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The reference method for the measurement of ground level ozone concentration is based on UV absorption at the mercury wavelength 253.7 nm. The ozone absorption cross-section value at this wavelength is the value measured by Hearn in 1961, and the expanded uncertainty of this value has recently been estimated as 2.12 %.
The NIST SRP ozone reference standard operates on the principle of UV absorption, and acts as the primary standard for numerous national and international ozone monitoring networks (including the WMO/GAW stations). Several replicas of this instrument are maintained by the BIPM, one of them being the reference for international comparisons of national ozone standards coordinated by the BIPM (BIPM.QM-K1).
During the first international comparison (CCQM-P28) piloted by the BIPM in 2006, twenty-three UV absorption based ozone standards at ground level were compared with two realizations of an independent method for primary ozone concentration measurements based on gas phase titration systems developed by the NIES (Japan) and the BIPM (BIPM GPT system). The 2 %-3 % bias observed between the methods requires explanation and confirmation of the ozone absorption cross-section value, which represents the major uncertainty component in measurements based on UV photometry.
In 2007, the BIPM started a laboratory programme aimed at performing new measurements of the ozone absorption cross-section with an improved accuracy. Efforts are being made on two major sources of uncertainty in such measurements: the ozone purity and the knowledge of the light path length. For this purpose, the BIPM developed a laser ozone photometer, capable of measuring ozone concentrations in the same range as the SRP with an improved accuracy, obtained by using a laser instead of a lamp as the light source in order to better control the light path length.
This instrument has already been used to perform measurements of relative absorption cross-sections of ozone at three wavelengths in the Hartley band. Results1 show an agreement with published data provided that the appropriate reference absorption cross-section is used. The instrument will also be used in conjunction with a set-up currently under construction, in which the concentration of pure ozone will be assessed by pressure measurements, allowing new absolute measurements of the ozone absorption cross-section at the same three wavelengths.
Ozone cross-section in the Hartley band as measured by Daumont et al. in 1992 (blue).
Reference value for ozone photometers measured by Hearn in 1961 (red).
1To be published
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