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Preprints
https://doi.org/10.5194/acp-2020-1110
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-1110
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  06 Nov 2020

06 Nov 2020

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This preprint is currently under review for the journal ACP.

Co-emission of volcanic sulfur and halogens amplifies volcanic effective radiative forcing

John Staunton-Sykes1, Thomas J. Aubry2,4, Youngsub M. Shin1, James Weber1, Lauren R. Marshall1, Nathan Luke Abraham1,3, Anja Schmidt1,2, and Alex Archibald1,3 John Staunton-Sykes et al.
  • 1Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, UK
  • 2Department of Geography, University of Cambridge, Cambridge, UK
  • 3National Centre for Atmospheric Science, UK
  • 4Sidney Sussex College, Cambridge, UK

Abstract. The evolution of volcanic sulfur and the resulting radiative forcing following explosive volcanic eruptions is well understood. Petrological evidence suggests that significant amounts of halogens may be co-emitted alongside sulfur in some explosive volcanic eruptions, and satellite evidence indicates that detectable amounts of these halogens may reach the stratosphere. In this study, we confront an aerosol-chemistry-climate model with four stratospheric volcanic eruption emission scenarios (56 Tg SO2 ± 15 Tg HCl & 0.086 Tg HBr and 10 Tg SO2 ± 1.5 Tg HCl & 0.0086 Tg HBr) in order to understand how co-emitted halogens may alter the life cycle of volcanic sulfur, stratospheric chemistry and the resulting radiative forcing. The eruption sizes simulated in this work are hypothetical Volcanic Explosivity Index (VEI) 7 (e.g. 1257 Mt. Samalas) and VEI 6 (e.g. 1991 Mt. Pinatubo) eruptions, representing 1 in 500–1000 year and 1 in 50–100 year events respectively, with plausible amounts of co-emitted halogens based on satellite observations and volcanic plume modelling. We show that co-emission of volcanic halogens and sulfur into the stratosphere increases the volcanic ERF by 24–30 % compared to sulfur-only emission. This is caused by an increase in both the forcing from volcanic aerosol-radiation interactions (ERFari) and composition of the stratosphere (ERFclear,clean). Volcanic halogens catalyse the destruction of stratospheric ozone which results in significant stratospheric cooling (1.5–3 K); counteracting the typical stratospheric radiative heating from volcanic sulfate aerosol. The ozone induced stratospheric cooling prevents aerosol self-lofting and keeps the volcanic aerosol lower in the stratosphere with a shorter lifetime, resulting in reduced growth due to condensation and coagulation and smaller peak global-mean effective radius compared to sulfur-only simulations. The smaller effective radius found in both co-emission scenarios is closer to the peak scattering efficiency radius of sulfate aerosol, thus, co-emission of halogens results in larger peak global-mean ERFari (6–8 %). Co-emission of volcanic halogens results in significant stratospheric ozone, methane and water vapour reductions, resulting in significant increases in peak global-mean ERFclear,clean (> 100 %), predominantly due to ozone loss. The dramatic global-mean ozone depletion simulated in both co-emission simulations (22 %, 57 %) would result in very high levels of UV exposure on the Earth's surface, with important implications for society and the biosphere. This work shows for the first time that co-emission of plausible amounts of volcanic halogens can amplify the volcanic ERF in simulations of explosive eruptions; highlighting the need to include volcanic halogens fluxes when simulating the climate impacts of past or future eruptions and providing motivation to better quantify the degassing budgets and stratospheric injection estimates for volcanic eruptions.

John Staunton-Sykes et al.

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