Standard reference photometer for the assay of ozone in calibration atmospheres

Abstract

A photometric instrument for the assay of ozone from 0 ppbv to 1 000 ppbv in calibration atmospheres has been designed, constructed, and evaluated.The absorption cross-section of ozone at 254 nm (Hg line) is the major component of uncertainty of the instrument.A number of workers have determined this cross- section to be about 1 .147 x 10' 17 cnr/molecule (or equivalently 308.32 atm' 1 cm' 1 , base e), with a standard uncertainty of 1 .0%.To a lesser extent, the uncertainty of the instrument also depends on the uncertainty of the temperature, pressure, and transmittance measurements on the ozone sample.Based upon ozone calibration needs of the U.S. EPA, the design specifications called for an instrument with a standard uncertainty of 2 ppbv in the range of 1 ppbv to 100 ppbv and 2 % in the range of 100 ppbv to 1000 ppbv.Thirty-one of these instruments, called Standard Reference Photometers (SRP), have been built since 1983; each SRP has met or exceeded the design specifications.Two of these instruments are maintained at the National Institute of Standards and Technology (NIST), one serving as NIST's principal ozone standard, and the other serving as a backup and quality system check instrument.Ten additional instruments of the same design are maintained at various EPA or state operated laboratories across the United States to facilitate local access to authoritative ozone standards.The current network of SRPs also includes instruments maintained in Australia, Austria, Canada, the Czech Republic, France, Germany, Portugal, Spain, Switzerland, Sweden, Taiwan R.O.C. and the United Kingdom.Each of the SRPs provides ozone assays with a certified uncertainty similar to that of a Standard Reference Material (SRM) obtained from NIST. 1.

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