A Harmonized Nitrous Oxide (N2O) Ocean Observation Network for the 21st Century

Bange, Hermann W.; Arévalo-Martínez, Damian L.; de la Paz, Mercedes; Farías, Laura; Kaiser, Jan; Kock, Annette; Law, Cliff S.; Rees, Andrew P.; Rehder, Gregor; Tortell, Philippe D.; Upstill-Goddard, Robert C.; Wilson, Samuel T.

doi:10.3389/fmars.2019.00157

REVIEW article

Front. Mar. Sci., 02 April 2019

Sec. Ocean Observation

Volume 6 - 2019 | https://doi.org/10.3389/fmars.2019.00157

This article is part of the Research Topic Oceanobs'19: An Ocean of Opportunity View all 136 articles

A Harmonized Nitrous Oxide (N₂O) Ocean Observation Network for the 21st Century

$\r\nHermann W. Bange*$ Hermann W. Bange^1*

Damian L. Arévalo-Martínez¹^†

Mercedes de la Paz²^‡

Laura Farías³^§

Jan Kaiser⁴^‖

Annette Kock¹^¶

Cliff S. Law^5,6^#

Andrew P. Rees⁷^Δ

Gregor Rehder⁸^♢

Philippe D. Tortell⁹

Robert C. Upstill-Goddard¹⁰^↓

Samuel T. Wilson¹¹^↑

¹GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
²Instituto Español de Oceanografía (IEO), Centro Oceanográfico de A Coruña, A Coruña, Spain
³Department of Oceanography, Center for Climate and Resilience Research (CR2), University of Concepción, Concepción, Chile
⁴Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
⁵National Institute of Water and Atmospheric Research, Wellington, New Zealand
⁶Department of Chemistry, University of Otago, Dunedin, New Zealand
⁷Plymouth Marine Laboratory, Plymouth, United Kingdom
⁸Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany
⁹Department of Earth, Ocean and Atmospheric Sciences – Department of Botany, The University of British Columbia, Vancouver, BC, Canada
¹⁰School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, United Kingdom
¹¹Daniel K. Inouye Center for Microbial Oceanography: Research and Education, University of Hawai’i at Mānoa, Honolulu, HI, United States

Nitrous oxide (N₂O) is an important atmospheric trace gas involved in tropospheric warming and stratospheric ozone depletion. Estimates of the global ocean contribution to N₂O emissions average 21% (range: 10 to 53%). Ongoing environmental changes such as warming, deoxygenation and acidification are affecting oceanic N₂O cycling and emissions to the atmosphere. International activities over the last decades aimed at improving estimates of global N₂O emissions, including (i) the MarinE MethanE and NiTrous Oxide database (MEMENTO) for archiving of quality-controlled data, and (ii) a recent large-scale inter-laboratory comparison by Working Group 143 of the Scientific Committee on Ocean Research (SCOR). To reduce uncertainties in oceanic N₂O emission estimates and to characterize the spatial and temporal variability in N₂O distributions in a changing ocean, we propose the establishment of a harmonized N₂O Observation Network (N2O-ON) combining discrete and continuous data from various platforms. The network will integrate observations obtained by calibrated techniques, using time series measurements at fixed stations and repeated hydrographic sections on voluntary observing ships and research vessels. In addition to exploiting existing oceanographic infrastructure, we propose the establishment of central calibration facilities in selected international laboratories to improve accuracy, and ensure standardization and comparability of N₂O measurements. Final data products will include a harmonized global N₂O concentration and emission fields for use in model validation and projections of future oceanic N₂O emissions, to inform the global research community and policy makers.

Introduction

Nitrous oxide (N₂O; laughing gas) is an atmospheric trace gas, which accounts for 6% of tropospheric warming by greenhouse gasses, and is a major ozone-depleting compound in the stratosphere (Ravishankara et al., 2009; IPCC, 2013; WMO, 2014). Emission estimates indicate that the oceans may contribute 10 to 53% of combined natural and anthropogenic N₂O sources (Anderson et al., 2010; Ciais et al., 2013). N₂O has been measured in the water column of all major ocean basins, in most marginal seas and in numerous estuaries (Kock and Bange, 2015; Murray et al., 2015), with measurements from the surface mixed layer down to 9800 m in the deep Izu-Ogasawara Trench (Kawagucci et al., 2018). These and other studies show that N₂O concentrations may vary over three orders of magnitude from the open ocean to coastal shelves and semi-enclosed basins. Concentrations range from <1 nmol L^-1 in the permanently anoxic deep basin waters of the Black Sea and Cariaco Trench (Hashimoto et al., 1983; Westley et al., 2006) to ≈1000 nmol L^-1 in coastal near-surface waters off Peru (Arévalo-Martínez et al., 2015) and ≈1500 nmol L^-1 in the suboxic deep waters of the Baltic Sea (Rönner, 1983). Some estuaries may reach similarly high concentrations (Barnes and Upstill-Goddard, 2011).

While the oceans are clearly a major natural contributor to atmospheric N₂O, quantitative estimates remain highly uncertain (Buitenhuis et al., 2018). This uncertainty reflects the low number of marine N₂O measurements to date, as compared to, for example, CO₂ [see e.g., (Bakker et al., 2016)], and the lack of information on (i) seasonal and inter-annual variability, (ii) land-ocean gradients, (iii) the effects of small scale/mesoscale features (Grundle et al., 2017) and (iv) extreme events such as storms (Naik et al., 2008). There is also uncertainty in the relative importance of the various biological processes driving the production and consumption of N₂O in oceanic waters, and their potential responses to changing oceanic conditions (Bange et al., 2010). Likewise, the influence of sea ice on N₂O emissions from high-latitude ecosystems is currently unknown (Vancoppenolle et al., 2013). Randall et al. (2012), for instance, showed that sea ice formation and melting cycles can reverse the direction of the N₂O fluxes across the ocean/atmosphere interface. Yet, the overall impact of these processes on the annual cycle is still unclear.

Oceanic N₂O production and consumption principally occurs in subsurface and deep waters. Microbial nitrification (N₂O is a by-product of ammonia oxidation to nitrite), partial denitrification (reduction of nitrate to N₂O), and nitrifier-denitrification (i.e., nitrifier switching to nitrite reduction under low O₂ conditions) are considered to be the main oceanic N₂O production pathways, whereas, the main N₂O sink is via reduction to N₂ by denitrification in anoxic waters (Bange et al., 2010). Extreme accumulation of N₂O resulting from nitrification and/or denitrification has been found at oxic/anoxic boundaries within oxygen minimum zones (OMZ) of the eastern tropical North/South Pacific Ocean and the Arabian Sea, and also in coastal shelf waters (Bange et al., 2010). In addition, several studies indicate N₂O production via nitrification in surface waters of the open ocean (Dore and Karl, 1996; Law and Ling, 2001; Morell et al., 2001) and in estuaries (Barnes and Upstill-Goddard, 2011), as well as its possible consumption during microbial N₂O fixation (Farías et al., 2013; Cornejo et al., 2015).

Environmental changes such as ocean warming (and associated changes in stratification and ice coverage), acidification, deoxygenation, and eutrophication due to increasing anthropogenic inputs of nutrients (via rivers and atmospheric deposition), may significantly alter N₂O production and consumption, its distribution patterns and, ultimately, its release to the atmosphere (Kroeze et al., 2005; Zhang et al., 2010; Suntharalingam et al., 2012; Rees et al., 2016; Myllykangas et al., 2017). Indeed, model projections that account for ocean warming and atmospheric nitrogen deposition show a net decrease of 4 to 24% in future global oceanic N₂O emissions during the 21st century (Martinez-Rey et al., 2015; Landolfi et al., 2017; Battaglia and Joos, 2018). One model projection suggests that the decrease of N₂O emissions in the 21st century might be followed by a substantial increase of the N₂O emissions in the 22nd century (Battaglia and Joos, 2018). The large degree of uncertainty in future N₂O emission projections results partly from the limitations of existing N₂O concentration data used in model parameterizations and validation. These current data sets are not yet cross-calibrated (their comparability is limited due to missing standard measurement protocols), and are biased by poor spatio-temporal coverage of the ocean (Kock and Bange, 2015).

The importance of additional, routine oceanic N₂O measurements is recognized by the Global Ocean Observing System (GOOS) program, which recently added N₂O to its list of Essential Ocean Variables (EOV)¹.

To reduce uncertainties in current global N₂O marine emission estimates, better constrain and understand temporal and spatial variability, and improve future projections of N₂O concentrations in a changing ocean, we propose the establishment of a harmonized Global N₂O Ocean Observation Network (N2O-ON).

Observation Network Components

Measurement Techniques

The analysis of N₂O at the sea surface and in the ocean interior differs in both measurement approach and the required analytical precision. While water column N₂O concentrations are usually determined using discrete seawater samples, state-of-the-art surface water measurements increasingly use air-water equilibration systems coupled to optical sensors in a continuous mode. In this section we briefly review the development of marine N₂O observations, discuss a coordinated approach to method calibration, and identify emerging technologies that should contribute to improved data quality and spatio-temporal coverage within N2O-ON.

Discrete Measurements

The first study of oceanic N₂O distributions took place nearly 60 years ago in the South Pacific Ocean (Craig and Gordon, 1963), and was followed by measurements in the North Atlantic Ocean during the late 1960s/early 1970s (Junge and Hahn, 1971). A later study in the Sargasso and Caribbean Seas introduced the concept of “Δ(N₂O)” [= c_measured(N₂O) – c_equilibrium(N₂O)], to quantify the difference between the observed and air equilibrium concentration of dissolved N₂O, and thus examine net N₂O production/consumption (Yoshinari, 1976). The development of a rigorously calibrated electron capture detector (ECD) coupled with gas chromatography (GC) facilitated precise and reliable N₂O measurements (Rasmussen et al., 1976; Cohen, 1977; Elkins, 1980; Weiss et al., 1981). Since those pioneering studies, the increasing availability and comparatively low cost of such instrumentation facilitated a significant increase in data availability.

An important next step was the fundamental work on N₂O solubility in seawater (Weiss and Price, 1980), which promoted the development of equilibration techniques for high-resolution surveys of the surface ocean (Weiss et al., 1992) (see section “Continuous Surface Measurements”) and water column N₂O (Butler et al., 1989; Butler and Elkins, 1991). Today, GC-ECD analysis, coupled to headspace equilibration or purge-and-trap techniques, is used by the majority of laboratories worldwide for quantifying dissolved N₂O in discrete seawater samples (Wilson et al., 2018). Even so, mass spectrometric analysis of N₂O is becoming increasingly wide-spread (Capelle et al., 2015; Babbin et al., 2017; Bourbonnais et al., 2017) and may become increasingly important in the future.

An inter-laboratory comparison of oceanic N₂O measurements was recently conducted by the Scientific Committee on Oceanic Research (SCOR) international Working Group (WG) 143². Discrete water samples from the subtropical Pacific Ocean and the Baltic Sea were distributed to participating laboratories (Wilson et al., 2018) for a comparison of accuracy and precision. The samples represented a range of N₂O concentrations, from low concentrations in the oligotrophic open ocean to high concentrations in highly productive and suboxic coastal waters. Recommendations arising from the inter-comparison include (Wilson et al., 2018):

(i) calibration of working gas standards against primary standards,

(ii) incorporation of internal controls (i.e., air-equilibrated seawater) alongside routine sample analysis, and

(iii) the production of high and low N₂O concentration reference seawater for calibrating N₂O measurements across the full range of seawater N₂O concentrations.

Primary gas standard mixtures obtained from atmospheric monitoring agencies will ensure consistency between ocean observations and global atmospheric monitoring networks such as NOAA’s Earth System Research Laboratory/Global Monitoring Division (ESRL/GMD³), NASA’s Advanced Global Atmospheric Gasses Experiment (AGAGE⁴) and the European Integrated Carbon Observing System (ICOS⁵). With accompanying guidelines for discrete measurements in preparation, these recommendations should lead to significant advances in precision and accuracy, thereby improving the inter-comparability of dissolved N₂O measurements and facilitating the detection of seasonal and inter-annual N₂O variability in the near future. Detecting inter-annual N₂O signals is a major goal of N2O-ON, and will require a precision of better than 0.02 nmol L^-1 (<0.2%). This value is derived from the expected change in N₂O solubility due to an annual surface ocean warming of 0.01°C, and an annual increase of 1 nmol mol^-1 (ppb) in the atmospheric N₂O dry mole fraction, setting the salinity to 35 assuming no changes in oceanic N₂O sources and sinks.

Continuous Surface Measurements

In addition to the discrete analysis of N₂O, measurements are also conducted by continuous sampling from the shipboard underway seawater supply. Such measurements are made at a fixed depth (generally between 2 and 10 m below the sea surface) and are often accompanied by atmospheric measurements. These underway measurements have benefited from recent technological advances in cavity-enhanced absorption spectroscopy (CEAS), which facilitate rapid and precise N₂O detection at very low atmospheric mole fractions (i.e., in the sub-ppb range). CEAS analyzers coupled to continuous seawater/gas equilibrators (Arévalo-Martínez et al., 2013; Grefe and Kaiser, 2014; Erler et al., 2015; Zhan et al., 2018) are now frequently used to determine N₂O temporal and spatial variability in surface layers of open and coastal oceans, see e.g., (Arévalo-Martínez et al., 2015; Brase et al., 2017; Grefe et al., 2018; Wells et al., 2018). In addition to CEAS, Fourier Transform Infrared (FTIR) analysis coupled to continuous seawater/gas equilibration (Müller D. et al., 2016) has been developed. A ship-board comparison of five analytical systems (incl. four CEAS systems and one FTIR system) for continuous dissolved N₂O measurements was conducted in the Baltic Sea as part of the activities of SCOR WG 143, demonstrating good agreement between measurements obtained from the different systems. Only recently, a Pumped Profiling System (PPS), connected with a liquid degassing membrane coupled with CEAS has allowed real-time, high-resolution, vertically resolved measurements of sub-surface N₂O (Troncoso et al., 2018). N2O-ON will encourage the wider use of these and emerging new technologies where they can contribute to improvements to data quality, measurement frequency and spatial resolution.

Measurements in the Marine Boundary Layer

Accurate estimates of N₂O flux densities across the ocean/atmosphere interface require measurements of the N₂O mole fraction in the atmospheric boundary layer above the ocean, as well as ocean surface N₂O concentrations. Atmospheric dry mole fraction can be converted into seawater saturation concentration as a function of seawater temperature, salinity and ambient pressure using an established solubility equation (Weiss and Price, 1980). Atmospheric N₂O dry mole fractions are often measured in parallel with continuous underway measurements on research vessels and on vessels of opportunity (VOS: also often referred to as “Voluntary Observing Ship” routes) (Arévalo-Martínez et al., 2013). As for seawater measurements, N2O-ON advocates the routine rigorous calibration and quality control of accompanying atmospheric data.

The relatively inert nature of the N₂O molecule results in a long tropospheric residence time, leading to well-mixed and regionally invariant global mole fractions (Prather et al., 2015). Consequently, where high quality ship-based atmospheric measurements are unavailable, N2O-ON will encourage use of high quality data from land-based global atmospheric monitoring networks; for example tropospheric N₂O dry mole fractions from ESRL/GMD (see text footnote 3) or AGAGE (see text footnote 4). This will enable the extrapolation of individual campaign results to regional or global scales. Satellite-based N₂O measurements show promise to augment atmospheric data collection at land-based monitoring stations, but these remote sensing observations currently have intrinsically large measurement errors, making them unsuitable for quantifying air-sea N₂O exchange (Xiong et al., 2014; Bernath et al., 2017). With further improvements, however, such approaches have the potential to inform N2O-ON in the future.

Future Enhancements

New CEAS-based instruments allow high quality N₂O isotopolog measurements (Harris et al., 2013). N2O-ON will identify an observational framework that will facilitate deployment of these instruments on selected sustained observation lines to provide additional constraints on the global atmospheric N₂O budget (Rahn and Wahlen, 2000; Bernard et al., 2006; Park et al., 2012), and to potentially provide greater insight into the mechanisms of oceanic N₂O production and consumption (Sutka et al., 2006; Yamagishi et al., 2007).

Although the development of CEAS can considerably improve N₂O monitoring capabilities (see above), the estimation of sea/air N₂O flux densities remains challenging because of the intrinsic temporal and spatial variability in surface ocean N₂O concentrations, and the variability of existing gas exchange parameterizations (Garbe et al., 2014), which reflect the complexity in environmental controls of air-sea gas exchange. Unraveling this complexity, and thereby refining gas exchange parameterizations, is the focus of considerable ongoing research beyond the scope of N2O-ON. However, techniques such as the eddy covariance (EC) method that directly evaluate air-sea fluxes circumvent the need for such parameterizations (Businger, 1986). Going forward, the use of direct flux techniques such as EC in combination with N₂O analysis by CEAS will be encouraged by N2O-ON as a means of enhancing our understanding of N₂O fluxes across the sea surface on a range of temporal and spatial scales.

Observation Platforms

N2O-ON will exploit established and new observation platforms to improve the characterization of spatial and temporal variability in oceanic N₂O concentrations.

Research Vessels

To date, the majority of surface and water column N₂O data have been obtained on board research vessels from discrete samples collected in Niskin bottles on a CTD Rosette (see section “Discrete Measurements”), or from underway surface measurements via a continuous seawater supply (see section “Continuous Surface Measurements”). While the significant contribution of research vessels is beyond question and will be supported by N2O-ON, such vessels have a limited spatial and temporal footprint, with most sampling campaigns not repeated regularly and mainly occurring during the summer. N2O-ON will address this limitation by promoting the use of additional measurement platforms and sustained observational campaigns.

Repeat Hydrographic Lines and Time-Series Stations

Repeat hydrographic sampling programs are important in evaluating variability at the ocean-basin scale and for establishing variability on timescales from seasonal to decadal. For example, N₂O has been measured biannually since 2012 in repeat hydrographic/geochemistry surveys on GO-SHIP⁶ section A25 between Portugal and Greenland (de la Paz et al., 2017). The Atlantic Meridional Transect⁷ is an example of an annually repeated cruise on which N₂O measurements have been made over two decades (Forster et al., 2009; Rhee et al., 2009; Grefe and Kaiser, 2014). N₂O has been repeatedly measured during the annual Chinese Arctic and Antarctic Expeditions (CHINARE) to the Arctic and Southern Oceans, see e.g., (Zhan and Chen, 2009; Zhan et al., 2015, 2017). Beside these examples, there are few published time-series measurements of open ocean water column N₂O distributions from repeat hydrographic sections (Nevison et al., 1995; Fenwick and Tortell, 2018). Extending and optimizing the distribution and sampling frequency of repeat hydrographic lines is an important future aspiration for N2O-ON, both for open-ocean and coastal regimes.

Temporal variability is also investigated through regular data collection at a small number of fixed time-series stations, which are usually located close to land. Examples include stations off Goa (India), in Saanich Inlet (Vancouver Island, British Columbia), off central Chile, off Hawai’i in the North Pacific subtropical gyre, in the Eckernförde Bay (southwestern Baltic Sea) and in the Strait of Gibraltar (Naqvi et al., 2010; de la Paz et al., 2015; Farías et al., 2015; Capelle and Tortell, 2016; Wilson et al., 2017; Capelle et al., 2018). Considering the important role of coastal regions in the global N₂O cycle (Bange, 2006; Anderson et al., 2010; Ciais et al., 2013), extending the spatial coverage of fixed time-series stations within a coordinated network is a major aspiration of N2O-ON.

VOS Lines

Autonomous measurement systems on established, regular international VOS routes are restricted to near-surface measurements, and thus do not provide depth-resolved N₂O data. Nevertheless, they do have the potential to deliver a comprehensive picture of the temporal and (limited) spatial variability in surface water N₂O distributions. A pilot VOS line N₂O study in the North Atlantic Ocean between Liverpool, United Kingdom, and Halifax, Canada, was conducted in January 2017 by GEOMAR for the EU InGOS program⁸. The EU BONUS INTEGRAL program⁹ will establish N₂O surface measurements on two VOS lines in the Baltic Sea between Lübeck/Travemünde (Germany) and Helsinki (Finland) as well as to Kemi (Finland) at the northern tip of the Baltic Sea. The successful long-term operation of CEAS-based measurements of dissolved non-CO₂ greenhouse gasses has already been demonstrated for methane in the Baltic Sea (Gülzow et al., 2011, 2013). Nevertheless, autonomously monitoring of N₂O on VOS lines requires a clean and maintained seawater supply, the oversight of analytical and emergency systems, and rapid instrument turnaround and cleaning during port calls. Although this is logistically challenging, particularly in remote ocean regions, the increased spatio-temporal coverage offered by measurement of near-surface N₂O on VOS routes should be encouraged as a component of N2O-ON.

Other Sampling Platforms

To date, there are no autonomous underwater sensors available for long-term in-situ N₂O monitoring in either the open or coastal ocean. Addressing this gap will require small, robust (resistant to high-pressure, hydrogen sulfide and biofouling) rapid response sensors with low power requirements for long-term deployment. Once developed, these sensors have the capability to decipher oceanic N₂O distributions with unprecedented spatio-temporal resolution. Potential sensor platforms include Bio-Argo floats¹⁰, gliders, coastal/deep sea moorings and mooring arrays, cabled observatories, drifting buoys and lander systems. We advocate a strong focus on the future development of such sensors and their subsequent integration into N2O-ON.

Data Management

MEMENTO

MEMENTO (The MarinE MethanE and NiTrous Oxide database¹¹), launched in 2009 (Bange et al., 2009), archives quality-controlled N₂O data from the open and coastal oceans (including estuaries, fjords etc.) (Kock and Bange, 2015). MEMENTO also publishes N₂O data sets, making them publicly and freely available. Regular updates include new datasets, additional meta-information, and the implementation of improved data quality control. As MEMENTO expands, it will adopt best practices for quality control according to the recommendations resulting from inter-comparison exercises (Wilson et al., 2018) and in accordance to existing databases such as the Surface Ocean CO₂ Atlas (SOCAT¹²) and the Global Ocean Data Analysis Project for Carbon (GLODAP¹³). N2O-ON and MEMENTO are clearly complementary and the routine archiving of quality-controlled data in MEMENTO is an intrinsic requirement of N2O-ON.

Ancillary Data

To evaluate the N₂O data derived from N2O-ON, additional standard hydrographic data (i.e., water temperature, salinity, depth) are important. In addition, chemical (i.e., dissolved O₂ and nutrient concentrations, and pH) and meteorological (i.e., air temperature, pressure, wind speed) data should ideally be collected. Most, if not all, of these variables are measured on a routine basis during research cruises, at some time-series stations and on some repeat hydrographic lines (see section “Observation Platforms”). VOS lines could be equipped with continuously operating systems such as the FerryBox¹⁴. N2O-ON will formally identify a suite of mandatory ancillary measurements and recommend appropriate measurement and/or sample collection alongside N₂O where possible.

Baseline Measurements

Resource constraints (both financial and personnel) preclude the extensive monitoring of N₂O concentrations across the entire global ocean. For this reason, a primary goal of N2O-ON is to develop a highly strategic sampling approach. In Figure 1, N₂O seasonal distributions derived from MEMENTO clearly show severe under-sampling of many ocean regions during various seasons, and it is precisely these regions that should be the target of near-term sampling efforts within N2O-ON. The following regions were specifically identified:

- the North Atlantic during December – February,

- the South Atlantic Ocean during March – August,

- the North Pacific Ocean during September – February,

- the South Pacific Ocean during all seasons,

- the North and South Indian Ocean during all seasons,

- the Southern and Arctic Oceans during all seasons, and

- selected marginal seas and major estuaries.

FIGURE 1

Figure 1. Maps of the distribution of N₂O measurements in (A) December, January, February; (B) March, April, May; (C) June, July, August and (D) September, October, November. Red lines indicate continuous surface measurements. Blue dots indicate locations of N₂O depth profiles. Data are from MEMENTO as of October 04, 2018 (https://memento.geomar.de/de).

N2O-ON will coordinate N₂O baseline measurements on VOS, establishing these along major international shipping routes crossing the gyres of the major basins of the Atlantic, Pacific and Indian Oceans (Figure 2). We propose the establishment of repeat hydrographic lines using research vessels and/or VOS lines to measure N₂O in the surface waters and water column of the Eastern Boundary Upwelling Systems (EBUS) and the Arabian Sea. This could exploit VOS lines transiting international shipping routes along the west coasts of North and South America, northwest and southwest Africa, and in the Arabian Sea. Moreover, routine N₂O measurements should be incorporated into the FRAM Ocean Observing System (Soltwedel et al., 2013) in order to close some of the large data gaps in the Arctic Ocean (Figure 2).

FIGURE 2

Figure 2. N₂O baseline measurements proposed for N2O-ON. Blue lines/red points indicate currently active repeated oceanographic sections/fixed time-series stations (see text). Dashed green lines indicate prospective VOS lines to be equipped with systems for continuous measurements. Green shaded areas mark key regions for the establishment of new time-series stations.

Incorporating N₂O into the suite of measurements of some established repeat hydrographic sections, such as GO-SHIP¹⁵ or GEOTRACES¹⁶, could provide a basin-scale approach to resolve N₂O variability in the ocean interior, thus forming an important N2O-ON collaborative activity. N2O-ON will also encourage the regular monitoring of N₂O in shelf areas and estuaries, which are prone to changes in redox-sensitive biogeochemistry due to enhanced anthropogenic and climatic impacts. Such activities would ideally be managed by local oceanographic institutes and/or relevant universities.

Established N₂O time-series (see Section “VOS Lines”) at fixed station sites need to be continued. N2O-ON will identify additional sites to be established in the EBUS off Oregon/California, Peru, Mauritania and Namibia, in the northeast Indian Ocean (Bay of Bengal), and at some strategic coastal and enclosed basin sites to form a comprehensive and coordinated network. In addition to being important N₂O sources to the atmosphere, these regions benefit from proximity to the necessary infrastructure provided by local/regional oceanographic institutes.

Summary and Outline of N₂O-ON

Surface N₂O concentration data can now be obtained with unprecedented precision. The inherent error in the CEAS technique is small relative to error in associated measurements (e.g., temperature correction to the seawater supply, non-steady state in the equilibration chamber, etc.). Even so, a harmonized data set requires a mechanism for inter-calibration, mutual agreement on metadata information and standard post-processing operations, as has been established for the global ocean surface CO₂ network SOCAT (Pfeil et al., 2013). Enhancing the accuracy and consistency of discrete dissolved N₂O concentration measurements requires the availability of liquid standards derived from strict preparation protocols, for example by the equilibration of seawater with air at known temperatures and salinity (Capelle et al., 2015; Wilson et al., 2018), or through the distribution of certified reference materials covering the range of concentrations expected in the oceanic environment (Wilson et al., 2018). The availability of a suitable reference material has been crucial in quantifying the oceanic carbon system (Dickson et al., 2007) with the required precision and accuracy to detect and evaluate long-term trends [e.g., (Müller J. D. et al., 2016)].

To improve and harmonize N₂O measurements in a changing ocean, we suggest establishing a Global N₂O Ocean Observation Network (N2O-ON) as outlined in Figure 3. In addition to exploiting existing oceanographic infrastructure (research vessels, VOS/repeat hydrographic lines etc.), we propose to establish central calibration facilities (CCF) in selected laboratories around the world to secure the comparability of N₂O measurements, and provide data sets with maximum accuracy. The CCF will: (1) enable the precise calibration of N₂O gas standards; (2) produce certified seawater reference material; (3) provide and maintain standard operating procedures for both surface and water column measurements; and (4) supply sampling bottles and equipment for research campaigns. Moreover, the N2O-ON calibration facilities will conduct regular internal comparison exercises to ensure long-term and high-level calibration performance. MEMENTO will archive all N₂O data and make them publicly available following stringent quality checks. MEMENTO will also publish the N₂O data sets with digital object identifiers (doi’s) to ensure appropriate referencing and tracking. Final N2O-ON data products, such as global N₂O concentration maps, emissions, budgets and trends, will be used in modeling studies for projections of future trends in oceanic N₂O emissions and advising policy makers and global climate assessments (Ciais et al., 2013). We advocate the establishment of regular workshops and courses to support all of these activities and to train the additional next generation of researchers who will be required to help realize the goals of N2O-ON.

FIGURE 3

Figure 3. Proposed scheme for the Global N₂O Ocean Observation Network. MEMENTO stands for “MarinE MethanE and NiTrous Oxide” database: https://memento.geomar.de/de.

Data Availability

No datasets were generated or analyzed for this study.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Funding

We thank SCOR for its generous support of WG 143 activities which were funded, in part, by a grant from the United States National Science Foundation (Grant OCE- 1840868) to SCOR. We also thank the EU FP7 project InGOS (Grant Agreement # 284274) for the support of the inter-laboratory comparison. LF was supported by the grants FONDAP 1511009 and FONDECYT N°1161138. MdlP received financial support from the INICIO project (CTM2015-74510-JIN). JK acknowledges support from grant NE/K002473/1 awarded by the United Kingdom Natural Environment Research Council (NERC). MEMENTO is currently supported by the Kiel Data Management Team at GEOMAR and the BONUS INTEGRAL Project which receives funding from BONUS (Art 185), funded jointly by the EU, the German Federal Ministry of Education and Research, the Swedish Research Council Formas, the Academy of Finland, the Polish National Centre for Research and Development, and the Estonian Research Council.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This article arose from meetings made possible by the SCOR WG 143 “Dissolved N₂O and CH₄ measurements: working toward a global network of ocean time series measurements of N₂O and CH₄.” We also thank Carolin Löscher for her comments on an early version of the manuscript and two reviewers for their helpful comments.

Footnotes

References

Anderson, B., Bartlett, K., Frolking, S., Hayhoe, K., Jenkins, J., and Salas, W. (2010). Methane and Nitrous Oxide Emissions from Natural Sources. Washington, DC: United States Environmental Protection Agency.

Google Scholar

Arévalo-Martínez, D. L., Beyer, M., Krumbholz, M., Piller, I., Kock, A., Steinhoff, T., et al. (2013). A new method for continuous measurements of oceanic and atmospheric N₂O, CO and CO₂: performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR). Ocean Sci. 9, 1071–1087. doi: 10.5194/os-9-1071-2013