Observation- and Model-Based Estimates of Particulate Dry Nitrogen Deposition to the Oceans

Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO₃^-) and ammonium (NH₄⁺) to the surface waters of the open ocean, with potential impacts on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, ~2900 observations of aerosol NO₃^- and NH₄⁺ concentrations, acquired from sampling aboard ships in the period 1995 - 2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NO_x) on mineral surfaces. Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes, however these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux are therefore very difficult to validate for dry deposition. Here the available observational data were averaged over a 5° × 5° grid and compared to ACCMIP dry deposition fluxes (ModDep) of oxidised N (NO_y) and reduced N (NH_x) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NO_y, NH_x and particulate NO₃^- and NH₄⁺, and surface-level particulate NO₃^- and NH₄⁺ concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO₃^- and NH₄⁺) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model - observation ratios, weighted by grid-cell area and numbers of observations, (R_A,n) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO₃^- concentrations (R_A,n = 1.4 - 2.9) and under-estimates NH₄⁺ concentrations (R_A,n = 0.5 - 0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH₄⁺ in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (R_A,n = 0.6 - 2.6 for NO₃^-, 0.6 - 3.1 for NH₄⁺). Values of R_A,n for NH_x CalDep - ModDep comparisons were approximately double the corresponding values for NH₄⁺ CalDep - ModDep comparisons due to the significant fraction of gas-phase NH₃ deposition incorporated in the TM4 and ACCMIP NH_x model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model - observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species concentrations and this cannot be achieved if model products only report dry deposition flux over the ocean.