SO₂ sequestration in large volcanic eruptions: High-temperature scavenging by tephra

It is well accepted that the climate impact of large explosive volcanic eruptions results from reduction of solar radiation following atmospheric conversion of magmatic SO₂ emissions into H₂SO₄ aerosols. Thus, understanding the fate of SO₂ in the eruption plume is crucial for better assessing volcanic forcing of climate. Here we focus on the potential of tephra to interact with and remove SO₂ gas from the eruptive plume. Scavenging of SO₂ by tephra is generally assumed to be driven by in-plume, low-temperature reactions between H₂SO₄ condensates and tephra particles. However, the importance of SO₂ gas-tephra interaction above the dew point temperature of H₂SO₄ (190-200°C) has never been constrained. Here we report the results of an experimental study where silicate glasses with representative volcanic compositions were exposed to SO₂ in the temperature range 25-800°C. We show that above 600°C, the uptake of SO₂ on glass exhibits optimal efficiency and emplaces surficial CaSO₄ deposits. This reaction is sustained via Ca²⁺ diffusion from the bulk to the surface of the glass particles. At 800°C, the diffusion coefficient for Ca²⁺ in the glasses was in the range 10^-13-10^-14cm²s^-1. We suggest that high temperature SO₂ scavenging by glass-rich tephra proceeds by the same Ca²⁺ diffusion-driven mechanism. Using a simple mathematical model, we estimated SO₂ scavenging efficiencies at 800°C varying from <1% to 73%, depending mainly on exposure time and tephra matrix glass Ca²⁺ content. Our results imply that large explosive eruptions with a deep magma fragmentation depth are likely to be affected by significant gas-tephra interaction at temperature above 600°C. This SO₂ sequestration mechanism should be considered when constraining volcanic S budgets, and when coupling the initial magmatic SO₂ content and the induced climatic response in some of the Earth's largest eruptions.