The NIST Standard Reference Photometer (SRP), developed jointly with the U.S. Environmental Protection Agency (USEPA), has provided an infrastructure for the calibration and traceability of ozone measurements within the U.S. since 1983. NIST has also provided SRPs to 22 international laboratories in 18 countries including several SRPs to the International Bureau of Weights and Measures (BIPM) to serve as an ozone reference standard under the International Committee of Weights and Measures (CIPM). In 2005, the BIPM completed a pilot study on ozone comparability (CCQM-P28) in which 23 laboratories participated. The first official key comparison (BIPM.QM-K1) on ozone at ambient levels has recently been completed and will be repeated every two years.
The concentration of ozone in the atmosphere continues to be an important global issue both scientifically and politically. Stratospheric ozone protects the Earth from harmful UV radiation, while tropospheric ozoneis a major health concern and contributes to global climate change as a greenhouse gas. The objective of this project is to maintain the U.S. national standard for ozone measurements and provide traceability to government laboratories, academia, and commercial organizations. We conduct research to improve the SRP system and operational control as needed, and provide new SRPs to government laboratories around the world.
An in-depth study by the BIPM of systematic biases in the SRP has revealed a temperature measurement bias causing an underestimate of ozone mole fractions by an estimated 0.4%, and an optical measurement bias causing an overestimate of ozone mole fractions by an estimated 0.5%. NIST has developed an instrument modification upgrade to greatly reduce or eliminate these biases. Twenty SRPs have been upgraded so far and four new SRPs have been produced with the new instrument modification. Data show the average optical measurement bias to be – 0.67% and the average temperature measurement bias to be + 0.37%, with a net average change in SRP measurement of – 0.30%. Overall agreement of all bias in upgraded SRPs has shown improvement from 0.72% to 0.33%.