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Das Kapitel geht der entscheidenden Rolle von Referenzmaterialien für Meerwasser (RM) bei der Überwachung des Karbonatsystems nach, das für das Verständnis der Ozeanversauerung und ihrer Auswirkungen auf marine Ökosysteme von entscheidender Bedeutung ist. Er beleuchtet den historischen Kontext und aktuelle Methoden zur Messung wichtiger Variablen wie des gelösten anorganischen Kohlenstoffs (DIC), der Gesamtalkalinität (TA), des pH-Wertes und des CO2-Partialdrucks (pCO2) und betont die Bedeutung genauer und präziser Messungen. Der Text diskutiert die Herausforderungen, vor denen das gegenwärtige Produktionssystem steht, insbesondere die Abhängigkeit von einem einzigen Labor, und die Störungen, die durch globale Ereignisse wie die COVID-19-Pandemie verursacht werden. Es schlägt ein neues Modell für die Produktion und Zertifizierung von Meerwasser-RMs vor, das mehrere regionale Zentren und Partnerschaften mit nationalen Metrologie-Instituten (NMI) umfasst, um globale Zugänglichkeit, Widerstandsfähigkeit und metrologische Rückverfolgbarkeit sicherzustellen. Das Kapitel untersucht auch die Vorteile und Herausforderungen dieses neuen Modells, einschließlich der Notwendigkeit eines kontinuierlichen Zugangs zu sauberem Meerwasser, angemessener Einrichtungen und spezieller Ausrüstung. Er schließt mit der Betonung der Vorteile eines global verteilten Produktionssystems, das die Qualität und Verfügbarkeit von Meerwasserressourcen verbessern, globale Umweltüberwachungsbemühungen unterstützen und zur Erreichung nachhaltiger Entwicklungsziele beitragen würde.
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Abstract
Ocean inorganic carbon research is crucial to quantify the global ocean uptake of atmospheric carbon dioxide (CO2), understand its spatiotemporal variability and mechanisms that control this process, and monitor ocean acidification. This requires high-quality measurements of the seawater carbonate system that rely on the availability of reference materials (RMs). The COVID-19 pandemic highlighted the fragility of the production system of the seawater RMs for the carbonate system, currently depending on one single laboratory. With that in mind, a new model for seawater RMs for the carbonate system, centered on regional hubs, is being developed to create a more resilient system. Challenges associated with establishing new production centers, such as funding their startup and ongoing costs, ensuring the quality and stability of the materials, and staff training are discussed. Opportunities to minimize the cost of these RMs and to supply certified or indicative values for currently uncertified carbonate system variables are explored. Additionally, a vision to integrate the new model into the global metrology landscape whereby the materials are comparable and metrologically traceable to the International System of Units is highlighted. As more laboratories are seeking to undertake seawater carbonate system measurements, access to these RMs is ever more critical.
13.1 Introduction on Seawater Inorganic Carbon Research and Reference Materials (RMs) for the Seawater Carbonate System
The global ocean has absorbed around 20% to 30% of atmospheric carbon dioxide (CO2) emissions accumulated since the industrial revolution and serves as an overall sink for excess anthropogenic atmospheric CO2 (Gruber et al., 2019; Friedlingstein et al., 2022). The increased reservoir of oceanic CO2 modifies ocean chemistry leading to changes collectively known as ocean acidification that are mainly traced by decreasing seawater pH and saturation states of biologically essential minerals such as the calcium carbonate polymorphs, calcite, and aragonite (Riebesell et al., 2000; Kroeker et al., 2013; Pörtner et al., 2014; Mostofa et al., 2016; Doney et al., 2020).
The increase in the oceanic CO2 reservoir and its consequences (i.e., ocean acidification) are monitored by measuring four key variables of the seawater carbonate system: total dissolved inorganic carbon (DIC, the sum of the concentrations of all inorganic carbon species), total alkalinity (TA, a measure of the buffering capacity of seawater), pH (pH = -log10a[H+]), and partial pressure of CO2 (pCO2, a measure of dissolved CO2 in seawater) (Fig. 13.1). Measuring any two of these variables, along with auxiliary determinations of salinity, temperature, pressure, and dissolved inorganic nutrient concentrations, allows for the complete characterization of the seawater carbonate system, which includes the amount of carbonate and bicarbonate ions, and calcium carbonate saturation states. Measurements of DIC and TA are routinely performed by coulometric titration and potentiometric titration, respectively (Dickson et al., 2007), and are well-established within the monitoring efforts, with accuracies of around 2 µmol kg−1. Discrete samples of seawater pH are typically measured with spectrophotometric pH indicator dyes calibrated over the range of salinities and temperatures expected in the open ocean, with pH accuracies of 0.01 (Clayton & Byrne, 1993). Spectroscopic methods (e.g., cavity ring-down spectroscopy or infrared spectroscopy) or gas chromatography are used to quantify pCO2 with accuracies of around 2 µatm for surface waters (Wanninkhof & Thoning, 1993; Neill et al., 1997).
Fig. 13.1
Representative profiles of the seawater carbonate system variables in different ocean basins. pH is presented on the total hydrogen ion scale at in situ conditions of temperature and pressure (pHT (P & T in situ)). The fugacity of CO2 (fCO2) is the partial pressure of CO2 corrected for the non-ideality of gases. Data from GLODAPv2.2022 (Lauvset et al., 2022): North Pacific Ocean (N. Pacific, navy right-pointing triangles) profile from station 324 of cruise 49NZ20071008 (expocode; expocode refers to the Expedition Code of the cruise and is a series of numbers and letters guaranteed to be unique and constructed by combining the country code and platform code from the ICES (International Council for the Exploration of the Sea) library (https://vocab.ices.dk/) with the date of departure in the format YYYYMMDD), South Pacific Ocean (S. Pacific, dark green circles) profile from station 86 of cruise 096U20160426, North Atlantic Ocean (N. Atlantic, teal squares) profile from station 83 of cruise 06MT20010507, South Atlantic Ocean (S. Atlantic, cyan upward-pointing triangles) profile from station 66 of cruise 33RO20110926, Indian Ocean (Indian, yellow six-pointed stars) profile from station 503 of cruise 49NZ20031209, Arctic Ocean (Arctic, pink diamonds) profile from station 76 of cruise 06AQ20150817, Southern Ocean (Southern, purple downward-pointing triangles) profile from station 24 of cruise 06AQ20141202, and Mediterranean Sea (Mediterranean, rosy five-pointed stars) profile from station 324 of cruise 06MT20110405
Efforts to monitor changes in the oceanic inventory of CO2 began in the 1970s using large-scale repeat hydrography cruises starting with the GEOSECS (Geochemical Ocean Sections Study) program (Takahashi et al., 1982), which laid the groundwork for later programs such as WOCE (World Ocean Circulation Experiment) and JGOFS (Joint Global Ocean Flux Study) in the late 1980s and 1990s, respectively. Nowadays, GO-SHIP (Global Ocean Ship-based Hydrographic Investigations Program) maintains the global survey of select hydrographic sections with modern standard practices and methods (Sloyan et al., 2019). On the other hand, in recognition that an understanding of the variability over a range of time scales from seasonal to interannual was required to better understand the connection between climate and ocean biogeochemistry, sustained ocean time series were promoted around the world in the late 1980s and early 1990s as part of the international WOCE/JGOFS effort where designated stations such as the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time series (HOT) were sampled monthly (Sabine et al., 2010). Currently, seawater carbonate chemistry monitoring focuses on increasing the spatiotemporal coverage of observations to also improve our understanding of short-scale variability. For this, autonomous systems have been developed to routinely measure seawater carbonate system variables (Bushinsky et al., 2019). Global ocean monitoring has expanded through the development of regional programs and the founding of the Global Ocean Acidification Observing Network (GOA-ON) (Newton et al., 2015). Ultimately, measurements of the seawater carbonate system variables are populated into quality-controlled data products, such as SOCAT (Surface Ocean CO2 Atlas) (Bakker et al., 2016) and GLODAP (Global Ocean Data Analysis Project) (Olsen et al., 2020), that inform bodies such as the Intergovernmental Panel on Climate Change (IPCC), an international cooperative established by the World Meteorological Organization (WMO), and the United Nations Environment Program (UNEP), which helps to shape global environmental policies.
Thanks to the collection of systematic measurements of the seawater carbonate system, it has been observed that, with an increased reservoir of oceanic CO2, the pH of the surface ocean is decreasing between 0.0013 and 0.0026 per year depending on the study area and the frequency of the observations (Bates et al., 2014). While these massive efforts have greatly expanded our understanding of the seawater carbonate system and have initiated ocean acidification as one of the global indicators of ocean health (World Meteorological Organization, 2021), the oceanic reservoir of CO2 still represents one of the largest uncertainties in the global carbon budgets (Friedlingstein et al., 2022). For this reason, accurate and precise seawater carbonate system measurements are essential to monitoring the ongoing progression of climate change and assessing the health of marine ecosystems.
Despite numerous technological advances over the last several decades, ship-based hydrography remains the only method for obtaining high-quality measurements over the full water column, especially for the deep ocean below 2 km (52% of global ocean volume). The precision and accuracy of ship-based measurements of the seawater carbonate system variables have improved by an order of magnitude since the 1990s (García-Ibáñez et al., 2022; Lauvset et al., 2022) due to the creation and adoption of standard operating procedures (SOPs) for measurements of DIC, TA, pH, and pCO2 (Dickson et al., 2007), and the availability of reference materials (RMs) for TA and DIC (Dickson, 2010). Over these past three decades, a single laboratory managed by Prof. Andrew Dickson at Scripps Institution of Oceanography (SIO) in the United States has provided over 150,000 bottles of well-characterized seawater RMs for TA and DIC to the international oceanographic community. These seawater RMs are produced from modified local seawater (Pacific Ocean) with lower than average salinity and characterized for TA and DIC (Dickson, 2010), with values for salinity and dissolved inorganic nutrients. The demand for the seawater carbonate system RMs grew to a point where by 2015 production capacity exceeded 10,000 bottles annually, with approximately 70% of the RMs being shipped outside the U.S. (A. Dickson, pers. comm).
The COVID-19 pandemic highlighted the fragility of the current production and distribution system of the seawater RMs for the carbonate system (Catherman, 2021). The dramatic reduction in the supply of RMs during the pandemic forced many groups to produce in-house or working RMs to overcome the shortage of RMs. This is a challenging task since there are no standard protocols for their production, thus resulting in non-uniform production and compliance measures and uncertainty. In Europe, ICOS (Integrated Carbon Observation System) performed initial attempts for regional distribution of in-house or working RMs within the ICOS network. The ICOS-lead production showed that it is possible to produce stable in-house or working RMs. However, it still needs work to assign values to these working RMs and not compromise the quality of the seawater carbonate system measurements. Most, if not all, of these working RMs are supplied with reported repeatability of measurement values for their only uncertainty source.
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With the expected increase in the use of RMs for the seawater carbonate system by the current users (Acquafredda et al., 2022) and the additional needs coming from the increased focus on ocean and coastal observing and the introduction of mitigation measures to curb ocean acidification, the demand is there for seawater RMs for the carbonate system. Additionally, the increasing need to serve national targets and actions in the global assessment (e.g., Sustainable Development Goals and national action plans to limit carbon and greenhouse gas emissions within the Paris Agreement) requires trustworthy data collected using global Best Practices that incorporate RMs in the workflow. Therefore, a robust and sustained production and a global supply of seawater RMs for the carbonate system are crucial. In this chapter, we propose a model of production and certification of those seawater RMs based on discussions among the seawater carbonate system research community.
13.2 Community Needs
The term RM is used to broadly categorize materials that are both sufficiently homogeneous and stable with reference to specified property values. The current seawater RMs for the carbonate system are produced at Prof. Andrew Dickson’s laboratory at SIO in the U.S. using Pacific seawater, with an average salinity of 33.449 ± 0.135, average DIC of 2038 µmol kg−1 ± 39 µmol kg−1, and average TA of 2226 µmol kg−1 ± 27 µmol kg−1 (based on properties from batch #119 onwards, which ensures the same seawater source and calibration methodologies (OCADS website 2023), with ± indicating one standard deviation; Fig. 13.2). However, those seawater RMs are used to ensure measurement accuracy on seawater samples not only from the Pacific Ocean but from the global ocean (Acquafredda et al., 2022), which presents a wider range of salinity, DIC, and TA values (Fig. 13.2). In some instances, for example, in the high-salinity and high-TA Mediterranean waters, seawater carbonate system properties vary from the RM by more than 370 µmol kg−1 for TA and 280 µmol kg−1 for DIC (Fig. 13.2), quantities 100 times larger than the uncertainty of those measurements. The mismatch of properties of the seawater RMs for the carbonate system and the seawater samples analyzed can pose some issues, especially for laboratories that use the seawater RMs to calibrate their equipment due to the lack of other calibration techniques (see Sect. 13.4). Therefore, having multiple RM compositions will increase the measurement accuracy of those groups by providing more than one point to confirm their calibrations.
Fig. 13.2
Average seawater properties and standard deviations (STD) of different ocean basins and the seawater reference materials (RMs; properties from batch #119 onwards (OCADS website, 2023)). Oceanographic data from GLODAPv2.2022 (Lauvset et al., 2022), with the following sample distribution: North Pacific Ocean < 500 m water depth: 65,940 samples, > 500 m water depth: 74,381 samples; South Pacific Ocean < 500 m water depth: 28,860 samples, > 500 m water depth: 40,369 samples; North Atlantic Ocean < 500 m water depth: 44,941 samples, > 500 m water depth: 58,629 samples; South Atlantic Ocean < 500 m water depth: 17,593 samples, > 500 m water depth: 28,843 samples; Indian Ocean < 500 m water depth: 22,781 samples, > 500 m water depth: 32,541 samples; Arctic Ocean < 500 m water depth: 39,477 samples, > 500 m water depth: 17,776 samples; Southern Ocean < 500 m water depth: 21,388 samples, > 500 m 23,127 samples; Mediterranean Sea < 500 m water depth: 617 samples, > 500 m water depth: 528 samples. The Arctic and Southern Oceans are defined as North of 60 ºN and South of 60 ºS, respectively
In 2021, the current uses of RMs by the seawater carbonate system research community were assessed through surveys issued by the Interagency Working Group on Ocean Acidification (IWGOA), a U.S. federal subcommittee under the Office of Science and Technology Policy (Acquafredda et al., 2022), and the GOA-ON Mediterranean Hub, a network that connects Mediterranean scientists who are working and are interested in ocean acidification in the Mediterranean Sea (Hassoun et al., 2022). The results from both surveys emphasized the need for better and sustained access to the widely used seawater RMs for the carbonate system with values for DIC and TA, with the majority of research groups surveyed expecting to increase their consumption of these RMs over time (Acquafredda et al., 2022). Furthermore, higher demand is expected from the increased focus on ocean and coastal observing (Dobson et al., 2022) and the introduction of marine carbon dioxide removal (mCDR) applications (National Academies of Sciences, Engineering, & Medicine, 2022).
Community surveys (Acquafredda et al., 2022; Hassoun et al., 2022) also highlight the demand for seawater RMs for pH through the high number of research groups using the current seawater RMs to check the accuracy of their pH measurements, even though those RMs only include values for TA and DIC. Some laboratories routinely calculate RM pH from DIC and TA and use these values to quality control their discrete pH measurements. Despite the high number of seawater pH measurements, currently, there is no single recommended measurement procedure, nor is there an internationally accepted RM for seawater pH measurement that enables different laboratories to reliably achieve comparable measurements. This can jeopardize the detection of the long-term anthropogenically-driven changes in ocean pH over multi-decadal timescales. This is especially important as pH was designated within the United Nations Sustainable Development Goals (SDGs) Target 14.3 to “Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels” as a climate indicator 14.3.1 (United Nations, 2015).
To increase the number of laboratories performing seawater carbonate system measurements with a known uncertainty, not only will the community need increased production of existing seawater RMs, but to satisfy the goals of the SDGs, developing countries need access to more affordable RMs. This objective can be achieved in multiple ways, for example, by producing RMs with a higher uncertainty, lowering the costs associated with the production of RMs, or subsidizing a portion of the RM costs. Additionally, there is a general need for SOPs on the production and value assignment of in-house or working RMs that are often used to check equipment drift, thus reducing the number of RMs used. SOPs for in-house or working RMs would ensure higher measurement quality from the global community. Prof. Andrew Dickson and the EuroGO-SHIP infrastructure are starting to draft those SOPs, which will likely include a protocol for their preparation and guidance on the determination of their long-term stability, value assignment, and uncertainty quantification (A. Dickson and T. Steinhoff, pers. comm.).
13.3 Integrating Seawater RM Production and Certification into the Global Metrology System
The improved quality of seawater carbonate system measurements is directly attributed to the availability of carefully characterized seawater batches of RMs using measurement capabilities grounded in metrological principles to minimize uncertainties and guarantee consistent traceability between batches. Recent discussions, such as the organization of the BIPM-WMO Metrology for Climate Action workshop (October 2022; Metrology for Climate Action, 2022), have highlighted the need to merge expertise from the oceanographic monitoring community and the global metrological community to ensure that these critical measurements are adequately optimized to quantify changes in the environment, are reproducible over long timescales, and are ideally metrologically traceable to the International System of Units (SI). One of the goals moving forward is to connect existing RM programs, such as the seawater RM carbonate system program, to the global metrology system.
The International Bureau of Weights and Measures (BIPM: abbreviated from the French: Bureau International des Poids et Mesures) is a global body established by the Metre Convention, an international treaty signed in 1875 by 17 nations, which established the basis for an internationally agreed-upon system of measurements (Fig. 13.3). As of June 2024, the BIPM consists of 64 Member States and 36 Associate States and Economies (see Fig. 13.4, where dark-shaded countries are BIPM Member States and medium-shaded countries are BIPM Associate States and Economies). Within this structure, the International Committee for Weights and Measures (CIPM: abbreviated from the French: Comité International des Poids et Mesures), managed by 18 representatives from Member States, promotes global uniformity of units of measure by supervising the research of the BIPM. This metrological research is actualized through 10 Consultative Committees (CCs). Most relevant to the seawater carbonate system community is the work conducted by the CC for Amount of Substance: Metrology in Chemistry and Biology (CCQM), which houses 12 working groups consisting of representatives from National Metrology Institutes (NMIs) specialized in various areas of metrology, such as the Electrochemical Analysis Working Group (EAWG), the Gas Analysis Working Group (GAWG), the Inorganic Analysis Working Group (IAWG), the Organic Analysis Working Group (OAWG), or the Isotope Ratio Working Group (IRWG).
Fig. 13.3
Organization of the CGPM (General Conference on Weights and Measures; abbreviated CGPM from the French: Conférence Générale des Poids et Mesures), CIPM, and BIPM
The world’s regional metrology organizations (RMOs) as of June 2024 where dark-shaded countries represent BIPM Member States, medium-shaded countries represent BIPM Associate States and Economies, and light-shaded countries represent non-BIPM members participating in the RMO
CCQM working groups convene to discuss measurement challenges, assess measurement capabilities, and prove their performance on high-quality certification methods through their participation in interlaboratory comparisons called key comparisons (KCs). Ideally, every five years, each working group cycles through KCs specifically designed to assess Calibration and Measurement Capability (CMC) claims for a given analyte in a specified matrix within a designated range of amount content. The KCs are designed to reflect typical RMs issued by the participating NMIs. Obtaining CMCs for a specified measurand is subject to an international peer-assessed approval process, which requires the review of the institute’s quality management system and usually requires participation in KCs. In the design of KCs, metrologists recognize that demonstration of capabilities to support CMC claims can be performed independent of certification method or technique. In some comparisons, for example, within the EAWG, analysts test their expertise in measuring the activity of hydrogen ion in buffers using techniques ranging from primary pH measurements (Harned cells) to secondary differential cells and glass electrodes (Spitzer et al., 2013). From a metrological viewpoint, while the true value of a material can never be known, the use of high-quality measurements whereby all sources of uncertainty have been accounted for will likely minimize errors between the measurement result and the true value. The KCs are critical for maintaining quality in RM production as they shed light on potential biases and uncertainties in the measurement process.
Six Regional Metrological Organizations (RMOs) have been established (Fig. 13.4) to implement regional strategies in addition to serving the BIPM by organizing supplemental comparisons, monitoring quality management systems, and providing guidance to the BIPM CCs for KCs. These RMOs consist of BIPM Member States and Associate States and Economies, other countries and stakeholders within the region, and entities including NMIs beyond the region who hold invested economic interests in helping to strengthen measurement capabilities for the RMO. For example, within AFRIMETS, its “Ordinary” voting members consist of three groups: BIPM Member States (South Africa, Kenya, Egypt, and Morocco), BIPM Associate States and Economies (Tanzania, Mauritius, Namibia, Botswana, Ethiopia, and Ghana), and legal metrology institutes (LMIs) not associated with BIPM. AFRIMETS Associate Members (nonvoting) include NMIs such as Physikalisch-Technische Bundesanstalt (PTB, Germany) and LMIs outside of Africa, which may aid in strengthening the regional measurement capabilities and/or provide traceability to their national standards. The structure enables the RMOs to organize their comparisons and, at times, provide guidance to help associate members improve their capabilities such that they qualify for full BIPM membership. The organization and structure of the global metrology system are established to ensure confidence that the standards needed to drive the global economy have the highest quality and lowest uncertainties.
One example of regional coordination is through the European Association of National Metrology Institutes (EURAMET), a collective of representatives from 38 states as of June 2024. In 2014, EURAMET established the European Metrology Network (EMN) for Climate and Ocean Observation to bring together expertise from 44 NMIs and Designated Institutes (DIs) along with academic groups and businesses to collaborate on a set of projects to improve, for example, the quality of Essential Climate Variables by increasing linkages of the variables to SI units and improving the accuracy of measurements (Woolliams et al., 2019). The group to-date has been successful in outlining the key stakeholders in the areas of terrestrial, atmospheric, and oceanographic monitoring and has worked with these stakeholders to understand the critical need to provide certified RMs (CRMs) and RMs to support the monitoring of Essential Ocean Variables (EOVs). In addition to the supply of RMs, this group is promoting the idea that laboratories providing data to national and global repositories should seek accreditation for their measurement capabilities in line with the guidance outlined in ISO/IEC 17025 (International Organization for Standardization and International Electrotechnical Commission, 2017) or ISO 17034 (International Organization for Standardization, 2016). This network is a leading example of the integration of metrological perspectives with global environmental monitoring.
Within the BIPM, CCQM has a range of activities closely related to the development of seawater standards and, particularly, for the determination of both TA and DIC. The determination of TA requires the accurate standardization of a hydrochloric acid titrant in a background of sodium chloride. The CCQM EAWG organized a KC on the standardization of hydrochloric acid titrant (CCQM-K73), with the last reported comparison conducted in 2020 (Pratt et al., 2013; Bastkowski et al., 2020). For DIC measurements, a seawater sample of known mass is acidified to convert all the inorganic carbon species (H2CO3, HCO3−, and CO32−) to CO2 gas. The gas is extracted from seawater using a vacuum line, which isolates and purifies CO2 through a series of sublimation steps. The quantity of CO2 evolved from the original seawater sample is determined using digital manometry, which relies on accurate volume, pressure, and temperature determinations. The final steps in this measurement process to quantify CO2 are quite similar to the determinations of CO2 in air or gas mixtures performed by members of the CCQM GAWG. In 2019, the GAWG conducted a Pilot Study CCQM-P188 in parallel with KC CCQM-K120 on the determination of CO2 in air. This study provided comparisons of CO2 values obtained by FTIR (Fourier-transform infrared spectroscopy) and GC-FID (gas chromatography with flame ionization detection) (Flores et al., 2019). In both instances, the KCs give insight into uncertainties associated with these measurements and help assess the participants’ abilities to provide certified values on similar materials. We expect that to envelop ocean carbon RMs in the global metrology system, studies such as these will be evermore important in solidifying the ability of NMIs or other laboratories to certify seawater carbonate system RMs. Ideally, future KCs directly examining CO2 in seawater should be conducted; however, the challenge is gaining support within the NMIs to prioritize these measurement capabilities.
13.4 Classification of RMs and Their Uses
While the linkages between the agreed-upon SI units and national standards are established through the work of NMIs and DIs, the connection between the SI units and users can be understood through a hierarchy of RMs (Fig. 13.5). Metrological traceability provides connections between the SI units and materials routinely adopted by end-users (International Organization for Standardization, 2007). For the seawater carbonate system research community, this can be understood as a chain progressing from primary CRMs to RMs and then to Quality Control Materials (QCMs). National governments (NMIs) are confirmed as authorized bodies through their participation in the BIPM where their CMC claims are established; thus, they can ensure standardization to the top level of the traceability chain. Additionally, they routinely adhere to guidance outlined in ISO 17034 (International Organization for Standardization, 2016) and ISO/IEC 17025 (International Organization for Standardization & International Electrotechnical Commission, 2017) for RMs producers and testing and calibration laboratories, respectively.
Fig. 13.5
General traceability chain of the seawater carbonate system measurements illustrating the relationship between calibrator type, measurement procedures, and entity responsible for implementation where NMI is the national metrology laboratory and ARML is an accredited reference (calibration) laboratory. Solid lines represent known links in the traceability chain, while dotted lines represent contingent linkages and usage by the community
Within chemical metrology, at the highest level, direct linkages to the SI units are created using primary reference measurement procedures (RMPs) to value-assign primary CRMs (high purity materials). Here we use the term ‘primary CRM’ in the same sense in which the term ‘primary measurement standard’ has been defined within the VIM–International Vocabulary of Metrology–(BIPM et al., 2012). The criterion for producing primary CRMs is strict. Primary RMPs, used to value-assign primary CRMs, are performed without reference to a calibrator having assigned values of the same kind being measured. For example, to establish metrological traceability to the amount of substance, there are only a few measurement procedures, such as gravimetry and coulometry, and freezing point depression, which qualify as primary RMPs (Milton & Quinn, 2001). Secondary CRMs, which are often useful for customers who require a matrix-based material with a similar composition to their samples, are often calibrated against a primary calibrator using secondary RMPs. In cases where the primary RMPs are applicable, the certification of secondary CRMs can be performed solely with the high-caliber method. However, when the high-caliber method is not an option, secondary RMPs, which combine at least two different approaches, and which can include measurements performed by interlaboratory comparison of expert laboratories, can be used to establish certified values. The CRM label (or SRM for NIST and ERM for EURAMET) requires that the material is not only homogeneous and stable, but also that the true value lies within the stated uncertainty to a specified level of confidence as given in a certificate supplied to the user. At NIST (National Institute of Standards and Technology; U.S.), certified values have “the highest confidence in that all known or suspected sources of bias and imprecision have been considered and any contributions they may make to measurement uncertainty have been quantified and are expressed in the reported uncertainty.”
RMs lacking certified values consist of a broader category whose only specifications are that the material is sufficiently homogeneous and stable with reference to specified property values. The values for the materials are assigned using well-established methods, which have been calibrated sufficiently based on the user community’s fitness-for-purpose (Magnusson & Örnemark, 2014), and full uncertainty budgets are not required. As the traceability chain progresses from the SI units (or other agreed-upon standards) to RMs and QCMs, the uncertainty for each successive level gets incorporated into the overall uncertainty budget at the next level in the chain. RMs are often matrix-based and are lower in cost as compared to CRMs, while the CRM prices are higher in exchange for lower uncertainties and greater confidence in the reliability of the certified values.
The main purpose of using RMs is to obtain the correct analysis result. Valid analysis results can be obtained by using primary CRMs for calibration, method validation, and instrument qualification. Matrix-based CRMs and RMs are ideal for recovery studies. The lower costs of RMs and QCMs make them ideal for implementing quality control throughout the analysis process. Ideally, calibrations should be conducted using high-purity CRMs. In the absence of these materials, the void can be filled by calibrating with a matrix-based material. The accuracy and validity of the calibration should be checked with a material that is different from the material used for calibration. The CRM for quality control should preferably not be equal to one of the calibration points and should not be too close to the calibration upper-lower limit values.
The current seawater RMs for the carbonate system were designed to improve quality control and to provide harmonization of analysis across time and space. These materials allowed laboratories worldwide to determine the accuracy and validity of the analysis results as compared to the single RM producer and have enabled the community to assess change over time (where change is seen in the environments studied and even changes in laboratory personnel). The methods used by Prof. Andrew Dickson’s laboratory at SIO were developed to bottle and characterize the materials to ensure the stability and homogeneity of every batch produced. One reality is that even though the materials are often referred to by the community as “CRMs”, based on practices defined within the field of metrology and based on the ISO definitions, these materials do not fit the current criteria for CRM. They are indeed RMs proven to be stable and homogenous but with non-certified values. To qualify as CRMs, they would require a somewhat more rigorous value assignment process, including the development of uncertainty budgets that incorporate all significant components of uncertainty. The SIO RMs are currently reported with measurement repeatability as the only source of uncertainty. The traceability chain to the SI units may or may not be established. With the newly proposed model for RM production, we hope that in the future, these materials can fully qualify as secondary CRMs.
13.5 Re-Envisioning the Production and Certification Model of the Seawater RMs for the Carbonate System
The last few years have illustrated that significant geopolitical and global public health crises can rapidly interrupt or halt regular distribution networks and supply chains. With that in mind, a new production and certification model for seawater RMs for the carbonate system is being discussed, taking as one of the main focuses providing sufficient resilience in the system to minimize the impact of any restricted movement between nations and regions.
The proposed production and certification model for seawater RMs for the carbonate system with improved resilience would consist of at least three globally distributed production and certification centers (regional hubs; Fig. 13.6). For each regional hub, production would be separated from certification, with the former likely occurring in institutions close to the sea (ensuring continuous access to clean low-nutrient seawater) and the latter likely occurring at NMIs (ensuring compliance with metrological standards). Production centers would perform stability and homogeneity assessments of the RM batches and would send a selected number of bottles within each batch to the certification centers where the batches would be value-assigned for the seawater carbonate system variables. The certification for each batch would also include the uncertainty budget. Procedures should conform to guidelines outlined in ISO 17034 (International Organization for Standardization, 2016) and the relevant ISO guides and standards to which it refers. Metrological traceability should all be well-documented, and homogeneity and stability testing should follow rigorous experimental and statistical design principles.
Fig. 13.6
Proposed final and transitional new model for production and certification of seawater RMs for the carbonate system
A model for the proposed program could follow the established NIST Traceable Reference Material (NTRM) Program for Gas Standards (Dorko et al., 2015). The program was designed to help meet the air quality monitoring requirements set forth by the U.S. EPA (U.S. Environmental Protection Agency) with the Clean Air Act. The demand for gas standards, similar to the seawater RMs for the carbonate system, far exceeded the ability of NIST to supply. Specialty gas companies, who produce the bulk supply of gas standards, work directly with NIST to provide SI traceable values on those gas standards and ensure their quality. NIST provides guidance and oversight of the production of NTRM gas standards using protocols defined by NIST, which include producer batch preparation, producer stability and homogeneity analyses, producer data validation, NIST batch value assignment, and evaluation of uncertainty according to the Guide to the Expression of Uncertainty in Measurement (GUM) (Williams, 2016). NIST staff members periodically conduct quality assessments of producer facilities, and NIST serves as the assessment laboratory for the mandatory EPA Protocol Gas Verification Program (PGVP), or EPA blind audit as it is commonly called, which was put in place to assure the accuracy of EPA Protocol gas standards produced and disseminated to end users by specialty gas companies utilizing NTRM standard.
In the oceanographic community, we envision that a similar structure is needed to aid in the sustained production of high-quality seawater RMs for the carbonate system and, in doing so, seawater RM programs will become integrated into the existing metrological infrastructure. NMIs, in partnership with production centers, would ideally participate in relevant KCs to ensure comparability of measurement capabilities and mutual recognition of the certificates provided with CRMs. Quality management systems for all participating bodies will undergo regular audits. NMIs and production centers would work together to ensure the user community has access to the best guidance for producing in-house or working RMs. These efforts will not only support the existing oceanographic research community but will also enhance efforts to expand support for monitoring CO2 in coastal regions, which presents its challenges due to the proximity to terrestrial inputs.
In thinking of the new vision and knowing the challenges associated with setting up production centers, it is imperative to derive a transitional production model. Given the current impossibility of reproducing SIO’s laboratory elsewhere in the near future, the transitional production and certification model is envisioned where the SIO’s bottling facility is maintained to provide continuity in the seawater RM supply and will work in partnership with a metrology laboratory (most probably NIST in the U.S.) to begin certifying the RMs (Fig. 13.6). NIST is developing their measurement capabilities to certify seawater for TA and DIC and in doing this they will work closely with SIO to understand the measurement processes they have developed since the start of their RM program. Since the seawater carbonate system research community’s measurements have been traceable to the SIO’s laboratory for the past three decades, an important component of the transition will involve bilateral comparisons between SIO and NIST on every aspect of the measurement process (e.g., recovery of total CO2 from a known mass of seawater and determination of total CO2 from a purified CO2 gas sample). These comparisons will document the concordance of the measurement approaches taken by both laboratories and will ensure sustained quality and continuity in data acquired by the seawater carbonate system research community. For example, if the measurement approach of NIST yields values that are not statistically in agreement with SIO, further investigation into the quality of the new measurement process and the SIO approach will be required. It is also important to note that the traceability of each variable will be assessed given the resources at the NMI (or NIST in the transition phase). In fact, the traceability of values for each variable extends beyond the simplified chain represented in Fig. 13.5, which excludes specific SI units and reference measurement procedures. For example, TA traceability could be established through coulometric titration, which is traceable to the SI units for current, time, and mass using Faraday’s constant. Similar assessments will be conducted at the NMI to fully establish traceability of both TA and DIC measurements. With both parameters, the complete uncertainty budget will be derived.
During the implementation of the transitional model, regional hubs will concurrently be created in areas where there is access to seawater and a willingness to provide facilities and staff. These hubs will strengthen their capabilities to supply laboratories within their network with stable and homogeneous batches of seawater. With the help of either a centralized laboratory (such as NIST) or the implementation of an interlaboratory certification model, these hubs can supply batches of seawater with non-certified values for DIC and TA. In the absence of multiple qualified certification laboratories, a central NMI (most probably NIST in the U.S.) during this period would help to ensure consistency in the value assignment of these RMs on a global scale.
It is necessary to create best practices for RM production and certification for the regional hubs. Each producer must ensure that every batch is adequately tested for homogeneity and stability before certification. Once certification is performed, periodic stability assessments are needed to demonstrate the continued quality of the RMs based on the best assessment of seawater parameter stability. Additionally, guidance on the uncertainty quantification of each batch and, ideally, the pathway to SI traceability is critical to the ongoing success of these efforts. The certification testing process would ideally be refined through assessments involving the scientific community.
13.5.1 Advantages and Challenges of the New Model
The global distribution of production and certification for the seawater RMs for the carbonate system would result in a more resilient system with fewer bottlenecks and better statistics. Having multiple production and certification centers would improve international availability in addition to ensuring resilience. Besides, the new production scheme of seawater RMs for the carbonate system would ensure a wider composition of the RMs, with RMs produced with natural seawater from different ocean basins, therefore covering one of the identified community needs (see Sect. 13.2). Creating regional hubs would also help reduce the price of RMs by reducing logistics and distribution costs. Furthermore, the collaboration with NMIs would enable the use of the global metrological infrastructure in support of this new model of production and certification of seawater RMs for the carbonate system.
The partnership between oceanographic institutions, leading the production of RMs, and NMIs, leading the certification, would help to tie measurements into the global metrology system to ensure long-term traceability and extend the provision of RMs to other seawater carbonate system variables, such as pH and pCO2, which currently lack seawater RMs. This would particularly benefit developing countries because pH is one of the most widely measured variables of the seawater carbonate system in areas where other equipment is not readily available. On the other hand, additional nonroutine variables that are not part of the seawater carbonate system, such as the 13C/12C isotope ratio, could also be certified in the seawater RMs. Access to technology to define isotope ratios is increasing in some laboratories since these ratios are tracers of water masses and biological processes.
The new scheme of production and certification of seawater RMs for the carbonate system comes with its challenges. The first one is the establishment of production centers. Those centers would require continuous access to clean low-nutrient seawater, adequate space, and equipment. This is especially important given the expected future demand for seawater RMs for the carbonate system. The current producer produces between 7 and 9 batches per year requiring approximately 720 L per batch. If a continuous supply of seawater is not possible, an integrative effort with international cruise programs, such as GO-SHIP (Sloyan et al., 2019), could provide a relatively continuous supply of open ocean seawater for RM production. In addition to seawater access, new production centers will need dedicated space to prepare RMs and store them once they are produced. For example, the current producer has over 370 m2 of chemical laboratory space solely reserved for this purpose. However, an alternate model is one where the existing laboratory facilities are used. Additionally, specialized equipment is needed for batch sterilization and bottling (including the bottles) and performing acceptance testing to ensure stability and homogeneity assessments of the RMs. The new production centers would also need personnel exclusively dedicated to the production of the RMs. The current producer of RMs needs at least three full-time staff.
Beyond the establishment of regional production centers, another level of global organization will be needed to ensure the RMs produced by all centers are comparable. The robustness of the regional hubs will require periodic stability testing and a commitment or requirement for each production center to participate in interlaboratory exercises (proficiency testing). This quality assurance of the RMs produced by different hubs would reduce the user’s concern of decreased data quality derived from using RMs produced by different manufacturers. Additionally, if the certification and traceability are established using NMIs or designated institutes, it will be critical to evaluate their measurement capabilities within KCs organized by the BIPM. Currently, one NMI (NIST) is developing its measurement capabilities for both DIC and TA, but an ideal model will involve multiple NMIs establishing these measurement procedures. The proposed regional model (Fig. 13.6) considers the current CMC claims documented by the BIPM such that within each region, at least two NMIs have technical capabilities that could be applied to certify seawater for both TA and DIC.
Finally, the biggest challenge to developing the proposed new model of production and certification of seawater RMs for the carbonate system is the investment needed. The required investment likely will come from either a governmental source or a commercial entity, and perhaps a combination of both. The current seawater RMs for the carbonate systems are partially subsidized by the National Science Foundation (NSF in the U.S.) (Dickson, 2010), allowing an affordable cost per bottle of ca. 65 USD (excluding shipping and customs). A similar subsidizing scheme would need to be in place for the proposed future production and certification model, so users are not cost-prohibited from acquiring RMs. Partnerships with NMIs may help to defer some of the analytical costs as there are NMIs, such as NIST in the U.S., PTB in Germany, and LNE in France, currently researching measurement services related to seawater carbonate system measurements. In the future, these measurement services could be provided within the global distribution network on a fee-per-service basis. As such, it will still be necessary to have partners willing to provide investments to both establish and likely supplement these distribution centers. Additionally, a subsidizing scheme to reduce RM prices for developing countries would significantly improve the accessibility of seawater RMs for the carbonate system to a wider number of laboratories, ensuring the production of high-quality measurements of the variables of the seawater carbonate system worldwide. One of the ways RM supply could be subsidized is by reducing or eliminating importation taxes, which can represent a price increase of around 20% of the bottle’s original price. The Ocean Foundation (ToF; https://oceanfdn.org/) is interested in working on the elimination of duties of RM shipments through regional conventions (ToF pers. comm.). The reduced cost of seawater RMs for the carbonate system would improve our ability to meet the United Nations SDG Target 14.3 “Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels”, and would contribute to a better understanding of how the ocean regulates our climate. The funding for a more resilient production and certification model for seawater RMs is fundamental to achieving climate-quality observations that allow the detection of long-term trends associated with ocean acidification (Newton et al., 2015).
13.5.2 Remaining Challenges
There are several ongoing innovations that are needed to improve the quality of the existing seawater carbonate system RMs. For example, finding alternative packaging to the glass bottles, which are currently used for the TA and DIC RMs, will lower costs and improve production capabilities, which are currently limited by the glass bottle supply chain. SIO encourages customers to return the bottles to receive a discount on subsequent orders. The glass bottles are not always returned and thus represent an ongoing bottleneck in the production process. Aside from supply chain nuances, the glass bottle contributes 40% of the RM weight and results in a substantial expense in shipping costs. Alternative lightweight, flexible, and/or single-use packaging will considerably reduce the costs of the RMs.
The preservation method is another area of research to be explored. Currently, mercury (II) chloride (HgCl2) is used to effectively maintain the RM values for at least three years. However, this preservative is actively discouraged due to its toxicity (e.g., the Minimata Convention of Mercury), and it is a banned chemical that cannot be shipped into some countries. Therefore, finding alternative preservation for these RMs could increase their availability in nations that currently cannot receive such items. The potential preservative should be a biocide and not compromise the acid–base chemistry in seawater.
Alternatives to solve both packaging and preservative problems are being explored. For example, JAMSTEC (Japan Agency for Marine-Earth Science and Technology), in collaboration with KANSO TECHNOS CO., LTD, is developing non-toxic seawater RMs for TA and DIC using sterilized natural seawater by autoclaving and bottling in aluminum bottles similar to those used for sodas (Murata, 2010). Other laboratories have explored the possibility of pasteurization for seawater dissolved inorganic nutrient samples (Daniel et al., 2012); while another potential solution comes through the reformulation of the seawater RMs for the carbonate system to be in salt form, such as the method explored for dissolved inorganic nutrient standards (Pagliano et al., 2022).
Certifying values for pH and pCO2 in the available seawater RMs for the carbonate system represents another challenge. For pH, technical issues are particularly problematic in seawater. First, seawater has a high ionic strength, which causes problems when using conventional pH calibration standards as the activity of hydrogen ion changes with ionic strength and, therefore, salinity. For this reason, spectrophotometric indicator dyes have often been recommended for seawater pH measurements because they can be calibrated over a range of temperatures and salinities using synthetic seawater buffers. The indicator dyes present their own challenges due to the presence of impurities, which cause pH-dependent offsets between the actual and measured pH (e.g., Carter et al., 2013, 2018; Fong & Dickson, 2019; Álvarez et al., 2020; Takeshita et al., 2021). Additionally, a massive effort is required to “certify” a batch of purified indicator dye against a range of artificial seawater buffers prepared at multiple pH values and salinities. When potentiometric electrode measurements are the only option available, electrodes can be calibrated using these in-house artificial seawater buffers. The challenge is that the buffer preparation is somewhat complex requiring seven different components, some of which require assays prior to use. With pCO2, the biggest challenge is supplying users with a seawater matrix standard in a form easily integrated into existing measurement systems. As the efforts move forward such that these RMs are produced in multiple locations, we must also be aware that innovation is needed to improve the products supplied to the user community.
13.6 Conclusion
The seawater community will greatly benefit from the restructuring of the existing seawater RM model for the carbonate system. Over the past few years, it has become evident that the oceanographic community cannot depend on one single laboratory to provide global access to seawater RMs for the carbonate system. A new production and certification model, which involves the establishment of regional hubs in partnership with SIO and NMIs, is being discussed. Although this new model faces inherent challenges (e.g., finding locations close to seawater sources that have the facilities, personnel, and technical expertise to provide values on the materials), there is a great benefit to expanding the production to a global scale. Users will have access to multiple sources of seawater, which can provide matrix standards similar in composition to their areas of study. We also believe the new model may help to reduce the costs of production by eliminating some of the current shipping costs, which can represent a price increase of around 40% of the RM’s original price. Partnerships with NMIs will ensure these RMs are connected to the global metrology system, are metrologically traceable, and that batches produced across the globe are delivered at a consistent quality. Only time and effort will determine if this new model provides to this important community, whose data are intrinsically linked to our assessment of climate change, long-term sustained access to these critical RMs.
13.7 Competing Interests
The authors have no conflicts of interest to declare that are relevant to the content of this chapter.
Acknowledgements
This activity is supported by the International Ocean Carbon Coordination Project and the author team acknowledges funding to IOCCP provided by a grant to SCOR from the U.S. National Science Foundation (OCE-2140395).
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