Isotopic fractionation of sulfur during COS hydrolysis

Carbonyl Sulfide (COS) is the most abundant sulfur-containing gas in the atmosphere, and it is used as a proxy for terrestrial gross primary productivity (GPP). There are uncertainties in the COS fluxes estimations that limit this approach. Oceans are the major source of COS to the atmosphere. In the oceans, the COS is produced by photochemical reactions and "dark production", whose mechanism is not well understood. Hydrolysis is the major process that removes COS from the ocean's surface. Identifying the sulfur isotope values (δ³⁴S) and the isotopic fractionation (e) associated with these major sources and sinks could decrease the uncertainties in the fluxes, based on an improved COS global model with an isotopic mass balance [1]. In the current study, we aim to determine the e  during the hydrolysis process of COS (e_h).  We use a purge and trap system coupled to a GC/MC-ICPMS to measure δ³⁴S values during hydrolysis under different pH, salinity (S), and temperature, representing various oceanic conditions. We calculate from our δ³⁴S and COS concentration measurements a e_h of −2.6 ± 0.3‰ in natural seawater from the Gulf of Aqaba (pH 8.2, 22 , S=41‰). Using an artificial solution at similar pH and temperature conditions (pH 8.0, 22 , S=0.2‰) we found e_h of −2.3 ± 0.2‰, hence, salinity has no significant effect on the fractionation. Using the same artificial solution at 4   we found e_h  of −3.9 ± 0.2‰, thus fractionation increases with decreasing temperatures, as can be expected from theory. We will also report the effect of acidity on e_h from experiments in pH of 4 and 9 (at 22 ). This information on the e_h will help us to understand the contribution of COS hydrolysis to the oceanic source and in the future to establish an isotope mass balance model to decrease the uncertainty of this major source.

[1] Davidson, Chen, Alon Amrani, and Alon Angert. "Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes." Proceedings of the National Academy of Sciences 118.6 (2021): e2020060118.