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 TM4-ECPL (TM4) model: 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, weighted by grid-cell area and numbers of observations, (RA,n) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO3- concentrations (RA,n = 1.4 - 2.9) and under-estimates 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.
Highlights
Global emissions of inorganic nitrogen (i.e. all nitrogen (N) species, excluding N2) to the atmosphere have likely increased by factors of 3–4 since the onset of industrialisation in the mid-19th century (Duce et al, 2008; Galloway et al, 2008)
Major sources include the emission of nitrogen oxides (NOx) as a by-product of combustion (Galloway et al, 2004) and ammonia (NH3) emissions resulting from fertiliser application and intensive livestock-rearing practices (Bouwman et al, 1997)
This paper describes the database of aerosol N species’ concentrations that was assembled and the results of comparing this database to the models at the global scale, as well as in three specific regions: the tropical eastern Atlantic (TEAtl), the northern Indian Ocean (NInd) and the margins of the northwest Pacific (NWPac)
Summary
Global emissions of inorganic nitrogen (i.e. all nitrogen (N) species, excluding N2) to the atmosphere have likely increased by factors of 3–4 since the onset of industrialisation in the mid-19th century (Duce et al, 2008; Galloway et al, 2008). Ongoing implementation of emission controls (mostly affecting NOx) and global economic development will lead to further changes in both the magnitude and spatial distribution of nitrogen emissions over the coming decades Dentener et al, 2006; Lamarque et al, 2013a) Nitrogen deposition impacts both terrestrial and marine ecosystems. N is a limiting nutrient for primary producers over ∼ 70 % of the global ocean (Duce et al, 2008). Its deposition enhances primary productivity in low-nitrogen marine ecosystems Export of atmospheric N into sub-oxic or anoxic zones of, for example, the Arabian Sea will lead to non-linear effects on the marine and atmospheric N cycle through the processes of denitrification and N2O production and consumption (Suntharalingam et al, 2012; Landolfi et al, 2013; Somes et al, 2016)
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