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

Abstract. Nitrogen (N) emissions to the atmosphere have increased by a factor of 3–4 through anthropogenic activity since the Industrial Revolution. This has led to large increases in 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 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. Surface particulate NO3− and NH4+ concentrations simulated by the TM4-ECPL (TM4) model were compared to observed aerosol concentrations. Dry deposition fluxes of these species predicted by TM4 (ModDep) were compared with equivalent fluxes calculated from the observed concentrations (CalDep) using two commonly applied methods for the determination of CalDep. CalDep was also compared to total dry deposition fluxes of oxidised N (NOy) and reduced N (NHx) from TM4 and the ACCMIP multi-model mean product. Comparison in the three study regions suggests that TM4 over-estimates NO3− concentrations and under-estimates NH4+ concentrations, 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 and it was not possible to assess objectively the relative merits of the two methods for estimating CalDep. Comparisons of NH4+ CalDep to NHx ModDep were impaired by the significant fraction of gas-phase NH3 deposition incorporated in the TM4 and ACCMIP model products. All of the comparisons (of concentration and deposition) suffered due to the large uncertainty in dry deposition velocities used in the models and in the calculation of CalDep. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades.