Photochemical and physical modeling of carbonyl sulfide in the ocean

Abstract

Carbonyl sulfide (COS) concentrations in the oceanic mixed layer were simulated using dynamic models that include parameterizations for photochemical and dark production, hydrolysis, air‐sea gas exchange, and vertical mixing. The results were compared with measurements taken at and below the ocean surface from four cruises. The description of the physics of vertical mixing and especially of the mixed layer depth has a great impact on model performance. A one‐dimensional K‐profile parameterization (KPP) model is shown to reproduce surface values quite accurately and to provide a reasonable approximation of the depth profiles. Surface values can also be approximated by a simple mixed‐layer box model, but the missing removal from the surface due to mixing and the integration over the entire mixed layer cause some significant deviations from the observations, especially when there are large diel variations in the mixed layer depth. Rate constants for COS photoproduction that were fitted by the models displayed pronounced regional differences, which are attributed to variations in the concentration of chromophoric dissolved organic matter (CDOM), the precursor for COS photoproduction. Overall, an approximately second‐order relationship between the photoproduction rate constant and the CDOM absorbance is observed in good agreement with the current mechanistic understanding of COS photoproduction. This relationship can help to improve the simulation of COS depth profiles by the KPP model, when information about the depth distribution of CDOM is available. Foremost, it may provide a key to upscaling the various local models to the regional or global scale, where the variability of COS production rates so far remains the biggest obstacle.