Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies showa correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO2, that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of 32SO2, 33SO2, 34SO2, and 36SO2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereasmass-dependent oxidation, such as SO2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of amass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of 33S/34S and 36S/33S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.
Hattori, S., Departments of Environmental Chemistry and Engineering, Yokohama 226-8502, Japan, Departments of Environmental Science and Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan; Schmidt, J.A., Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark; Johnson, M.S., Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark; Danielache, S.O., Departments of Environmental Science and Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan, Faculty of Science and Technology, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan, Department of Earth and Planetary Sciences, Japan; Yamada, A., Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan; Ueno, Y., Department of Earth and Planetary Sciences, Japan, Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan; Yoshida, N., Departments of Environmental Chemistry and Engineering, Yokohama 226-8502, Japan, Departments of Environmental Science and Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan, Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
Recommended Citation:
Hattori S.,Schmidt J.A.,Johnson M.S.,et al. SO2 photoexcitation mechanism links mass-independent sulfur isotopic fractionation in cryospheric sulfate to climate impacting volcanism[J]. Proceedings of the National Academy of Sciences of the United States of America,2013-01-01,110(44)