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Global atmospheric greenhouse gas observations

The trace concentration of greenhouse gases in the atmosphere has been, and continues to be, a measurements and standards challenge when monitoring their behavior both globally and locally. Scientific interest, and concern for atmospheric GHG impact on Earth’s atmosphere, began in the early and mid-19th century. Initial measurements (Foote, 1856) of the heating of CO, by solar radiation were followed by robust, higher-accuracy measurements and investigations of atmospheric CO, by Tyndall (Jackson, 2018) in the mid-19th century. These results and experiments regarding the effect of solar radiation on the Earth led to quantitative investigations on solar heating of the atmosphere by CO, and water vapor, the most abundant greenhouse gases in the atmosphere (Arrhenius, 1896). Continuous measurements of atmospheric CO, mole fractions were initiated in 1957 by C.D. Keeling of Scripps Institution of Oceanography (SIO) (Scripps-CO,, 2019) at the Maima Loa, Hawaii observatory of the National Weather Sendee and subsequently the National Oceanic and Atmospheric Administration (NOAA). Figure 1 shows this high-accuracy, atmospheric CO, data record based on instrumental and flask sampling measurements. These results illustrate that stable, highly accurate measurement standards, initially developed by Keeling, are the basis for underpinning this and other long-term data records of atmospheric greenhouse gas behavior.

The widely recognized Keeling Curve serves as one of the most highly regarded assessment tools for long-term monitoring of atmospheric CO, mole fraction. It is a proxy for assessing the atmosphere’s warming capability globally. In addition to following the yearly increase in atmospheric CO, mole fraction, the sensitivity of the measurements, and the stability of the standards upon which these are based, also clearly show the annual variation (4 to 6 pmoTmol) in the northern hemisphere’s atmosphere. This 1% to 1.5% annual variation in the global atmospheric signal, as measured at Mauna Loa, is a demanding measurement supported by standard gas mixtures with mole fraction uncertainties of 0.1 to

0.2 pmol/mol. Early in his research efforts, Keeling recognized the need for stable and accurate standard gas mixtures to support the Mauna Loa CO, observations, and subsequently those made at several locations roughly along a longitude reaching from Pt. Barrow in Alaska to near the South Pole to monitor

Global greenhouse gas mole fraction measurement data

Figure 1. Global greenhouse gas mole fraction measurement data: Mauna Loa, Hawaii Observatory, https://www.esrl.noaa.


global atmospheric behaviors. This type of observing capability continues and has been expanded under the international efforts of the World Meteorological Organization’s Global Atmospheric Watch (GAW) program (GAW, 2019) aimed at monitoring atmospheric constituent behaviors.

As discussed below, the standards and measurements underpinning long-term, high-accuracy data records were pioneered by Keeling and continue to rely on efforts of the Scripps CO, Measurements Program (Scripps-CO,, 2019) and that of the Global Monitoring Division (GMD) of NOAA’s Earth Systems Research Laboratory (GMD. NOAA, 2019) (Zhao. Estimating uncertainty of the WMO Mole Fraction Scale for carbon dioxide in air, 2006). Sustaining a consistently accurate measurement standards capability has been, and continues to be. a central feature supporting reliability and confidence in monitoring of atmospheric greenhouse gas concentrations. GMD, designated as WMO's Central Calibration Laboratory (CCL) for these gases, provides real air mole fraction standards to the GHG monitoring and research community worldwide (see discussion of the term ‘real air'). As WMO’s CCL, GMD participates with others in the atmospheric monitoring community and in the international metrology community (BIPM/CCQM, 2017) to ensure consistency in the gas mixture standards upon which atmospheric monitoring is based. These standards comparison activities are largely conducted under the aegis of the WMO/GAW organization. Such joint community efforts ensure international recognition of these standard gas mixtures thr ough linkage to the International System of Units, the SI (BIPM, 2019) for consistency and accuracy among affected international communities.

Annual variations in data records such as these reflect the role of CO, exchange processes between Earth’s atmosphere, laud and oceans. As the primary carbon source for photosynthetic chemical pathways in vegetation, CO, is both removed fr om the atmosphere thr ough uptake by plant material (photosynthesis) and respired by plants, particularly when photosynthesis ceases during the night. Animals, soils and water-borne microbes also respire CO, as part of then normal biologic function. The globally aggr egated CO, uptake processes on laud and in the oceans account for approximately half the CO, currently emitted yearly fr om anthr opogenic sources (Allen. 2014). Similar annual cycles in mole fraction are also observed for other atmospheric greenhouse gases, e.g., methane (Basso, 2016; Christensen, 2003), although the process mechanisms giving rise to these differ. The differing magnitude of variations reflects GHG emission source and sink complexity globally, regionally, and locally, as do estimation and measurement system challenges for quantifying GHG atmospheric exchange fluxes over this broad and complex range of spatiotemporal scales and processes.

Greenhouse gas mole fraction standards—Histoty and methods

The need for a stable and accurate frame of reference upon which to base atmospheric observations became clear early in Keeling’s atmospheric CO, concentration investigations. He established high- accuracy standard gas mixtures as a reference upon which to base observations taken at widely differing locations and over extended time periods (Scripps-CO„ 2019). Keeling's development of these standards underpins the near continuous observations of atmospheric CO, content taken at Mauua Loa and subsequently expanded to other locations in the 1960s and 70s by SIO, NOAA's Earth Systems Research Laboratory' and others to better understand atmospheric behavior globally. The Mauna Loa data record is the longest term, lrigh-accuracy CO, record in existence. Recognized by the WMO early on, the Scripps CO, project became the forerunner of global atmospheric greenhouse gas observations. As part of this effort, Scripps served as the WMO’s Central Calibration Laboratory (CCL) for CO, and some related trace atmospheric constituents until the late 1990s when ESRL/GMD assumed those responsibilities (Zhao, 1997).

2.4.1 Standards realization methods

The making of greenhouse gas mole fraction standard gas mixtures is generally based on the two methods described below. These use bulk gas that, in some procedures, are combined with high purity greenhouse gases as additives in order to reach target mole fraction values. The bulk gas is most often drawn directly from the atmosphere and is termed “real air” as opposed to bulk gases derived by mixing pure components, nitrogen, oxygen, and argon, to nominal atmospheric constituency. As dry ah' mole fraction measurement standards, water vapor is removed at the inlet to the compression system using the atmosphere as a bulk gas source in standard gas mixture manufacturing. Currently, aluminum cylinders are widely used as these har e been shown after decades-long studies to contain high purity gas mixtures for extended time periods with minimal trace constituent mole fraction change. Before addition to a mixture, high- purity GHGs are generally subjected to extensive chemical analyses in order to identify and quantify any contaminants. Either the gravimetric or manometric method is used to make (gravimetric) or measure (manometric) and assign concentration values to individual or groups of standards.

The gravimetric technique is an additive method involving several high-accuracy mass measurements of the cylinder ultimately containing the gas mixture standard (Rhoderick, 2016; Hall, 2019). The procedure begins with several evacuations of the cylinder to remove any gas that would be unquantified components of the final mixture. The cylinder valve is then closed, and the cylinder weighed in the evacuated state. With mixture GHG target values determined, liigh-purity CO,, CH4, N,0 and/ or other trace gases of interest are added to the evacuated cylinder in targeted amounts. The cylinder is weighed, then the bulk gas is added to the cylinder in an amount necessary to attain the final target mass. Finally, the cylinder is weighed in its filled state. These cylinder mass values along with their uncertainty and other factors are combined in order to calculate the final mole fraction value(s) and uncertainty estimates. The accuracy of gravimetrically prepared standards depends on GHG purity analysis and mass measurement accuracy. Accuracy of environmental parameters, such as temperature, pressure, and humidity of the weighing laboratory environment, must also be considered in order to achieve accurate mass measurements. These standards are then the basis for assigning mole fraction values to secondary and tertiary standards or reference materials used to disseminate field standards. Tire great advantage of the gravimetric technique is its universal applicability. However, it has the disadvantage of lacking the internal procedural capability to check the primary standards for drift over tune. As with any contained mixture, a variety of processes can change the mole fraction of gas delivered from the container. For example, absorption desorption processes at cylinder walls or slow chemical reactions occurring in the contained gas har e been shown to change mole fraction values over tune (Miller, 2015; Scliibig, 2018). Intensive study of drift in mole fraction values has resulted in the widespread use of aluminum alloy cylinders, as these have been demonstrated to minimize change in these mixtures. However, surveillance measurements, particularly for primary standards, as well as then descendants, is required to ensure the integrity of assigned mole fraction r allies over extended tune periods.

The manometric method is a separation method. It quantifies a gas mixture's CO, mole fraction based on accurate measurements of gas volume, pressure and temperature dining separation process steps (Zhao. 2006). Carbon dioxide is separated from the bulk gas by passing a known volume of the gas mixture through cold traps in order to condense the minor constituents that har e boiling point (B.P.) temperatures higher than those of the nitrogen (B.P. = 77.3 K) and oxygen (B.P. = 90.2 K) that are the bulk of the mixture. However, for CO, quantification, the boiling temperatures of CO, (B.P. = 194.65 K). and N,0 (B.P. = 184.67 K) are sufficiently similar to warrant a separate measurement of N,0 content in the final sample, generally with gas chromatography, to account for its presence, if any. Residual water vapor (B.P. = 373.15 K) may also be condensed in the separated sample even though measures are taken to dekumidify the starting gas sample. Measures may be taken to either eliminate or quantify its contribution also (Meyer, 2018).

To quantify the mole fraction of CO, in the gas mixture, an initial sample is transferred to the largest (approximately 6 to 10 liters) of three volume-calibrated glass containers. After it is allowed to temperature equilibrate, the number of moles of the stalling mixture is determined based on accurate pressure and temperature measurements of this known volume of gas. The largest container is then connected to a smaller container via a manifold. The second container’s temperature is held at temperatures sufficient to condense the CO,, and perhaps some N,0 and residual water vapor, into this second container. This container is slowly wanned so that the remaining CO, evaporates and is re- condensed as it transfers into a third smaller volume, which is then closed to the manifold connecting the containers and associated instruments. This final gas sample is then allowed to thermally equilibrate to near room temperature. As with the initial gas sample, accurate pressure and temperature measurements of the gas in this final volume-calibrated container determine the number of moles of CO, of the sample. Nitrous oxide concentration measurement for this final sample is a final correction in the mole fraction value assignment procedure.

Comparison of gas mixtures prepared by gravimetric or manometric methods with one another is a means of assessing consistency between the methods and the accuracy of the mixtures as standard artifacts. Recent comparisons using reference mixtures compared with primary standards by NOAA/ GMD (manometric method) and NIST (gravimetric method) demonstrate the level of agreement currently obtainable between the two methods and laboratories. Agreement within 0.05%, 0.13%, and 0.06% respectively for CO,, CH4. and N,0 (Rhoderick, 2016) has been demonstrated for GHG reference materials of northern hemisphere ah. Periodic comparison efforts involving members of the international metrology community organized under the Metre Convention (BIPM, 2019) and members of the atmospheric monitoring community organized under the WMO’s GAW contribute to the stability and international recognition of these standards.

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