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

Abstract

Carbonyl sulfide (COS) and carbon disulfide (CS2) are important atmospheric gases photochemically generated from organic sulfur precursors in sunlit natural waters. This study examined these processes by evaluating COS and CS2 photoproduction from dimethyl sulfide (DMS) in the presence of dissolved organic matter (DOM). DOM was added because it photochemically produces various reactive intermediates (3CDOM*, •OH, 1O2, and H2O2) potentially involved in these reaction pathways. DMS‐amended synthetic waters at pH 8 were varied in terms of their DOM type and concentration, spiked with the 3CDOM* quenching agent, phenol, in certain cases, and subsequently irradiated over varying exposure times. Results indicated that various DOM types ranging from freshwater to open‐ocean DOM increased COS but did not alter CS2, which remained at nondetect levels. DOM type influenced COS only at higher concentrations (20 mg/L), whereas increasing DOM concentrations proportionally increased COS concentrations for all DOM types. Phenol addition lowered COS formation for reasons that remained unclear because phenol likely quenched 3CDOM* and DMS‐derived sulfur‐based radicals. Further comparisons with DMS‐spiked natural waters and cysteine (CYS)‐spiked synthetic and natural waters assessed previously indicated that COS formation from both precursors in natural waters was always greater than in waters containing DOM alone. Practitioner Points DMS‐ and DOM‐spiked synthetic waters formed COS but did not form CS2 during sunlight photolysis. In DMS‐spiked synthetic solutions, DOM type has a limited influence on COS formation whereas DOM concentration has a stronger influence on COS formation. COS formation in the DMS‐spiked synthetic waters was fairly proportional to the DOC concentration but was generally lower than COS formation in DMS‐spiked natural waters.

Effect of Nitrate on the Photochemical Production of Carbonyl Sulfide From Surface Seawater

“…Previous research has shown that cysteine does not form COS via direct photolysis due to the negligible absorption of sunlight by cysteine, but is more likely to be converted by indirect photolysis (Gharehveran et al, 2020). Prior research has suggested that cysteine can react with a variety of ROS (e.g., •OH, H 2 O 2 , 1 O 2 , and 3 CDOM*) (Chu et al, 2016;Trujillo et al, 2016), while 3 CDOM* and •OH may be involved in the formation of COS (Flock et al, 1997;Gharehveran et al, 2020). In the current study, when the samples were left in dark conditions for 2 hr again, COS concentrations exhibited a slow decline.…”

“…We selected cysteine as a model compound due to: (a) its ability to form considerable levels of COS and CS 2 compared to other precursors; and (b) it does not undergo direct photolysis. In addition, previous photochemical studies on COS have also extensively selected cysteine as a representative of DOS (Gharehveran et al, 2020;Gharehveran & Shah, 2018;Ossola et al, 2019). We first investigated the production of COS in seawater containing cysteine at room temperature (24°C) in dark conditions.…”

“…These produced radicals can form COS via several reactions, such as hydrogen abstraction, disproportionation, and β-cleavage (Mozziconacci et al, 2008). Previous reports indicate that •OH is not involved in the rate-limiting step during COS formation, while the excited triplet states of DOM ( 3 CDOM*) are more likely to participate (Gharehveran et al, 2020). However, the significant increase in COS concentrations under the nitrate-rich system observed in this study (Figures 3b and 3c) is closely related to nitrate.…”

“…We focused on evaluating the effect of nitrate on the photochemical production of COS from the photodegradation of cysteine, a typical DOS proxy, in natural seawater samples under solar irradiation for the first time. Previous research has adopted a variety of ROS scavengers (e.g., isopropyl alcohol for •OH) to identify the role of ROS in photochemical processes (Gharehveran et al, 2020;Han et al, 2017). By performing •OH scavenger addition experiments, we further evaluated the relative contribution of •OH induced by nitrate photolysis to the formation of COS, thus improving our understanding of the role played by nitrate in the COS photochemical formation.…”

“…Andreae and Feree, 1992). The addition of various types of organic sulfur precursors, such as dimethylsulfoniopropionate, dimethyl sulfoxide, dimethyl sulfide, methionine, cysteine, and glutathione, has been shown to produce COS during irradiation (Gharehveran et al, 2020;Zepp & Andreae, 1994). In other words, the photochemical degra dation of DOS to generate COS represents one of the known routes of organic sulfur-containing compounds in aquatic systems.…”