The current Federal Reference Method (FRM) for measuring concentrations of ozone in ambient air, described in EPA regulations at 40 CFR Part 50, Appendix D, is based on the dry, gas-phase, chemiluminescence reaction between ethylene and any ozone that may be present in air samples. This methodology is technically sound and has well served its role in ozone monitoring and as an FRM for many years since its establishment in 1971. However, the method now no longer meets the full needs of an FRM because no manufacturer of ozone-monitoring instruments currently offers an FRM analyzer for sale. Existing FRM analyzers are largely obsolete and poorly supported (if supported at all) by their manufacturers, and, consequently, all current ozone monitoring is carried out with other types of monitors (equivalent methods). A new FRM for ozone is needed to meet the needs that the existing FRM can no longer fulfill.