Accurate measurements of carbon monoxide in humid air using the cavity ring-down spectroscopy (CRDS) technique

Abstract. Accurate measurements of carbon monoxide (CO) in humid air have been made using the cavity ring-down spectroscopy (CRDS) technique. The measurements of CO mole fractions are determined from the strength of its spectral absorption in the near-infrared region (~1.57 μm) after removing interferences from adjacent carbon dioxide (CO₂) and water vapor (H₂O) absorption lines. Water correction functions that account for the dilution and pressure-broadening effects as well as absorption line interferences from adjacent CO₂ and H₂O lines have been derived for CO₂ mole fractions between 360–390 ppm and for reported H₂O mole fractions between 0–4%. The line interference corrections are independent of CO mole fractions. The dependence of the line interference correction on CO₂ abundance is estimated to be approximately −0.3 ppb/100 ppm CO₂ for dry mole fractions of CO. Comparisons of water correction functions from different analyzers of the same type show significant differences, making it necessary to perform instrument-specific water tests for each individual analyzer. The CRDS analyzer was flown on an aircraft in Alaska from April to November in 2011, and the accuracy of the CO measurements by the CRDS analyzer has been validated against discrete NOAA/ESRL flask sample measurements made on board the same aircraft, with a mean difference between integrated in situ and flask measurements of −0.6 ppb and a standard deviation of 2.8 ppb. Preliminary testing of CRDS instrumentation that employs improved spectroscopic model functions for CO₂, H₂O, and CO to fit the raw spectral data (available since the beginning of 2012) indicates a smaller water vapor dependence than the models discussed here, but more work is necessary to fully validate the performance. The CRDS technique provides an accurate and low-maintenance method of monitoring the atmospheric dry mole fractions of CO in humid air streams.