Constraints on global oceanic emissions of N₂O from observations and models

Abstract. We estimate the global ocean N₂O flux to the atmosphere and its confidence interval using a statistical method based on model perturbation simulations and their fit to a database of ΔpN₂O (n =  6136). We evaluate two submodels of N₂O production. The first submodel splits N₂O production into oxic and hypoxic pathways following previous publications. The second submodel explicitly represents the redox transformations of N that lead to N₂O production (nitrification and hypoxic denitrification) and N₂O consumption (suboxic denitrification), and is presented here for the first time. We perturb both submodels by modifying the key parameters of the N₂O cycling pathways (nitrification rates; NH₄⁺ uptake; N₂O yields under oxic, hypoxic and suboxic conditions) and determine a set of optimal model parameters by minimisation of a cost function against four databases of N cycle observations. Our estimate of the global oceanic N₂O flux resulting from this cost function minimisation derived from observed and model ΔpN₂O concentrations is 2.4 ± 0.8 and 2.5 ± 0.8 Tg N yr⁻¹ for the two N₂O submodels. These estimates suggest that the currently available observational data of surface ΔpN₂O constrain the global N₂O flux to a narrower range relative to the large range of results presented in the latest IPCC report.