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

Abstract

Carbonyl sulfide (COS) plays an important role in the sulfur cycle and climate change. Yet, much remains unknown about the photochemical mechanisms of COS in nutrient‐rich seawater. We measured the photochemical production rates of COS in the surface seawater of the Indian Ocean under sunlight irradiation. The photochemical production of COS was mainly initiated by ultraviolet (UV) radiation with UVA contributing approximately 68% to the total COS production. Using cysteine, a typical proxy of dissolved organic sulfur, the effect of enhanced nitrate concentration on COS formation was conducted in authentic seawater during simulated sunlight irradiation, indicating the enhancement of the COS formation with increasing nitrate concentrations. This result revealed that the generation of hydroxyl radical with nitrate photolysis plays a key role in the COS formation process. These findings improve our understanding of the marine COS photoproduction cycle and the impact of nitrate on the COS photochemical production in surface seawater.

Revealing the Marine Cycles of Volatile Sulfur Compounds and Their Biogeochemical Controls: A Case of the Western North Pacific

“…More than 80% of photoproduction of both the OCS and CS 2 was driven by UV radiation. Similar wavelength dependence of OCS photoproduction was also reported by Li et al through laboratory culture . This phenomenon may be due to the photosensitizers having wavelength-selective absorption and mainly absorbing UV radiation.…”

“…After normalizing by photon flux density, OCS and CS 2 photoproduction rates in the KOE significantly exceeded those in the NPSG (Table S5), which indicated abundant photosensitizers and/or precursors in the KOE. Figures S3 and c demonstrated that KOE upwelling likely influenced the availability of dissolved organic matter and nitrate for solar radiation exposure, enriching surface photochemical processes with photosensitizers and precursors. , Additionally, upwelling-associated nutrient influx boosted surface phytoplankton growth, furthering in situ organic matter production. Lennartz et al.…”

“…Plentiful photosensitizers (i.e., dissolved organic matter and nitrate) , and intense solar radiation were beneficial to the photoproduction of OCS/CS 2 in the KOE and NPSG, respectively. Abundant organic matter and a higher seawater temperature expedited the dark production of OCS in the KOE and NPSG, respectively.…”

“…Figures S3 and 1c demonstrated that KOE upwelling likely influenced the availability of dissolved organic matter and nitrate for solar radiation exposure, enriching surface photochemical processes with photosensitizers and precursors. 16,49 Additionally, upwelling-associated nutrient influx boosted surface phytoplankton growth, furthering in situ organic matter production. Lennartz et al also suggested upwelling regions as potential hotspots for OCS and CS 2 photoproduction.…”

“…Similar wavelength dependence of OCS photoproduction was also reported by Li et al through laboratory culture. 49 This phenomenon may be due to the photosensitizers having wavelength-selective absorption and mainly absorbing UV radiation. For example, CDOM dissolved in seawater is easily excited by UV radiation and forms an excited-state CDOM ( 3 CDOM*) to participate in the formation of COS and CS 2 .…”