A new model for the global biogeochemical cycle of carbonyl sulfide – Part 1: Assessment of direct marine emissions with an oceanic general circulation and biogeochemistry model

The global budget of tropospheric carbonyl sulfide (OCS) is believed to be at equilibrium because background air concentrations have remained roughly stable over at least the last decade. Since the uptake of OCS by leaves (associated with photosynthesis) and soils have been revised significantly upwards recently, an equilibrated budget can only be obtained with a compensatory source of OCS. It has been assumed that the missing source of OCS comes from the low-latitude ocean, following the incident solar flux. The present work uses parameterizations of major production and removal processes of organic compounds in the NEMOPISCES (Nucleus for European Modelling of the Ocean, Pelagic Interaction Scheme for Carbon and Ecosystem Studies) ocean general circulation and biogeochemistry model to assess the marine source of OCS. In addition, the OCS photo-production rates computed with the NEMO-PISCES model were evaluated independently using the UV absorption coefficient of chromophoric dissolved organic matter (derived from satellite ocean color data) and apparent quantum yields available in the literature. Our simulations show global direct marine emissions of OCS in the range of 573– 3997 GgS yr, depending mostly on the quantification of the absorption rate of chromophoric dissolved organic matter. The high estimates of that range are unlikely, as they correspond to a formulation that most likely overestimate photoproduction process. Low and medium (813 GgS yr) estimates derived from the NEMO-PISCES model are however consistent spatially and temporally with the suggested missing source of Berry et al. (2013), allowing us thus to close the global budget of OCS given the recent estimates of leaf and soil OCS uptake.