Abstract
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.
Highlights
Earth SystemAtmospheric carbon monoxide (SCOc)ieplnaycseansimportant roleH2O mole fractions between 0–4 %
Alaska from April to November in 2011, and the accuracy sorption spectroscopy (TDLASS)o(lSidachEseaert tahl., 1987), closed of the CO measurements by the cavity ring-down spectroscopy (CRDS) analyzer has been path Fourier transform infrared (FTIR) absorption (Griffith validated against discrete NOAA/ESRL flask sample mea- et al, 2012), gas chromatography combined with a mercuric surements made on board the same aircraft, with a mean dif- oxide detector or a flame ionization detector (GC/HgO or ference between integrated in situ and flask measurements GC/FID) (Novelli, 1999) and, more recently, quantum casof −0.6 ppb and a standard deviation of 2.8 ppb
In the case of CO measurements by the CRDS technique, there are a number of factors that complicate the correlation between CO mole fraction in wet air (COwet) and COdry: (1) the linear term in Eq (1) is not equal to −1 if water vapor is not accurately determined; (2) a nonlinear term exists as a result of pressure broadening; (3) residual line interference remains due to imperfect quantifications of adjacent absorption lines of CO2 and H2O
Summary
A major advantage of some recently available techniques (QCL/ICOS/CRDS) is that water vapor mole fractions can be simultaneously measured. This makes measurements of CO in humid air feasible as the H2O values can be used to correct for dilution and potential spectroscopic effects. Carbon monoxide is detected by QCL or ICOS in the mid-infrared region where the CO absorption is well separated from other absorption features; in contrast, the CRDS technique measures CO in the near-infrared region (∼ 1.57 μm) where there are significant interferences from adjacent absorption lines of CO2 and H2O at ambient mole fractions of CO, CO2, and H2O.
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