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    Pure ozone

    Standards for ozone at ambient level

    Ozone is a principal pollutant associated with photochemical smog. Ground-level ozone concentration is an important air-quality parameter which is monitored and reported world-wide. The reference method for ground-level ozone measurements is based on UV photometry, with replicates of the NIST Standard Reference Photometer (SRP) acting as primary standards for numerous national and international ozone-monitoring networks.

    The BIPM runs an international programme to ensure the equivalence of national ozone standards, underpinning the international comparability of ground-level ozone measurements. It coordinates the ongoing key comparison BIPM.QM-K1 for ozone at ambient level, consisting of a series of bilateral comparisons between national standards and the BIPM ozone reference standards. More than twenty National Metrology Institutes or Designated Institutes take part in this comparison. For those institutes that do not maintain a primary UV photometer, the BIPM also proposes the calibration of national ozone analysers.

    The high reactivity of ozone makes it unsuitable for preparation of gas standards by static gravimetric methods. It is one of the few gases for which the standard is an instrument based on a photometric principle, with the measurement of the amount of UV light absorbed by ozone inside the instrument's gas cells. Therefore all measurements of ozone amount fractions in ambient air are ultimately anchored to the value of the ozone absorption cross-section at the wavelength 253.65 nm. This value has been measured by many groups, including two independent measurements performed at the BIPM, and is currently under review by a Task Group of the CCQM's Gas Analysis Working Group.

    The BIPM maintains three NIST Standard Reference Photometers (SRP27, SRP28 and SRP31) as ozone reference standards to underpin its international comparison programme. The measurement principle in the NIST SRP is UV photometry, based on the measurement of the absorption of UV light emitted by a mercury lamp by ozone in air flowing alternatively through the two gas cells of the instrument.

    Since the start of the collaboration between NIST and BIPM, the instrument design has been modified to take into account the outcomes of a study of systematic biases and measurement uncertainties published in 2006[1]. Following that study, new SRPs constructed by the NIST include sealed gas cells in quartz with end windows slightly tilted to avoid multiple reflections of the UV light beam.

    The BIPM also maintains two other SRPs (SRP32 and SRP33, constructed at the BIPM as part of the NIST/BIPM collaboration), for its research and development programme. Recently, the BIPM and NIST have been cooperating to upgrade the electronics module and the control software of the instrument.

    Related links:

    1. Viallon J., Moussay P., Norris J.E., Guenther F.R., Wielgosz R.I., A study of systematic biases and measurement uncertainties in ozone mole fraction measurements with the NIST Standard Reference Photometer, 2006 Metrologia 43 441

    The property of ozone to strongly absorb UV radiation, notably in the Hartley Band, and the relative ease of reproducing a mercury line at 253.65 nm (in air), has led to the value of the ozone absorption cross-section at this wavelength becoming particularly important for global ozone atmospheric monitoring. Ozone concentration measurements performed with UV photometers such as the NIST Standard Reference Photometer maintained by the BIPM are all anchored to a value that was adopted by convention during the Quadriennal Ozone Symposium in 1984.

    During the first international comparison of ozone standards (CCQM-P28) conducted by the BIPM in 2003-2005, twenty three laboratories reported results based on UV absorption, and two (the National Institute for Environmental Studies of Japan and the BIPM[1]) reported results based on Gas-Phase Titration systems (GPT). Gas-Phase Titration involves reacting ozone in air with nitrogen monoxide (NO), and measuring either the loss of nitrogen monoxide or the gain of the reaction product, nitrogen dioxide (NO2), to deduce the ozone concentration in the sample. The 2 % to 3 % bias observed between the methods (GPT reporting higher ozone concentrations) could be explained by a biased ozone absorption cross-section value, which represents the major uncertainty component in measurements based on UV photometry.

    The conventional value of the ozone absorption cross-section adopted in 1984 has been called into question by a number of independent measurements, which are generally performed by measuring the light absorption at 253.65 nm of pure ozone samples maintained at low pressure in a gas cell. This method was first implemented at the BIPM after the development of a laser-based ozone photometer[2], which was then used a light source in subsequent measurements performed on pure ozone samples[3]. The result was compared with that of measurements based on GPT[1], which were also used to deduce a value of the ozone absorption cross-section via the comparison with the BIPM SRP. Both methods gave lower values than the conventionally adopted one.

    These values, together with all independent values published since 1953, are under review by an Ozone Cross-Section Task Group, established by the CCQM's Gas Analysis Working Group with the task of recommending a value and uncertainty for the ozone cross-section at 253.65 nm to be used in ozone reference photometers and for comparisons of these standards in BIPM.QM-K1. The Task Group is expected to report its findings in 2017.

    1. Viallon J., Moussay P., Flores E., Wielgosz R.I., Ozone cross-section measurement by gas phase titration, Anal. Chem., 2016, 88, 10720-10727
    2. Petersen M., Viallon J., Moussay P., Wielgosz R.I., Relative measurements of ozone absorption cross-sections at three wavelengths in the Hartley band using a well-defined UV laser beam, J. Geophys. Res., 2012, 117, D05301
    3. Viallon J., Lee S., Moussay P., Tworek K., Petersen M., Wielgosz R.I., Accurate measurements of ozone absorption cross-sections in the Hartley band, Atmos. Meas. Tech., 2015, 8, 1245-1257

    For those Member State NMIs that do not hold primary standards for ozone in air, the BIPM offers calibrations of secondary standard ozone analysers.

    Interested laboratories are invited to contact Dr J. Viallon for the relevant calibration request forms.