the intergovernmental organization through which Member States act together
on matters related to measurement science and measurement standards.
The comparisons in gas metrology piloted by the BIPM fall into two types: those using gravimetric (static) methods, and those using dynamic methods. Further information about the comparisons and the CCQM pilot studies piloted by the BIPM are available through the tabs above. The following comparisons piloted by the BIPM are registered in the KCDB:
Carbon dioxide (CO2) in air
Changes in carbon dioxide concentration in the atmosphere are of high environmental and political importance, with increasing levels of emissions and background monitoring. Performance criteria for CO2 monitoring networks are being set, with the most stringent of these being for background ambient levels where data compatibility from different monitoring sites should be at the level of
The BIPM, in collaboration with the NIST, is organizing an international comparison, CCQM-K120) of primary CO2 in air standards, which contains two parts: part (a) for standards in the range
Prior to the comparison the BIPM used primary standards of the NIST, NOAA and NPL to develop comparison methods based on Gas Chromatography with a Flame Ionization Detector (GC-FID) and on Fourier Transform Infrared spectroscopy (FTIR). Measurement uncertainties as low as
The BIPM is developing an independent system to assign the CO2 amount fractions in gas standards based on pressure, volume and temperature measurements: the CO2-PVT measurement system. This facility will support the CCQM-K120 comparison and planned ongoing comparisons for CO2 in air (BIPM.QM-K2).
Methane (CH4) in air
Methane contributes 18.1% of the overall global radiative forcing and is the second most important greenhouse gas after carbon dioxide. Atmospheric methane concentrations are now 254% of the pre-industrial level. Measurements of CH4 in the atmosphere are calibrated with gas standards prepared by gravimetry by the addition of pure CH4 into a matrix of dry air. The most stringent compatibility goals for global methane monitoring are set at
In 2010 the BIPM in collaboration with NIST developed accurate methods for methane in air standard comparisons based on Gas Chromatography with a Flame Ionization Detector (GC-FID) and Cavity Ring-Down Spectroscopy (CRDS). Standards prepared in whole air and synthetic air were compared, to demonstrate that proper preparation of the latter could avoid well-known biases that could occur with the CRDS measurement technique.
The BIPM facilities were successfully used in 2013 for the coordination of CCQM-K82, for methane in air standards ranging from
Nitrous oxide (N2O) in synthetic air
Although nitrous oxide is present at much lower levels than CO2 (about a thousand times) in the atmosphere, it has a Global Warming Potential (GWP) that is 296 times that of CO2. The most important N2O sources are agriculture, combustion processes (including catalytic conversion in cars), and industry.
As for CO2 and CH4, N2O measurements are calibrated with standards prepared by static gravimetry, with relative uncertainties between 0.2% and 1%. After a first international comparison coordinated by KRISS in 2008 (CCQM-K68), efforts have concentrated on further reducing gravimetric uncertainties, as world-wide measurements of N2O need to be compatible to better than 0.03% to be meaningfully interpreted. The BIPM is currently preparing the repeat comparison CCQM-K68.2019 in collaboration with KRISS.
KRISS has provided a Gas Chromatography analyser with an Electron Capture Detector (GC-ECD) to the BIPM, together with a set of standards prepared at KRISS to validate the future comparison method. In addition, an instrument based on Infrared Spectroscopy with a Tuneable Diode Laser (TILDA-CS), will also be used to compare the standards.
Nitrogen monoxide (NO) in N2
A facility for the comparison of primary nitrogen monoxide gas standards is maintained at the BIPM. The NO facility enables the analysis of primary reference NO/N2 mixtures in the range (30 to 70) µmol/mol, with a measurement uncertainty of about
The system is entirely automated by software developed at the BIPM. In 2005-2006 it was used as the central analytical facility for the international comparison of nitrogen monoxide gas standards CCQM-P73, involving 12 NMIs and coordinated by the BIPM. The results illustrated the reductions in uncertainties that can be achieved through a comparison with measurements performed at a central facility. Since then it has been maintained to undertake the key comparison CCQM-K137 on NO standards at
The facility was also used to label secondary gas standards for the ozone Gas Phase Titration (GPT) facility, thereby providing traceability of the GPT system to the primary NO/N2 standards. By comparison with the more traditional method for measuring ozone by UV absorption, this allowed the calculation of an independent value of the ozone absorption cross-section currently under review by a Task Group of the CCQM's Gas Analysis Working Group.
Formaldehyde (HCHO) in N2
In addition to being an important indoor air pollutant, formaldehyde is a ubiquitous component of both the remote atmosphere and polluted urban atmospheres. Dynamic standards as well as static gas mixtures of around
The BIPM formaldehyde primary facility has the capability to generate a level of formaldehyde in nitrogen that can be adjusted between 1 µmol/mol and 10 µmol/mol, using continuous accurate measurements of mass loss from a permeation tube coupled with a dilution system. This system is very similar to the one used to generate NO2 in nitrogen mixtures. However in the case of formaldehyde, the BIPM has validated two different raw materials: paraformaldehyde which leads to the loss of formaldehyde when heated at
One obvious source of bias in the system is the presence of impurities. Great efforts have been made to work with pure gases, for which purity is assessed using a Fourier Transform Infrared (FTIR) spectrometer together with a selected suite of software for quantification of impurities. The BIPM formaldehyde facility is completed by a Cavity Ring-Down Spectroscopy (CRDS) analyser. The FTIR and the CRDS are both used as comparators between gas concentrations from the permeation facility and the certified gas cylinders.
Nitrogen dioxide (NO2) in N2
Nitrogen dioxide is a major air pollutant and precursor to the formation of greenhouse gases, and its atmospheric concentrations are regulated and monitored globally. NO2 being reactive at the concentration of interest, NO2 standards are often calibrated with dynamic generation systems such as the primary facility maintained at the BIPM.
The facility is based on the constant permeation of NO2 through a fluoropolymer membrane (permeation tube) in a flow of dry nitrogen. The mass of the permeation tube is constantly monitored by a magnetic suspension balance to deduce the mass loss rate. Together with a flow control and measurement device, this allows the determination of the NO2 amount fraction in the nitrogen flow. The principle and the performances of the facility were reported in 2012.
The BIPM NO2 facility is used to dynamically generate gravimetric mixtures of NO2 in nitrogen in the range (1 to 15) µmol/mol. It was the basis for the reference value in the international comparison CCQM-K74 coordinated by the BIPM.
During this comparison, the facility was also used to compare the accuracies achievable using Fourier Transform Infrared (FTIR) measurements and synthetic calibration procedures, with traceability to line-strength measurements of the gases of interest, and of particular use when no gas mixture calibration standards are available[3, 4]. A recent example was the case for nitric acid (HNO3), which is the major impurity in standards of NO2 in nitrogen. A complete FTIR method based on synthetic calibration spectra has been developed and validated on NO2, and applied to quantify HNO3 in the mixtures.
Ozone (O3) in air
BIPM.QM-K1 is a key comparison for ozone reference standards and capabilities coordinated by the BIPM. It is run as an ongoing key comparison. Since its start in 2007, 21 National Metrology Institutes or Designated Institutes have been taking part in this comparison, including the World Calibration Center for surface ozone of the World Meteorological Organization (WMO). BIPM.QM-K1 is indeed recognized as the way to demonstrate metrological traceability of surface ozone measurements in the WMO guidelines for continuous measurement of ozone in the troposphere.
The comparison protocol and associated forms can be downloaded from this page. Completed registration forms should be sent to the comparison coordinator, Dr J. Viallon (BIPM).
The BIPM was also the coordinating laboratory for a previous CCQM comparison of national surface-ozone standards, CCQM-P28, completed in March 2005.