Linking production of carbonyl sulphide to the composition of the marine dissolved organic matter pool

Sulfur containing trace gases impact Earth’s climate, and the ocean is a major natural source for the most abundant sulfur gas in the atmosphere, carbonyl sulfide (COS). Understanding and quantifying COS oceanic emissions is relevant for closing the gap in the atmospheric COS budget, which currently impedes conclusions about trends in stratospheric aerosol formation and gross primary production on a global level. The main production process of COS in surface seawater, photochemical production, remains one of the largest uncertainties in quantifying marine COS emissions. Precursors of COS are dissolved organic molecules, which form an enormously diverse mixture. How the composition of this diverse pool influences COS production has not yet been fully understood.

Here we present continuously measured COS concentrations in seawater and the marine boundary layer to quantify the relationship between photochemical production of COS in various marine regions: Along a North-South Atlantic transect from Bremerhaven to Cape Town and at two stations in the North Sea, one located in the open sea area close to Heligoland, one in the Wadden Sea area close to Spiekeroog Island. Samples of dissolved organic matter (DOM) were taken twice daily for subsequent analysis of the molecular composition and optical properties of the large precursor pool for COS photo- and dark production. Concentrations of COS in surface seawater showed distinct diurnal cycles with considerable day-to-day variations. The marine DOM pool differed with respect to sulfur containing molecules mainly between locations, and less along diurnal cycles. Our results will help to improve mechanistic models of marine COS cycling, especially with regard to the currently existing uncertainties in the global emission estimate of COS.