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

摘要

Accurate measurements of carbon monoxide (CO) in humid air have been madeusing the cavity ring-down spectroscopy (CRDS) technique. The measurementsof CO mole fractions are determined from the strength of its spectralabsorption in the near-infrared region (~1.57 μm) afterremoving interferences from adjacent carbon dioxide (CO₂) and watervapor (H₂O) absorption lines. Water correction functions that accountfor the dilution and pressure-broadening effects as well as absorption lineinterferences from adjacent CO₂ and H₂O lines have been derivedfor CO₂ mole fractions between 360–390 ppm and for reported H₂Omole fractions between 0–4%. The line interference corrections areindependent of CO mole fractions. The dependence of the line interferencecorrection on CO₂ abundance is estimated to be approximately ?0.3 ppb/100 ppmCO₂ for dry mole fractions of CO. Comparisons of watercorrection functions from different analyzers of the same type showsignificant differences, making it necessary to perform instrument-specificwater tests for each individual analyzer. The CRDS analyzer was flown on anaircraft in Alaska from April to November in 2011, and the accuracy of theCO measurements by the CRDS analyzer has been validated against discreteNOAA/ESRL flask sample measurements made on board the same aircraft, with amean difference between integrated in situ and flask measurements of?0.6 ppb and a standard deviation of 2.8 ppb. Preliminary testing of CRDSinstrumentation that employs improved spectroscopic model functions forCO₂, H₂O, and CO to fit the raw spectral data (available since thebeginning of 2012) indicates a smaller water vapor dependence than themodels discussed here, but more work is necessary to fully validate theperformance. The CRDS technique provides an accurate and low-maintenancemethod of monitoring the atmospheric dry mole fractions of CO in humid airstreams.