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The NIST Standard Reference Photometer for Ozone Measurement Traceability


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.


Photograph of the NIST Standard Reference Photometer showing the component modules consisting of optical bench with sample and reference tubes, ozone generator module with controls and tubing, and control module with numeric LED displays.

The NIST Standard Reference Photometer (SRP), developed jointly with the USEPA to provide measurement traceability for ozone measurements. 

Credit: Lane Sander

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.

Major Accomplishments

  • 42 NIST SRPs have been produced with several more in progress
  • 24 laboratories in 19 countries maintain an SRP  
  • USEPA has maintained a network of 11 SRPs, traceable to NIST, for 25 years
  • SRP used as BIPM ozone reference standard for international traceability


    1: On loan to UBA-Germany; 2: Delivered October 2001; 3: Built by BIPM staff at BIPM

Additional Technical Details

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%.


  1. Viallon, J., Idrees, F., Moussay, P., Wielgosz, R., Norris, J. E., and Hodges, J. T., "Final report, on-going key comparison BIPM.QM-K1, ozone at ambient level, comparison with NIST, June 2019," Metrologia, 57, (2020).
  2. Hodges, J. T., Viallon, J., Brewer, P. J., Drouin, B. J., Gorshelev, V., Janssen, C., Lee, S., Possolo, A., Smith, M. A. H., Walden, J., and Wielgosz, R. I., "Recommendation of a consensus value of the ozone absorption cross-section at 253.65 nm based on a literature review," Metrologia, 56, (2019). 
  3. Rhoderick, G. C., Kelley, M. E., Miller, W. R., Norris, J. E., Carney, J., Gameson, L., Cecelsld, C. E., Harris, K. J., Goodman, C. A., Srivastava, A., and Hodges, J. T., "NIST Standards for Measurement, Instrument Calibration, and Quantification of Gaseous Atmospheric Compounds," Analytical Chemistry, 90, 4711-4718 (2018).
  4. Leiva, M. A., Araya, C., Mancilla, C., Seguel, R., and Norris, J. E., "Uncertainty of ozone measurements with the primary standard reference photometer (SRP45)," Talanta, 86, 71-81 (2011).
  5. Viallon, J., Moussay, P., Norris, J. E., Guenther, F. R., and Wielgosz, R. I., "A study of systematic biases and measurement uncertainties in ozone mole fraction measurements with the NIST Standard Reference Photometer," Metrologia, 43, 441-450 (2006).
Created January 24, 2009, Updated November 7, 2023