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 (NO<sub>3</sub><sup>−</sup>) and ammonium (NH<sub>4</sub><sup>+</sup>) to the surface waters of the open ocean, with potential impacts on m...
| Main Authors: | , , , , , , , , , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/8189/2017/acp-17-8189-2017.pdf |
| Summary: | Anthropogenic nitrogen (N) emissions to the atmosphere have increased
significantly the deposition of nitrate (NO<sub>3</sub><sup>−</sup>) and ammonium
(NH<sub>4</sub><sup>+</sup>) 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<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup> 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<sub><i>x</i></sub>) on mineral surfaces.
<br><br>
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<sub><i>y</i></sub>) and reduced N (NH<sub><i>x</i></sub>)
and to the following parameters from the Tracer Model 4 of the Environmental Chemical Processes Laboratory (TM4): ModDep for
NO<sub><i>y</i></sub>, NH<sub><i>x</i></sub> and particulate NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup>, and
surface-level particulate NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup> concentrations.
As a model ensemble, ACCMIP can be expected to be more robust than TM4,
while TM4 gives access to speciated parameters (NO<sub>3</sub><sup>−</sup> and
NH<sub>4</sub><sup>+</sup>) 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 (<i>R</i><sub><i>A</i>, <i>n</i></sub>), 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 NO<sub>3</sub><sup>−</sup> concentrations (<i>R</i><sub><i>A</i>, <i>n</i></sub> = 1.4–2.9) and
underestimates NH<sub>4</sub><sup>+</sup> concentrations (<i>R</i><sub><i>A</i>, <i>n</i></sub> = 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 NH<sub>4</sub><sup>+</sup> 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 (<i>R</i><sub><i>A</i>, <i>n</i></sub> = 0.6–2.6 for NO<sub>3</sub><sup>−</sup>, 0.6–3.1
for NH<sub>4</sub><sup>+</sup>). Values of <i>R</i><sub><i>A</i>, <i>n</i></sub> for NH<sub><i>x</i></sub> CalDep–ModDep
comparisons were approximately double the corresponding values for
NH<sub>4</sub><sup>+</sup> CalDep–ModDep comparisons due to the significant fraction
of gas-phase NH<sub>3</sub> deposition incorporated in the TM4 and ACCMIP NH<sub><i>x</i></sub>
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. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |