Atmospheric Input of Chemicals to the Ocean

Lead agency: World Meteorological Organization

History of WG 38

Working Group 38 was formed in 2008 because of growing concern about the impact of atmospheric deposition of both natural and anthropogenic substances on ocean chemistry, biology, and biogeochemistry as well as climate. It has held meetings at the University of Arizona, Tucson, AZ, in 2008, at IMO in London in 2010, in Malta in 2011, and at the University of East Anglia, Norwich, UK in 2013. Sponsors of those WG 38 efforts have included WMO, IMO, SCOR, SIDA, the European Commission Joint Research Centre, the University of Arizona, the International Environment Institute at the University of Malta, and the University of East Anglia, and the US National Science Foundation. Following the initial terms of reference, five scientific papers have been published in the peer-reviewed scientific literature, and these are listed at the end of this section.

The Nitrogen Study and its Terms of Reference

Although the early work of Working Group 38 did consider some aspects of the deposition and impacts of atmospheric nitrogen species on the ocean, it was recognized that this was a significant and complex scientific issue that required a more in depth study. Thus, at GESAMP’s 39th session in 2012, Members approved additional terms of reference for continued work of GESAMP WG 38 to address issues related to the impact of the atmospheric deposition of anthropogenic nitrogen to the ocean. An abbreviated form of the new Terms of Reference were as follows:

  • Update the geographical estimates of atmospheric anthropogenic nitrogen deposition to the global ocean;

  • Re-evaluate the magnitude and impact of atmospheric nitrogen deposition on marine biogeochemistry;

  • Provide a more reliable estimate of the impact of atmospheric anthropogenic nitrogen deposition on the production of additional nitrous oxide in the ocean and its subsequent emission to the atmosphere;

  • Evaluate the extent to which anthropogenic nitrogen, delivered to the coastal zone via rivers, is transported to the open ocean; and

  • Make a detailed estimate of the input and impact of anthropogenic nitrogen in the area of the Northern Indian Ocean and the South China Sea.

To address these new terms of reference, a highly successful and productive workshop on “The Atmospheric Deposition of Nitrogen and Its Impact on Marine Biogeochemistry” was held at the University of East Anglia in Norwich, United Kingdom, from 11 to 14 February 2013. Twenty-three scientists participated in the workshop. Sub-groups began the development of a number of scientific papers covering the task areas above. Six of these papers have been published in the peer-reviewed scientific literature and a seventh paper will be submitted in the latter part of 2017 (these are listed at the end of this section). This nitrogen workshop was supported by WMO, IMO, the University of East Anglia and the US National Science Foundation through SCOR.

Results of the Nitrogen Study

Atmospheric N deposition to the ocean was estimated using an atmospheric chemistry model, which explicitly includes organic nitrogen (ON) deposition. The deposition fields estimated by this model for the inorganic N are consistent with other independently developed model estimates. The total atmospheric Nr input to the ocean, including the shelf seas, is estimated to be 39 TgN y-1 in 2005, with >30 TgN y-1 of this reaching the open ocean (1Tg = 1012g or 106 tonnes). The models suggest that combustion processes are the primary source of oxidized nitrogen (HNO3 and NO3- predominantly)- 55% from terrestrial anthropogenic combustion, 11% from biomass burning and 11% from shipping, with the remainder from natural sources. Almost two thirds of the reduced nitrogen (NH3 and NH4+ predominantly)comes from agriculture. The ON of anthropogenic origin has increased from about 7% of the total ON in 1850 to about 30% of the total in 2005. However, the model suggests that about a quarter of the total input of reduced and organic N to the ocean arises from recycled material from the ocean itself, and therefore it does not constitute a net new N input to the ocean. The net atmospheric deposition to the oceans estimated here is thus significantly lower than that estimated by Duce et al. [2008], largely as a result of the now-recognized recycling of organic nitrogen and ammonia (NH3). N inputs to the oceans are particularly large downwind of major source regions, especially in the western north Atlantic and north Pacific.

The present global modeling study of the Nr atmospheric cycle and Nr deposition to the ocean is the first that evaluates past, present and future Nr atmospheric deposition accounting for ON primary sources as well as for secondary ON chemical formation as a N-dependent process. While the total Nr deposition to the ocean is not expected to change significantly by 2050, the relative importance of oxidized and reduced N is expected to change, with an increasing proportion of ammonium compared to nitrate, resulting from more efficient controls on terrestrial emissions of nitrogen oxides (NOx) compared to ammonia. This would result in a change in the acidity of the atmospheric deposition.

To evaluate more accurately the importance of atmospheric deposition of N to the ocean, a separate study was initiated to evaluate the riverine input of Nr to the ocean. It was determined that ~23 TgN y-1 as dissolved inorganic N enters the ocean via rivers, with ~17 TgN y-1 reaching the open ocean. There is an additional input of about 11 TgN y-1 from rivers as dissolved (DON), and the effectiveness of the shelf sea trapping of this organic component is uncertain. Atmospheric nitrogen inputs are thus more effective at delivering anthropogenic N to the open ocean than rivers, because of the role of the coastal ocean in trapping fluvial nitrogen. These estimated amounts of atmospheric and fluvial N inputs can be compared with our current estimate of 164 TgN y-1 for biological N fixation within the global ocean. Hence riverine and atmospheric inputs, which are both predominantly anthropogenic, are almost comparable in magnitude to natural inputs of N to the ocean predominantly by nitrogen fixation. An important uncertainty identified in the work of WG 38 is the extent to which this biological fixation might be suppressed by anthropogenic N inputs, since in terms of biochemical energy considerations planktonic organisms can be expected to use the most readily available N of nitrate and ammonium inputs rather than expend energy to fix N. Such suppression has the potential to provide a short term negative feedback that would reduce the impacts of atmospheric N deposition on the oceans, but the scale of local increases in N deposition and particularly ambient concentrations, which trigger this feedback, are very uncertain.

Models represent the only method to derive global scale estimates of atmospheric deposition to the oceans, but these models need evaluation. Atmospheric deposition is routinely measured at many land stations around the world, and this can be used to validate the models. However, the collection of such model validation data at sea is more challenging. To evaluate the performance of atmospheric N deposition models, a database of measured aerosol nitrate (NO3-) and ammonium (NH4+) concentrations obtained on-board ships over the global ocean was compiled and compared to model simulation results. The database contained measurements of ~2900 samples collected from 1995 – 2012 and details of access to this database are available within the summary of paper II.6 in this report. The model data comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities, vd, used to derive deposition fluxes from concentrations. When considering modelled dry Nr deposition, the uncertainty in vd probably implies that fluxes can be estimated to within no better than one order of magnitude. Uncertainties in vd have been a major limitation on estimates of the flux of material to the oceans for several decades and we provide recommendations to help improve the situation. Overall the agreement of model and data based deposition estimates is very good at large ocean basin scales, but becomes less good at higher spatial resolution. It is therefore important to consider the time and space scales of interest when determining the appropriate model products being used.

Our terms of reference included focusing particularly on some regions where atmospheric deposition is especially high and changing. In one regional study of the impact of N deposition, the analysis of datasets for atmospheric Nr deposition, satellite chlorophyll-a (Chl-a), and air mass back trajectories revealed that transport of N originating from the populated east coasts of China and Indonesia and its deposition to the ocean contributed approximately 20% of the annual biological new production in the South China Sea. The airborne contribution of Nr to new production in this region is expected to grow considerably in the coming decades. Another regional study as part of the WG38 activities focused on the northern Indian Ocean, which is another area of particular high and growing nitrogen deposition. The issue in this region is not just that this atmospheric deposition can stimulate increased ocean productivity, but also because the circulation of deep waters within the northern Indian Ocean gives rise to one of the largest low oxygen regions of the world ocean. This is a natural phenomenon, but the increased atmospheric Nr deposition in this region can stimulate increased phytoplankton production, and the subsequent sinking of extra organic matter into deep ocean waters are predicted to increase this globally important region of low oxygen. Model calculations conducted as part of the WG38 activities discussed below confirm this prediction and the associated increased production of the greenhouse gas N2O.

Most ocean primary productivity is driven by internal recycling of the vast reservoirs of nutrients in the deep ocean. This productivity ultimately sustains life in the oceans and also mediates a vast exchange of carbon dioxide between the oceans and atmosphere. Ocean productivity depends on the availability of light and a range of nutrients beside N, particularly phosphorus and iron, and the iron also has an important atmospheric source. WG 38 deliberations focused on N, but the ocean modelling incorporated the role of all the factors limiting productivity. We estimate that atmospheric N deposition increases global ocean carbon sequestration via the biological pump by 0.15 Pg C y-1, which is important within the context of an estimated ocean annual total C (as CO2) uptake of 2.2+0.5 Pg C y-1. This estimated enhancement of carbon uptake is smaller than an estimate made earlier by members of WG 38 [Duce et al. (2008)], reflecting lower net atmospheric nitrogen deposition estimates, particularly due to recycling of ammonia and organic nitrogen between the atmosphere and ocean. Increases in atmospheric deposition of nutrients have the potential to increase ocean productivity and thereby the sinking of organic matter into the deep ocean, thus increasing areas of low oxygen in the deep ocean, causing increases in denitrification (the breakdown of organic matter in areas of low oxygen using nitrate rather than oxygen as an oxidising agent) and the resulting emission of the greenhouse gas N2O. However, we estimate that the increase in this N2O flux will be small (1 to 3%) on the global scale, but perhaps 5-20% in the Arabian Sea region under likely scenarios over coming decades to 2050. A response has been identified in which increased ocean denitrification can create a negative feedback on the longer timescales (100s to 1000s of years) causing a decline in ocean nitrogen inventory and thus limiting the potential impact of atmospheric Nr deposition on marine productivity. The significance of this feedback clearly merits further study, particularly in Earth system models evaluating the longer term functioning of the ocean system.

Through these papers significant advances have been made in our understanding of atmospheric nitrogen deposition and its impacts on ocean biogeochemistry. A number of scientific gaps remain,providing new opportunities and challenges. Therefore we recommend additional work on:

  • The sources of atmospheric organic nitrogen,

  • the magnitude and significance of recycling of ammonia and organic nitrogen from the oceans,

  • the parameterisation of deposition, particularly dry deposition

  • an improved data base, particularly on wet deposition over the oceans

  • the extent and thresholds for suppression of nitrogen fixation by ambient surface water dissolved nitrogen

  • the retention of nitrogen within shelf systems, and particularly how the rates of bacterial processes that convert fixed nitrogen back to dinitrogen gas (denitrification and the related anammox both of which convert Nr into inert nitrogen gas) vary with temperature

  • the impacts of possible changes in aerosol acidity over coming decades on the atmospheric delivery of nutrients to the oceans

  • increasing atmospheric carbon dioxide concentrations are already driving ocean acidification and this is likely to increase in coming decades. The impacts of the changing ocean pH on ocean emissions of atmospherically important gases including nitrogen species, but also other gases, needs to be evaluated.

We also recommend further work in the following regions, which are particularly sensitive to likely changes in atmospheric deposition:

  • the Northwest Pacific where deposition fluxes are expected to grow and where there may already be impacts from the current inputs,

  • the northern Indian Ocean, an important source region for N2O, which receives a large atmospheric input that is argued to already be increasing plankton productivity,

  • areas of the Mediterranean and North Atlantic where primary production is phosphorus- or iron-limited and hence where additional nitrogen deposition may lead to different nutrient biogeochemical responses to those in other ocean areas where nitrogen is the primary limiting nutrient.

Activities of WG 38 in 2017

From 27 February to March 2 two workshops took place at the University of East Anglia (UEA), Norwich, United Kingdom under the auspices of GESAMP Working Group 38 and sponsored by WMO, NSF, SCOR, SOLAS and UEA. These workshops focussed on the changes in the acid/base balance of the atmosphere and ocean, and their impacts on air-sea exchange.

Workshop on the Impact of Ocean Acidification on Fluxes of Atmospheric non CO2 Climate-Active Species

Earlier investigations on the impact of ocean acidification (OA) have primarily focused on changes in oceanic uptake of anthropogenic CO2, the resulting shifts in carbonate chemical equilibria and the consequences for marine calcifying organisms. Little attention has been paid to the direct impacts of OA on the ocean sources of a range of other gaseous and aerosol species (including N2O, CH4, DMS, and marine VOCs and halocarbons that are influential in regulating radiative forcing, atmospheric oxidising capacity (via OH and O3 cycling) and atmospheric chemistry. The oceanic processes governing emissions of these species are frequently sensitive to the changes in pH and ocean pCO2 accompanying ocean acidification. The direct and indirect influences of these oceanic processes (e.g., microbial metabolic rates, levels of surface primary production, ecosystem composition, etc.) on ocean fluxes of non-CO2 trace gases and aerosols, and the subsequent feedbacks to climate remain highly uncertain. The aim of this proposed project would be to review and synthesize the current science on the direct impact of OA on marine emissions of these other key species; identify the primary needs for new research to improve understanding of these processes and quantify the impact of OA on marine fluxes; publish the results in the open peer-reviewed scientific literature, and provide input to national and international research programs and relevant WMO programmes.

Workshop on Changing Atmospheric Nutrient Solubility

Atmospheric deposition of nutrients to the ocean is known to play a significant role in regulating marine productivity and biogeochemistry, in turn potentially impacting the drawdown of CO2 from surface seawater as well as the production of other climate-active gases (e.g., N2O and DMS). The specific impact is dependent on the nutrient in question, the location of the deposition (more significant impact where a particular nutrient is in short supply), and the bioavailability of the deposited nutrient. Bioavailability is largely governed by the chemical speciation of a nutrient and, in general, insoluble species are not bioavailable. For Fe and P, solubility increases during transport through the atmosphere. The causes of this increase are complex, but interactions of aerosol particles with acids appear to play a significant role. Past and future changes in anthropogenic emissions of acidic (SO2 and NOx) and alkaline (NH3) gases have had and likely will have an impact on the acidity of the atmosphere downwind of major urban/industrial sources, with potential consequences to the supply of soluble nutrients to the ocean. Concurrent with this change in acidity there are likely to be other changes which may also impact marine productivity rates and microbial species population composition. The aim of this proposed workshop would be to review and synthesize the current scientific information on solubility of key biogeochemical elements, their pH sensitivity and the biogeochemical controls on the pH sensitivity; identify the key future research needs that are necessary to reduce uncertainties in predictive capability in this area; publish the results in the open peer-reviewed scientific literature; and interact with and provide information to leading relevant international groups (e.g., SOLAS, IGAC, IMBER, SCOR) and WMO programmes such as GAW.

To access the report of these two workshops, please click on the link below:

Report of the two workshops of GESAMP Working Group 38 (27 Feb-2 March 2017)

Additional Activities in 2017

For the fourth year in a row WG 38 organized a session on atmospheric input of chemicals to the ocean for the 2017 European Geosciences Union meeting, held in Vienna, Austria in April – “Air-sea Exchanges: Impacts on Biogeochemistry and Climate”. A number of oral and poster papers at this session were presented by a combination of WG 38 members and other scientists. The initial outcomes from the 2017 Norwich workshops were also presented at this EGU session

Tim Jickells represented WG 38 (via a remote connection) at the GAW workshop on Measurement-Model Fusion for Global Total Atmospheric Deposition February 28 – March 2, 2017, Geneva, with a presentation on “Observation and Model based Estimates of Atmospheric Inputs to the Oceans”.

Tim Jickells represented WG 38 and GESAMP at the GAW Symposium 10-13 April 2017 in Geneva giving one presentation to the plenary session on the activities of GESAMP and then a presentation on “Observations and Modelling Needs to Understand the Impacts of Nitrogen Inputs to the Oceans at a side event on “How can GAW contribute to the N cycle assessment?”

Tim Jickells represented WG 38 and GESAMP at The Third Informal Meeting of the International Law Commission (ILC) on the Protection of the Atmosphere as part of the Dialogue with Scientists meeting in Geneva on May 5, 2017 giving a presentation entitled “Linkages between the oceans and the atmosphere” and participating in the subsequent discussion.

Potential Future Activities of Working Group 38

Possible Workshop for the Integrated Nitrogen Management System (INMS)

A future activity being considered by WG 38 is the assessment of the impact of nitrogen in certain regions of the marine environment as a contribution to the Integrated Nitrogen Management System (INMS). INMS is a global targeted research project with the aim to provide clear scientific evidence to inform future international nitrogen policy development. INMS’s core funding comes from the Global Environment Facility (GEF) (the environment funding mechanism of the United Nations System) with the United Nations Environment Program (UNEP) as the Implementing Agency and the UK Natural Environment Research Council (Centre for Ecology and Hydrology) as the Executing Agency acting on behalf of the International Nitrogen Initiative (INI). WG 38 is in an excellent position to bring together observational scientists and atmospheric modeling groups to address such nitrogen issues. The overall conclusions of the WG38 synthesis on atmospheric nitrogen inputs (which also included a substantial body of work on fluvial inputs), as summarised in the paper by Jickells et al. (2017), include the identification of regions of the ocean:

  • which are subject to particularly high atmospheric deposition loads,

  • which are likely to increase in the future, and

  • where oceanographic conditions lead to an expectation that these inputs may contribute to significant biogeochemical impact.

Two of the four regions highlighted by Jickells et al. were the Northern Indian Ocean and the North Pacific region including the East China Sea. These regions overlap with the INMS “demonstration regions” of South Asia and East Asia. These marine regions in many ways represent one of the ultimate repositories within the marine system for nitrogen mobilisation activities. Currently discussions are underway with INMS to see if workshop-type meetings to address atmospheric nitrogen input to and impact on these marine regions would be of help to INMS.

Recent Publications of Working Group 38

  • Okin, G., A. R. Baker, I. Tegen, N. M. Mahowald, F. J. Dentener, R A. Duce, J. N. Galloway, K. Hunter, M. Kanakidou, N. Kubilay, J. M. Prospero, M. Sarin, V. Surapipith, M. Uematsu, T. Zhu, “Impacts of atmospheric nutrient deposition on marine productivity: roles of nitrogen, phosphorus, and iron”, Global Biogeochemical Cycles, 25, GB2022, doi:10.1029/2010GB003858, (2011)

  • Hunter, K.A., P. S. Liss, V. Surapipith, F. Dentener, R. A. Duce, M. Kanakidou, N. Kubilay, N. Mahowald, G. Okin, M. Sarin, I. Tegen, M. Uematsu, and T. Zhu, “Impacts of anthropogenic SOx, NOx and NH3 on acidification of coastal waters and shipping lanes”, Geophysical Research Letters, 38, L13602, doi:10.1029/2011GL047720 (2011)

  • Kanakidou, M., R. Duce, J. Prospero, A. Baker, C. Benitez-Nelson F. J. Dentener, K.A. Hunter, N. Kubilay, P. S. Liss , N. Mahowald, G. Okin, M. Sarin, K. Tsigaridis, M. Uematsu, L.M. Zamora, and T. Zhu, “Atmospheric fluxes of organic N and P to the ocean”, Global Biogeochemical Cycles, 26, GB3026,doi:10.1029/2011GB004277, (2012)

  • Schulz, M., J. M. Prospero, A. R. Baker, F. Dentener, L. Ickes, P. S. Liss, N. M. Mahowald, S. Nickovic, C. Pérez García-Pando, S. Rodríguez, M. Sarin, I. Tegen, R.A. Duce, “The atmospheric transport and deposition of mineral dust to the ocean - Implications for research needs”, Environmental Science and Technology, 46, 10,390-10,404 (2012)

  • Hagens, M., K.A. Hunter, P.S. Liss, and J.L. Middelburg, “Biogeochemical context impacts seawater pH changes resulting from atmospheric sulfur and nitrogen deposition”, Geophysical Research Letters, 41, doi:10.1002/2013GL058796 (2014)

  • Kim, T.-W., K. Lee, R. Duce, and P. Liss, “Impact of atmospheric nitrogen deposition on phytoplankton productivity in the South China Sea”, Geophysical Research Letters, 41, 3156–3162, doi:10.1002/2014GL059665. (2014).

  • Somes, C.J., A. Landolphi, W. Koeve, and A. Oschlies, “Limited impact of atmospheric nitrogen deposition on marine productivity due to biogeochemical feedbacks in a global ocean model”, Geophysical Research Letters, 43, 4500–4509, doi: 10.1002/2016GL068335. (2016).

  • Kanakidou, M., S. Myriokefalitakis, N. Daskalakas, G. Fanourgakis, A. Nenes, A.R. Baker, K. Tsigaridis, and N. Mihalopoulos, “Past, Present, and Future Atmospheric Nitrogen Deposition”, Journal of the Atmospheric Sciences, 73, 2039-2047, doi:10.1175/JAS-D-15-0278.1 (2016).

  • Sharples, J., J. J. Middelburg, K. Fennel, and T. D. Jickells, “What proportaion of riverine nutrients reaches the open ocean”, Global Biogeochemical Cycles, 31, 39–58, doi:10.1002/2016GB005483. (2017).

  • Jickells, T.D., E. Buitenhuis, K. Altieri, A.R. Baker, et al., “A re-evaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean”, Global Biogeochemical Cycles, 31, 289–305, doi:10.1002/2016GB005586. (2017).

  • Baker, A.R., M. Kanakidou, K. E. Altieri, et al., “Observation- and model-based estimates of particulate dry nitrogen deposition to the oceans”, Atmospheric Chemistry and Physics, 17, 8189-8210, https://doi.org/10.5194/acp-17-8189. (2017).

  • Suntharalingam, P., Zamora, L. M., Sarin, M. M, Singh, A., et al., “Increasing inputs of anthropogenic nitrogen to the Northern Indian Ocean and impacts on marine N2O fluxes” to be submitted for publication to Environmental Research Letters in late 2017.

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