Update of Ozone Absorption Cross-Section Value for Calibration of Low Concentration Ozone Monitors
September 25, 2025
Ozone concentration measured by UV photometry is calculated by Beer-Lambert law using a constant known as the absorption cross-section.
- Since 1961, the value reported by Hearn (σ = 1.1476 × 10−17 cm2 molecule−1 at 253.65 nm) has been used internationally and served as the NIST-accepted reference in surface ozone measurement.
- In 2019, international collaborative work on surface ozone measurement led to a new consensus value CCQM.O3.2019 (σ = 1.1329 × 10−17 cm2 molecule−1 at 253.65 nm), which is 1.3% lower than the historical Hearn value.
- Since 1 January 2025, many international organisations including US EPA, ISO, ASTM, EN, JIS, AS have started the transition to this updated value in order to ensure consistency and comparability of ozone measurements worldwide.
Further information is available at: https://www.bipm.org/en/ozone
The calibration of BMT’s low concentration ambient ozone monitors is traceable to NIST via an internal transfer standard. The internal transfer standard was calibrated in 2024 against the SRP 29 using the Hearn 1961 value. In line with the international transition, adoption of the updated ozone absorption cross-section value is planned for Q4/2025. At that time, the internal transfer standard will then be adjusted to and verified against the updated value and all calibration certificates will reference CCQM.O3.2019 as the basis of traceability.
For high concentration ozone analysers no national standard currently exists. Calibration accuracy in this range relies on the quality of BMT’s own primary standards, which are based on UV photometry at 253.65 nm and reference the molar extinction coefficient defined in IOA Regulation 002/87 (F). Calibration certificates will continue to cite this regulation. BMT remains in contact with industry associations to monitor and discuss whether an update to this regulation may be considered in the future.
The updated absorption cross-section will increase reported ozone concentrations by about +1.3% compared to results based on the previous value. Example: An ozone concentration of 100.0 ppbv measured with an instrument calibrated to the Hearn (1961) value will correspond to 101.3 ppbv when calibrated to the CCQM.O3.2019 value.
For further information please contact us.
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