Preparation and Validation of Fully Synthetic Standard Gas Mixtures with Atmospheric Isotopic Composition for Global CO₂ and CH₄ Monitoring

Abstract

We report the preparation and validation of the first fully synthetic gaseous reference standards of CO2 and CH4 in a whole air matrix with an isotopic distribution matching that is in the ambient atmosphere. The mixtures are accurately representative of the ambient atmosphere and were prepared gravimetrically. The isotopic distribution of the CO2 was matched to the abundance in the ambient atmosphere by blending (12)C-enriched CO2 with (13)C-enriched CO2 in order to avoid measurement biases introduced by measurement instrumentation detecting only certain isotopologues. The reference standards developed here have been compared with standards developed by the National Institute of Standards and Technology and standards from the WMO scale. They demonstrate excellent comparability.

Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations

“…As a first approximation, when carbon dioxide is measured as the major isotopocule 12 C 16 O 2 , a 1‰ difference in δ 13 C values between the calibration and sample gases leads to a difference of 4 nmol mol −1 in the amount fraction of a 400 µmol mol −1 mixture, and 2 nmol mol −1 for the same difference in δ 18 O values. Blending carbon dioxide gases from different sources provides a method for matching to natural abundance isotope ratios for 13 C, with the mixing of industrially sourced carbon dioxide with highly enriched 13 CO 2 described [48] as well as mixing with pure carbon dioxide obtained from a gas well source [49] (δ 13 C at −1‰ versus Vienna Peedee Belemnite (VPDB)), with δ 18 O values better matched to atmospheric values using the latter source.…”

Effects of variation in background mixing ratios of N<sub>2</sub>, O<sub>2</sub>, and Ar on the measurement of <i>δ</i><sup>18</sup>O–H<sub>2</sub>O and <i>δ</i><sup>2</sup>H–H<sub>2</sub>O values by cavity ring-down spectroscopy

“…Three of the cylinders contained ultra high-purity N 2 , O 2 , and Ar (99.999 % purity, < 3 ppm H 2 O, and < 0.5 ppm total hydrocarbon content (THC); ALPHAGAZ 1, Air Liquide America Specialty Gases LLC, Houston, TX, USA). The fourth cylinder contained ultra high-purity air (< 1 ppm H 2 O, < 0.01 ppm THC, < 0.01 ppm CO, < 0.001 ppm NO x , < 0.001 ppm SO 2 ; Ultrapure Air, Scott-Marrin, Inc., Riverside, CA, USA) with the N 2 , O 2 , and Ar composition of the natural atmosphere (i.e., 78.1 % N 2 , 20.9 % O 2 , 0.9 % Ar; Brewer et al, 2014;Flores et al, 2015). In the experiments, background gas mixtures were dynamically mixed from these cylinders with the mass flow controllers (FC-260 with RO-28, Tylan-Mykrolis, Allen, TX, USA).…”

Gravimetrically prepared carbon dioxide standards in support of atmospheric research

“…Numerous laboratories make routine measurements of atmospheric carbon dioxide to better understand its sources, sinks, and temporal variability. These measurements are typically calibrated using high-pressure gas standards containing CO 2 in air (typically natural air with assigned CO 2 mole fractions), traceable to primary standards prepared or analyzed using absolute methods, such as manometry (Keeling et al, 1986;Zhao and Tans, 2006) and gravimetry (Machida et al, 2011;Rhoderick et al, 2016;Brewer et al, 2014), which provide traceability to the International System of Units (SI). The World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) initially adopted the Scripps Institution of Oceanography scale (Keeling et al, 1986) and subsequently the National Oceanic and Atmospheric Administration (NOAA) scale (Zhao et al, 1997), both of which are based on repeated manometric measurements of a suite of primary standards, for WMO-affiliated monitoring networks.…”

“…The gravimetric technique can employ multiple steps, in which the target gas (CO 2 in this case) is diluted to the desired amount fraction. For example, Brewer et al (2014) first prepared standards with mole fractions of a few percent, then made dilutions of those to the range needed for ambient monitoring (∼ 400 ppm). Others have opted to dilute CO 2 to part per million levels in one step (Machida et al, 2011).…”