Influence of dissolved organic matter on carbonyl sulfide and carbon disulfide formation from cysteine during sunlight photolysis

Abstract

Carbonyl sulfide (COS) and carbon disulfide (CS₂) are important atmospheric gases that are formed from organic sulfur precursors present in natural waters when exposed to sunlight. However, it remains unclear how specific water constituents, such as dissolved organic matter (DOM), affect COS and CS₂ formation. To better understand the role of DOM, irradiation experiments were conducted in O₂-free synthetic waters containing four different DOM isolates, acquired from freshwater to open ocean sources, and the sulfur-based amino acid, cysteine (CYS). CYS is a known natural precursor of COS and CS₂. Results indicated that COS formation did not vary strongly with DOM type, although small impacts were observed on the kinetic patterns. COS formation also increased with increasing CYS concentration but decreased with increasing DOM concentration. Quenching experiments indicated that ˙OH was not involved in the rate-limiting step of COS formation, whereas excited triplet states of DOM (³CDOM*) were plausibly involved, although the quenching agents used to remove ³CDOM* may have reacted with the CYS-derived intermediates as well. CS₂ was not formed under any of the experimental conditions. Overall, DOM-containing synthetic waters had a limited to no effect towards forming COS and CS₂, especially when compared to the higher concentrations formed in sunlit natural waters, as examined previously. The reasons behind this limited effect need to be explored further but may be due to the additional water quality constituents present in these natural waters. The findings of this study imply that multiple variables beyond DOM govern COS and CS₂ photoproduction when moving from freshwaters to open ocean waters.