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Are ozone concentrations in the atmosphere being underestimated?

An article just published in Analytical Chemistry[1] by the BIPM reports an independent value of the cross-section of ozone, which provides further evidence to suggest that the currently adopted conventional value of this quantity is too high, meaning that the reported ozone concentrations in the atmosphere are at present being underestimated.

In a previous publication[2], the BIPM had reported new values of the ozone absorption cross-section from measurements of the absorption of a well-defined UV light from an Argon-Ion laser in a pure ozone sample maintained at low pressure in a vacuum chamber. The results demonstrated lower values compared to the ones conventionally adopted since the 1970s.

In the new publication, a completely independent method was used, relying on gas phase titration (GPT) of ozone with nitrogen monoxide, following the well-known reaction taking place in the atmosphere: NO + O3 → NO2 + O2.

Realising this method can also be seen as a test of a metrological triangle in which three different traceability chains are being compared: NO, NO2 and O3 (Figure 1). NO reacted and NO2 gain are traceable to reference materials which are independently prepared by gravimetry. The amount of O3 that reacted, measured by an ambient ozone analyser, is traceable to a Standard Reference Photometer (SRP) for ozone, which is considered as a primary standard and relies on the measurement of the amount of UV light (at 253.65 nm) absorbed by the sample of ozone in dry air in the instrument's gas cells, which required knowledge of the ozone absorption cross-section value.

The titration of O3 with excess NO was performed in the gas phase at nmol mol-1 levels and was used to calculate two GPT-based ozone absorption cross-section values; one with traceability to NO standards, the other to NO2 (Figure 2).

These values, together with all independent values published since 1953, are under review by a Task Group of the CCQM's Gas Analysis Working Group which was established 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, and Wielgosz R, Ozone Cross-Section Measurement by Gas Phase Titration, Anal. Chem., 2016, 88 (21), pp.10720-10727.
  2. Viallon J, Lee S, Moussay P, Tworek K, Petersen M, and Wielgosz R, Accurate measurements of ozone absorption cross-sections in the Hartley band, Atmos. Meas. Tech., 2015, 8, pp.1245-1257.
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