A model study of sulfur mass-independent fractionation formation produced in a chamber photochemical experiment under reducing conditions

• Clarified S-MIF origin and mechanisms from chamber experiments. • Showed dynamic light-induced isotope effects via opacity changes. • Revealed sulfur isotope enrichment under reducing conditions. • Proposed a new OCS production pathway in chamber reactions. We developed a one-dimensional isotopic photochemical model that simulates dynamic changes in optical depth under self-shielding conditions and applied it to ultraviolet irradiation experiments involving a gas mixture of sulfur dioxide (SO 2 ) and carbon monoxide (CO) to investigate the origin and mechanisms of sulfur mass-independent isotopic signatures in the resulting chemical species. The chemical reaction network included 1205 reactions involving sulfur isotopologues. Modeled concentrations and final isotopic mixing ratios of the main species were compared with chamber experiments conducted under similar conditions. The modeled Sulfur Mass-Independent Fractionation (S-MIF) consistently showed larger Δ 33 S and Δ 36 S values than those observed experimentally. Δ 33 S values for OCS showed some level of agreement, whereas Δ 36 S values exhibited a larger discrepancy of up to −25‰. A built-in reaction-rate analysis tool revealed that the isotopic signatures in the carbonyl sulfide (OCS) product originate from SO 2 photodissociation and that OCS forms mainly through an OSSO intermediate. Additionally, the dynamic nature of light-induced isotopic effects was demonstrated by variations in opacity and isotopic fractionation triggered by concentration changes in ultraviolet-absorbing species over time. This study emphasizes the importance of accounting for both the spatial configuration of the experimental chamber and self-shielding conditions that are highly sensitive to concentration changes. Such considerations are essential for accurately reproducing and interpreting isotopic signals and for gaining deeper insight into the processes driving S-MIF under reducing conditions.