CCQM Task Group for Ozone Cross-Section

Terms of reference

• To establish and publish a timeline for implementing the change in the ozone cross-section value used world-wide for surface ozone measurements to the CCQM.O3.2019 value¹, taking into account the times needed for documentary change as well as implementation of the change in measurement instruments and networks, and actions necessary by various stakeholders. The ‘intention-to-change statement’ developed from the BIPM/CCQM-GAWG 2020 workshop, foresees a 3 to 5-year change process with the date of 1 January 2024, identified as a provisional start date.


• To identify regulations and documentary standards world-wide, in addition to ISO 13964, EN 14625 and US EPA 40 CFR Appendix D to Part 50, that would require change including their interrelationships. Identify the periods required for change of these documents and identify organizations/individuals able to initiate and keep track of the change process.


• To develop a website where all information on the change process can be disseminated from.


• To establish and maintain a database on interested stakeholders enabling updates on progress of the change process to be disseminated.


• To continue to promote best practice to the user community of flagging (providing metadata) on ozone mole fraction measurements, clearly identifying which cross-section value is used for measurement (CCQM.O3.2019 or Hearn 1961).


• To liaise with the atmospheric community and contribute to a peer-reviewed publication on the importance and challenges of ensuring uniformity of measurements of ozone concentrations in different parts of the atmosphere using different measurement techniques that utilize different parts of the electromagnetic spectrum.


• To develop practical guidance for users of instruments in the field on how to deal with the change in cross-section value, and processes to be followed to ensure their instruments are making measurements using the new cross-section value correctly, and how to report results in any transitional period.


• To develop practical guidance for manufacturers of instruments on how to deal with the change in cross-section value, and processes to be followed to ensure their instruments are making measurements using the new cross-section value correctly, and how type testing of instruments maybe affected by the change.


• To develop documents informing different communities, in different languages, of the rationale and impact of the change.


• To inform communities interested in trend analysis of ozone concentrations of potential step function effects in historic data and the need to consider cross-section meta data when analysing data for trends, especially over the transition period.

¹. This is the value in the 2019 publication:

The identifier CCQM.O3.2019 as a unique shorthand identifier to identify the value was decided by:

Units and values for the ozone absorption cross section at 253.65 nm (air) with appropriate significant digits and rounding for use in documentary standards, R.S. Davis, B. Niederhauser, J. Hodges, J. Viallon, R.I. Wielgosz