Standards for ozone at ambient level
The BIPM runs an international programme to ensure the equivalence of national ozone standards, underpinning the international comparability of ground-level ozone measurements.
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 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.
Ozone absorption cross-section
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.
Since then and after new measurements of the ozone absorption cross-section, a consensus value with the smallest uncertainty ever achieved was published in 2019.
Stakeholder were consulted during a BIPM Workshop on Accurate Monitoring of Surface Ozone (2020). Following this process, the CCQM Gas Analysis Working Group has issued a statement on its intention to change the Ozone Cross-Section Value used for Surface Ozone Standards and Measurement.
Global coordination has been going on since 2020 to implement the 2019 value, which will increase the measured atmospheric ozone concentrations. This is the task of a CCQM-GAWG Task Group on Ozone Cross-Section Change Management.
A specific website was created to inform stakeholders: https://www.bipm.org./en/ozone.
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