Des mesures exactes pour un air pur et un ciel bleu
In the lead-up to the International Day for Clean Air for Blue Skies on 7 September, the Bureau International des Poids et Mesures (BIPM) is highlighting how the science of measurement underpins efforts to monitor and improve air quality around the world. At the heart of this work is a global community committed to accurate, comparable ozone measurement — a cornerstone of clean air data and environmental monitoring.
The importance of accurate ozone measurement
Ozone is a key pollutant used to assess air quality, and measuring it accurately is vital for determining whether air is safe to breathe, identifying trends in pollution and ensuring public reporting is based on reliable science.
What is ground-level ozone?
Unlike the ozone layer in the upper atmosphere, which shields the Earth from UV radiation, ground-level ozone forms when sunlight reacts with pollutants near the Earth’s surface. It can irritate lungs and worsen respiratory conditions — which is why accurately measuring it is essential for air quality assessments.
Recognising the need for stronger alignment and reduced uncertainty in ozone measurement, in recent years the international metrology community has taken significant steps to improve calibration practices and update reference values.
For example, between 2022 and early 2025, the CCQM-GAWG Task Group on Ozone Cross-Section Change — composed of national metrology institutes and international partners — coordinated a global update to the ultraviolet ozone absorption cross-section used in ozone calibration.
The updated value, CCQM.O3.2019, adopted in 2025, is approximately 1.2% lower and six times more precise than the previously used standard from the 1960s.
This value is now being implemented around the world, reinforced by the BIPM.QM-K1 key comparison programme — an ongoing, peer-reviewed comparison between national metrology institutes that underpins international equivalence in ozone photometry and supports the CIPM Mutual Recognition Arrangement (MRA).
This work ensures that ozone measurements around the world are not only accurate but comparable — enabling regulatory agencies, researchers and policymakers to base their decisions on consistent and scientifically validated data.
See where the work is taking place
To mark the International Day for Clean Air for Blue Skies, experts from air quality measurement stations around the world have shared videos to highlight where they are “measuring ozone accurately for clean air and blue skies.” Watch the videos here.
Visit and explore
To learn more about the BIPM’s role in supporting ozone measurement, visit our dedicated page: Better science for more accurate measurements of ground-level ozone — featuring technical guidance, metadata protocols and the latest results from international comparisons.
See our video testimonials from the 'Clean Air for Blue Skies' campaign here.
Photo credits for the images in this article go to air quality and atmospheric monitoring institutions around the world, including stations in Ushuaia, Cape Grim and Sydney, Sonnblick Observatory, the Cabo Verde Atmospheric Observatory, the National Physical Laboratory, Mount Waliguan, CVC and Cali’s Carrera 127 station, LNE, Izaña, Hohenpeissenberg, Bukit Kototabang, Mace Head, Anmyeondo and Baengnyeongdo, inNET Monitoring AG, and national institutions in Mexico and the Netherlands.