In 2025, a quiet but consequential shift took place across environmental monitoring networks worldwide. Scientists, laboratories and monitoring stations began implementing a new ultraviolet ozone absorption cross-section value for ground-level ozone measurements — a change that strengthens the accuracy and comparability of air-quality data used every day by researchers, regulators and policymakers.

As the year draws to a close, the Bureau International des Poids et Mesures (BIPM) would like to recognise and thank the many scientists and institutions who made this transition possible. Their collective effort — spanning national metrology institutes (NMIs), atmospheric observatories, calibration laboratories and more — reflects years of collaboration and shared commitment to improving how ozone is measured, understood and reported around the world.

Watch the wrap-up video:

Why this transition matters

The newly implemented absorption cross-section value, CCQM.O3.2019, significantly reduces measurement uncertainty and improves consistency across global ozone monitoring networks. By strengthening comparability across sites and over time, the update enables more reliable trend analysis, research and evidence-based decision-making.

As Damian Smeulders, Director Reference Gases at National Measurement Institute, Australia noted during during the 2025 Gas Analysis Working Group:

“Air pollution and greenhouse gas emissions remain amongst the biggest environmental and health challenges of our time. Yet without consistent measurement standards, countries cannot fairly compare data, assess progress or coordinate responses. This initiative helps bridge that gap by strengthening global metrology for air quality.”

A shared effort, grounded in expertise

The transition to the new cross-section value reflects sustained technical work across the international metrology community, coordinated through the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology (CCQM), its Gas Analysis Working Group (GAWG) and the Task Group on Ozone Cross-Section Change Management, with coordination support from the BIPM.

For those implementing the change at monitoring sites and laboratories, the benefits are practical as well as scientific. As Dr. Andrew M Crotwell, from the Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder explained:

“This will improve comparability across sites and over time and reduce uncertainty. In turn, it allows trend analysis, research and health-related decisions to be based on more reliable measurements.”

Javis Nwaboh, Research Scientist at PTB Germany, highlighted how this work connects local measurements to global observing systems:

“Looking ahead, the focus is on scaling up collaboration and linking these high-precision measurements to wider networks, including satellite observations.”

The update is already being integrated into calibration services at NMIs, reflected in the BIPM.QM-K1 key comparison, and embedded in updated international standards, including ISO 13964 and ISO 10313.

Looking ahead

The adoption of the new ozone absorption cross-section is not an endpoint. In 2026 and beyond, international comparisons, training activities and continued cooperation through CCQM, GAWG, national institutes and atmospheric monitoring networks will further strengthen ozone measurement capability worldwide.

What this year has demonstrated most clearly is the value of sustained collaboration: when measurement scientists work together, the result is data that is more accurate, more comparable and more trusted — and ultimately more useful for protecting health, informing policy and safeguarding the air we breathe.

Delivering and sustaining this progress depends on collaboration well beyond any single institute or laboratory.