Towards an optical gas standard for ammonia measurements in air at ambient levels

We describe thorough metrological characterization of a cavity ring-down spectrometer, combined with proprietary spectral evaluation algorithm. Our measurements show good linearity, good accuracy and short response time of the instrument in the 50-400 nmol/mol amount fraction range. Our spectral evaluation algorithm enables decreasing the offset to 0.12 nmol/mol from the values around 0.6-0.9 nmol/mol reported using the built-in data evaluation of the commercial spectrometer. We have successfully eliminated cross-sensitivity by water vapor via an advanced spectral fitting approach using line parameters published in the HITRAN2020 database. Our spectrometer has been compared to a traceable reference gas standard and the results show that it is a good candidate to become an optical gas standard for ammonia amount fraction measurements under laboratory conditions.