Articles | Volume 17, issue 13
Atmos. Chem. Phys., 17, 8189–8210, 2017
Atmos. Chem. Phys., 17, 8189–8210, 2017

Research article 05 Jul 2017

Research article | 05 Jul 2017

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

Alex R. Baker1, Maria Kanakidou2, Katye E. Altieri3, Nikos Daskalakis2, Gregory S. Okin4, Stelios Myriokefalitakis2,a, Frank Dentener5, Mitsuo Uematsu6, Manmohan M. Sarin7, Robert A. Duce8, James N. Galloway9, William C. Keene9, Arvind Singh7, Lauren Zamora10,11, Jean-Francois Lamarque12, Shih-Chieh Hsu13,†, Shital S. Rohekar1,b, and Joseph M. Prospero14 Alex R. Baker et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, UK
  • 2Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, P.O. Box 2208, Heraklion, Greece
  • 3Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 4Department of Geography, University of California at Los Angeles, Los Angeles, CA, USA
  • 5European Commission, Joint Research Centre, Ispra, Italy
  • 6Center for International Collaboration, Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan
  • 7Geosciences Division, Physical Research Laboratory, Ahmedabad, India
  • 8Departments of Oceanography and Atmospheric Sciences, Texas A&M University, College Station, TX, USA
  • 9Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
  • 10Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 11Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park, MD, USA
  • 12NCAR Earth System Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
  • 13Research Center for Environmental Changes, Academia Sinica, Nankang, Taipei, Taiwan
  • 14Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL, USA
  • anow at: IMAU, University of Utrecht, Utrecht, the Netherlands
  • bnow at: School of Physics, Astronomy and Maths, University of Hertfordshire, Hatfield, UK
  • deceased, 10 October 2014

Abstract. 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 (NOx) 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 (NOy) and reduced N (NHx) and to the following parameters from the Tracer Model 4 of the Environmental Chemical Processes Laboratory (TM4): ModDep for NOy, NHx 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 (RA, n), weighted by grid-cell area and number of observations, were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 overestimates NO3 concentrations (RA, n =  1.4–2.9) and underestimates NH4+ concentrations (RA, 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 (RA, n =  0.6–2.6 for NO3, 0.6–3.1 for NH4+). Values of RA, n for NHx 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 NHx 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.

Short summary
Man's activities have greatly increased the amount of nitrogen emitted into the atmosphere. Some of this nitrogen is transported to the world's oceans, where it may affect microscopic marine plants and cause ecological problems. The huge size of the oceans makes direct monitoring of nitrogen inputs impossible, so computer models must be used to assess this issue. We find that current models reproduce observed nitrogen deposition to the oceans reasonably well and recommend future improvements.
Final-revised paper