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 degrees 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 degrees C. We show that above 600 degrees 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 degrees C, the diffusion coefficient for Ca2+ in the glasses was in the range 10(-13)-10(-14) cm(2) s(-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 degrees 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 degrees 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. (C) 2013 Elsevier Ltd. All rights reserved.