Ambient ozone measurements in the U.S. and many other countries are traceable to a National Institute of Standards and Technology Standard Reference Photometer (NIST SRP). The NIST SRP serves as the highest level ozone reference standard in the U.S. with units located at NIST, and many U.S. EPA laboratories. The International Bureau of Weights and Measures (BIPM) maintain a NIST SRP as the reference standard for international measurement comparability through the International Committee of Weights and Measures (CIPM). In total there are currently NIST SRPs located in 20 countries for use as an ozone reference standard. A detailed examination of the NIST SRP by the BIPM and NIST has revealed a temperature gradient and optical path-length bias inherent in all NIST SRPs. A temperature gradient along the absorption cells causes incorrect temperature measurements by as much as 2° C. Additionally, the RTD probe used for temperature measurements was found to inaccurately measure the temperature of the sample gas due to a self heating affect. Multiple internal reflections within the absorption cells produces an actual path-length longer than the measured fixed length used in the calculations for ozone mole fractions. Reflections from optical filters located at the exit of the absorption cells adds to this affect. Since all NIST SRPs are essentially identical, the temperature and path-length bias exists on all units by varying amounts dependent upon instrument settings, laboratory conditions, and absorption cell window alignment. This paper will discuss the cause of and physical modifications for reducing these measurement biases in NIST SRPs. Results from actual NIST SRP bias upgrades quantifying the affects of these measurement biases on ozone measurements will also be summarized.
Citation: Journal of the Air & Waste Management Association
Pub Type: Journals
bias, inter-comparison, ozone, standard reference photometer, upgrade