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Air quality

    Standards to underpin air-quality monitoring

    The quality of the air we breathe, both inside and outside our houses, is a major concern that has triggered the implementation of regulations at the local, national and regional levels. These regulations require continuous monitoring of a selection of air pollutants, together with calibration processes ensuring comparability and traceability of key air-quality measurements. The BIPM work programme includes the organization of a number of comparisons on important air-quality components to underpin corresponding national standards.

    Among the most monitored air pollutants are ozone and the nitrogen oxides (NOx, or NOx), which include nitric oxide (NO) and nitrogen dioxide (NO2). Of these three compounds, only nitric oxide allows the preparation of primary standard gas mixtures by weighing the individual components into a cylinder, providing NO is mixed in a matrix of nitrogen at the µmol mol–1 level. The BIPM maintains a facility to run international comparisons of NO in nitrogen standards prepared by National Metrology Institutes (NMIs) and Designated Institutes (DIs). It was first used in 2007 to organize the pilot study CCQM-P73 and will be used again in 2017 for the key comparison CCQM-K137.

    Nitric oxide standards are also used to calibrate NOx analysers for the measurement of nitrogen dioxide, which like ozone is a pollutant directly responsible for environment damages and health issues. Direct calibration for NO2 can be achieved through a dynamic generation system to produce NO2 standards on-line. Such a system is maintained at the BIPM and used to coordinate the international comparison CCQM-K74. Similar systems are also used to generate standards of Volatile Organic Compounds (VOCs), in particular formaldehyde (HCHO), which is mainly an indoor pollutant emitted by some resins, building materials, and glues. Standards of formaldehyde in nitrogen have been generated at the BIPM from two different chemicals, allowing the organization of the comparison CCQM-K90.

    Pilot Study:

    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 0.05 % – approximately the same as the uncertainty of gravimetric preparation of such mixtures. In addition, impurities in the standards are quantified with Fourier Transform Infrared spectrometry (FTIR) at the BIPM.

    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[1]. 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 30 µmol mol–1 and 70 µmol mol–1. At this latter concentration, preparing NO in nitrogen standards is seen as an example of a core capability of NMIs active in gas analysis.

    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.

    1. Wielgosz R.I., Esler M., Viallon J., Moussay P., Oh S.H., Kim B.M., Tshilongo J., Mokgoro I.S., Maruyama M., Mace T., Sutour C., Stovcik V., Musil S., Castorena A.P., Murillo F.R., Kustikov Y.A., Pankratov V.V., Gromova E.V., Thorn W., J., Guenther F.R., Smeulders D., Baptista G., Dias F., Wessel R.M., Nieuwenkamp G. and van der Veen A.M.H., Final report on CCQM-P73: International comparison of nitrogen monoxide in nitrogen gas standards (30-70) µmol/mol, 2008, Metrologia 45 Tech. Suppl. 08002

    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 1 µmol/mol have recently been developed at a number of National Metrology Institutes (NMIs), and their comparability demonstrated through the BIPM formaldehyde primary gas standards facility in the key comparison CCQM-K90.

    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[1]. 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 110 °C, and trioxane (the formaldehyde trimer), evaporated from a diffusion cell and then decomposed into formaldehyde in a converter. Although they lead to different impurities, both materials were found to be suitable to generate HCHO in nitrogen mixtures with typical relative uncertainties of 0.2%.

    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.

    1. Flores E., Idrees F., Moussay P., Viallon J., Wielgosz R., Highly accurate nitrogen dioxide (NO2) in nitrogen standards based on permeation, Anal. Chem., 2012, 84(23), 10283-10290