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

Abstract

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.

Climate change modulates the stratospheric volcanic sulfate aerosol lifecycle and radiative forcing from tropical eruptions

“…First, we only inject SO 2 to simulate a volcanic eruption in UKESM. Previous studies have shown that the co-injection of other volcanic products, including ash 64 , halogens 65 , 66 , and water 67 , would affect the sulfate aerosol cycle including the SO 2 lifetime or the self-lofting of the aerosol cloud. Second, we used atmosphere-only simulations, so that feedbacks related to the ocean response are not included.…”

Comment on cp-2021-170

“…The influence of ash on radiative forcing is currently unclear. For example, recent observations for the Kelud 2014 eruption suggest that ash exerted a radiative forcing of -0.08 W/m 2 three months after the eruption (Vernier et al 2016), even though the volcano erupted only 0.5 ± 0.2 x 10 11 kg of ash (Maeno et al 2019, Aubry et al, 2021. In comparison, we found that the Churchill eruption erupted 4.9 x 10 13 kg (3.9-6.1 x 10 13 kg) of ash, which might suggest a potentially strong radiative forcing from ash that is unaccounted for in our modelling.…”

Unknown Eruption Source Parameters Cause Large Uncertainty in Historical Volcanic Radiative Forcing Reconstructions

“…All rights reserved. Sykes et al, 2020). Further uncertainties are associated with the conversion of estimated sulfate aerosol burdens into SAOD and radiative forcing.…”

“…In addition, the RF efficiency per unit mass of emitted sulfur falls with the magnitude of SO 2 emission due to formation of larger sulfate aerosol particles (Pinto et al., 1989; Timmreck et al., 2010). The sulfate formation (e.g., size and amount) is also impacted by coincident emissions of other species such as water and halogens (e.g., LeGrande et al., 2016; Staunton‐Sykes et al., 2021). Further uncertainties are associated with the conversion of estimated sulfate aerosol burdens into SAOD and RF.…”