The BIPM has introduced a new on-demand Key Comparison of CO₂ gaseous standards in dry air, tailored to levels found from background to urban areas. This new service supports the global push to improve CO₂ emissions-monitoring and to ensure the precision of measurements worldwide.

Background

Carbon dioxide (CO₂) is the greenhouse gas with the largest contribution to global warming. With the rise of initiatives to measure emissions and to assess the impact of mitigation policies, the need for in situ monitoring sites—and therefore for high-quality CO₂ in air calibration standards—has grown.

To meet this need, the Bureau International des Poids et Mesures’ (BIPM) CO₂-PVT facility (based on pressure, volume, and temperature measurements of CO₂ in air and cryogenically extracted CO₂) allows National Metrology Institutes (NMIs) around the world to ensure the consistency of their CO₂ standards, facilitating uniform measurements and emission calculations.

Described in detail in a Metrologia publication, the CO₂-PVT facility:

• Enables the determination and comparison of amount fraction (concentration) values with those of gas cylinder standards.
• Operates over a wide range of CO₂ concentrations, from those found in background “clean” air to urban sites.
• Was developed by scientists at the BIPM Headquarters alongside visiting scientists from NIST (USA) and RISE (Sweden).

New Service: BIPM.QM-K2 Comparison

As of last month, the BIPM’s CO₂-PVT facility is now also offering an on-demand BIPM.QM-K2 comparison service. This new service allows Member States to demonstrate core competencies in gas metrology at their convenience, rather than waiting for a comparison organized by the CCQM Working Group on Gas Analysis (CCQM-GAWG).

Officially launched during the 50^th meeting of the CCQM-GAWG, held at KRISS on 21 October 2024, the new comparison is structured as a series of bilateral comparisons between each participant and the BIPM Headquarters.

How it works:

• The CO₂-PVT facility provides the Key Comparison Reference Value (KCRV) in each case and quantifies CO₂ amount fractions in air or nitrogen samples.
• The process involves precise measurements of the pressure, temperature, and volume ratio of the sample and of the CO₂ extracted from it by cryogenic trapping.
• Its performance was validated during the Pilot Study CCQM-P225, with standards from LNE, NOAA, NIM, NPL, and VSL.

Core Competencies Demonstration

Classified as a core comparison within the CCQM-GAWG Strategy, the BIPM.QM-K2 comparison allows NMIs and Designated Institutes (DIs) to demonstrate their expertise in:

• gravimetric preparation
• analytical certification
• purity analysis

Future Development: BIPM.QM-K5 Comparison

For NMIs and DIs developing CO₂ in air scale standards for the atmospheric monitoring community—where the internal consistency between standards needs to approach the 0.01 µmol/mol level—the BIPM will shortly launch the BIPM.QM-K5 comparison. This comparison will expand the availability of standards for national and international networks for CO₂ in air measurements, such as the WMO’s Global Greenhouse Gas Watch initiative.

The standards maintained at the BIPM and used as the reference for the BIPM.QM-K5 comparison have been analyzed with the BIPM.QM-K2 facility to ensure SI traceability within the uncertainty of the reference method.