the interruption of the continuous flow of freshly formed particles. The net effect of pulse or point emission of SO2 is to increase stratospheric aerosol residence time and radiative forcing. Pulse or point emissions of AM-H2SO4 has the opposite effect—decreasing stratospheric aerosol burden and radiative forcing by increasing coagulation. In summary, this study corroborates previous studies with uncoupled aerosol and radiation modules, suggesting that, compared to SO2 injection, the direct emission of AM-H2SO4 results in more radiative forcing for the same sulfur equivalent mass injection strength and that sensitivities to different injection strategies may vary for different forms of injected sulfur.

Sandro Vattioni et al.
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Status: final response (author comments only)
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Sandro Vattioni et al.
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This study is among the first modelling studies on stratospheric sulfate geoengineering which couple a size-resolved sectional aerosol module interactively to well-described stratospheric chemistry and radiation schemes in a global three-dimensional chemistry-climate model. We found that compared to SO2 injection, the direct emission of aerosols results in more effective radiative forcing and that sensitivities to different injection strategies may vary for different forms of injected sulfur.
This study is among the first modelling studies on stratospheric sulfate geoengineering which...
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