Observation- and model-based estimates of particulate dry nitrogen deposition to the oceans

• Published in: Atmospheric Chemistry and Physics
• Main Authors: A. R. Baker, M. Kanakidou, K. E. Altieri, N. Daskalakis, G. S. Okin, S. Myriokefalitakis, F. Dentener, M. Uematsu, M. M. Sarin, R. A. Duce, J. N. Galloway, W. C. Keene, A. Singh, L. Zamora, J.-F. Lamarque, S.-C. Hsu, S. S. Rohekar, J. M. Prospero
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Pacific; Indian; North Atlantic
• Online Access: https://doi.org/10.5194/acp-17-8189-2017; https://doaj.org/article/0b5c9c41a42840f1a480fc5d14909f69

Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO 3 − ) and ammonium (NH 4 + ) 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 NO 3 − and NH 4 + 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 Tracer Model 4 of the Environmental Chemical Processes Laboratory (TM4): ModDep for NO y , NH x and particulate NO 3 − and NH 4 + , and surface-level particulate NO 3 − and NH 4 + concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives ...