Standard Reference Gases: About the GML CCL

What is a CCL?

A World Meteorological Organization (WMO) Central Calibration Laboratory (CCL) is responsible for maintaining and distributing the WMO Mole Fraction scale for a specified gas in air. NOAA GML is the WMO, Global Atmosphere Watch (GAW) CCL for CO₂, CH₄, N₂O, SF₆, and CO. In support of GAW, GML offers calibration services for these gases for a fee.

The CCL scale for N₂O and SF₆ is maintained by Dr. Brad Hall (GML Halocarbons and other trace species (HATS) group)
The CCL scale for CO₂ is maintained jointly by Dr. Brad Hall (GML Halocarbons and other trace species (HATS) group) and Andrew Crotwell (GML Carbon Cycle Group)
The CCL scales for CH₄ and CO are maintained by Andrew Crotwell (GML Carbon Cycle Group)
The facility for preparing whole-air reference materials is maintained by Duane Kitzis (GML Carbon Cycle Group)

According to recommendations by the WMO/IAEA group of experts on carbon dioxide and related tracer measurement techniques (WMO 2020), a CCL will:

maintain the WMO Mole Fraction Scale for the respective gas in air by carrying out regular determinations of this primary scale with an absolute method. The scale is embodied in an adequate set of gas mixtures-in-air in large high-pressure cylinders (called WMO Primary Standards).
strive to distribute the scale with sufficient consistency to allow WMO/GAW stations and contributing programs to meet the WMO network compatibility goals.
carry out comparisons with independent primary scales, established either through gravimetric, manometric, or other means.
update its scale when warranted, as the gas mole fractions of the WMO Primary Standards may become better known over time through repeated absolute measurements and comparisons. Revisions of the WMO Scale by the CCL must be distinguished by name, such as WMO CO2_X2019, and the appropriate version number should be included in each standard calibration report. The CCL archives all earlier versions of the WMO scale.
provide complete and prompt disclosure of all relevant data pertaining to the maintenance and transfer of the primary scale, such as manometric calibration procedures and results, and an estimate of the expected uncertainty introduced by the calibration transfer procedure to each individual standard.
provide calibrated reference gas mixtures (gas mixtures-in-air) at the lowest possible cost.
provide for a backup in case a catastrophic event occurs.
organize, in collaborations with designated WMO World Calibration Centres, comparisons of laboratory calibrations by distributing sets of high-pressure cylinders to be measured by participating laboratories (called Round Robin comparisons).

GML Quality Manuals for Measurement Services in support of WMO/GAW

References:

WMO (2020), 20th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and Related Tracers Measurement Techniques (GGMT-2019), Jeju Island, South Korea, GAW Rep. No. 255, Geneva, Switzerland.