Mass-independent fractionation of sulfur isotopes during broadband SO2 photolysis: Comparison between 16O- and 18O-rich SO2

•We performed gas-phase ultraviolet photochemistry experiments using pure 16O- and 18O-rich SO2.•Products showed large mass-independent fractionation (S-MIF) effects.•Experiments with 18O-rich SO2 showed reduced S-MIF magnitude compared to those with 16O-rich SO2.•Modeling of the experiments suggested that the S-MIF was generated by a combination of mechanisms.•Modeling also suggested that S-MIF could be produced in an optically thin atmosphere.

This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190-235 nm) under pressure regimes (5-25 Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states.