IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 6 PL Organic Matter Photochemistry and the Sulfur Cycle Kristopher McNeill1 1 ETH Zurich, Institute of Biogeochemistry and Pollutant Dynamics, Universitätstrasse 16, 8092 Zürich, Switzerland Photochemical reactions are becoming more recognized for their role in the fate of dissolved organic sulfur (DOS) in natural waters, yet many questions remain. Recent work shows that sunlight converts a wide range of DOS compounds to sulfate and lowmolecular-weight S products, and that triplet-sensitized one-electron oxidation followed by SO2/S(IV)/SO4²– conversion may represent a general mechanism [1,2]. Building on these findings, we have begun to investigate how photochemistry shapes DOS cycling in the surface ocean and inland lakes. We combine field measurements, controlled irradiation experiments, and mechanistic studies to (i) assess whether photomineralization contributes to the “missing sink” of oceanic DOS, (ii) quantify DOS photooxidation across lakes with diverse sulfur geochemistry, and (iii) resolve oxidation pathways for representative biosynthetic, sulfurized, and petroleum-derived organosulfur compounds. This work will seek to clarify the magnitude and mechanisms of DOS photooxidation, improve predictions of organosulfur fate, and strengthen links between aquatic photochemistry, biogeochemical sulfur cycling, and climate-relevant sulfur fluxes. The results will be relevant to environmental chemistry, biogeochemistry, atmospheric chemistry, water treatment, and geoscience. References 1. R. Ossola, J. Tolu, B. Clerc, P. R. Erickson, L. H. E. Winkel, K. McNeill, Environ Sci Technol 53 (2019), 13191-13200. 2. R. Ossola, B. Clerc, K. McNeill, Environ Sci Technol 54 (2020), 13066-13076. Acknowledgement We gratefully acknowledge that funding for this work comes from the Swiss National Science Foundation, grant numbers 200020_188565 and 200021228211.
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