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

Abstract

Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO) and ammonium (NH) 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 and NH 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) 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) and reduced N (NH) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NO, NH and particulate NO and NH, and surface-level particulate NO and NH concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO and NH) 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, (R) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO concentrations (R = 1.4 - 2.9) and under-estimates NH concentrations (R = 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 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 (R = 0.6 - 2.6 for NO, 0.6 - 3.1 for NH). Values of R for NH CalDep - ModDep comparisons were approximately double the corresponding values for NH CalDep - ModDep comparisons due to the significant fraction of gas-phase NH deposition incorporated in the TM4 and ACCMIP ...

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…For instance, Baker et al (2010) noted that N inputs to the open ocean have increased by up to a factor of 3 over the last 150 years with more enhancements expected in the near future. But their calculations, as with others such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) (Baker et al, 2017), are difficult to validate since lack of data results in dry deposition calculations over the ocean to be based largely on emissions and transport modeling. Furthermore, there are major uncertainties in dry deposition velocities, which are needed to quantify deposition fluxes from concentrations (Baker et al, 2017).…”

“…But their calculations, as with others such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) (Baker et al, 2017), are difficult to validate since lack of data results in dry deposition calculations over the ocean to be based largely on emissions and transport modeling. Furthermore, there are major uncertainties in dry deposition velocities, which are needed to quantify deposition fluxes from concentrations (Baker et al, 2017). This latter issue applies broadly to all atmospheric constituents that can deposit to the surface, including of course sea salt aerosol as was demonstrated during EOPACE (Reid et al, 2001).…”

Seasonal Response of North Western Pacific Marine Ecosystems to Deposition of Atmospheric Inorganic Nitrogen Compounds from East Asia

“…The wet and dry depositions of airborne natural and anthropogenic material transported from continents to the ocean surface have become important pathways for supplying nutrients to the phytoplankton biomass, along with upwellings and river discharge 1 – 7 . Thus, ways to estimate both the atmospheric inputs of nitrogen to the oceans and the impact that atmospheric deposition has on ocean biogeochemistry have been investigated in several studies 2 , 4 , 6 , 8 – 12 .…”

Impacts of climate change and emissions on atmospheric oxidized nitrogen deposition over East Asia

“…There is a lack of deposition data over the ocean, making evaluation of the ACCMIP results across the oceans difficult. Recently, Baker et al (2017) conducted an intensive evaluation of the ACCMIP multi-model mean based on a large number of dry NO y deposition samples, i.e., a total of 770 samples collected over the Pacific, showing comparable spatial distributions between observations and AC-CMIP, such as a consistent northwest-southeast gradient with higher deposition flux closer to the coast ( Fig. 12 in Baker et al, 2017).…”

“…Recently, Baker et al (2017) conducted an intensive evaluation of the ACCMIP multi-model mean based on a large number of dry NO y deposition samples, i.e., a total of 770 samples collected over the Pacific, showing comparable spatial distributions between observations and AC-CMIP, such as a consistent northwest-southeast gradient with higher deposition flux closer to the coast ( Fig. 12 in Baker et al, 2017). In terms of wet deposition, considering the close relationship between wet deposition and precipitation (Kryza et al, 2012;Wałaszek et al, 2013), evaluation of precipitation is performed using the Tropical Rainfall Measuring Mission (TRMM; http://pmm.nasa.gov/trmm, last access: 18 January 2019) and Global Precipitation Climatology Project (GPCP) v2.3 (Adler et al, 2018) precipitation data.…”