Measurement of outdoor ozone: consequences of a revised absorption cross section

This Technical Brief reviews the importance of measuring ground level ozone in outdoor air and the challenges of regulating a secondary pollutant formed through photochemical reactions. It describes the principal measurement approach used in air quality monitoring - UV photometric analysis at 253.65 nm. The adoption of a newly recommended ozone absorption cross section value in 2025 is then discussed. This has been derived from a re-evaluation of historical and modern experimental data. This new value is 1.23% lower and with substantially reduced uncertainty when compared with the value previously used. The practical consequences of this change, including higher reported ozone mass concentrations and possible increases in regulatory exceedances, are considered.