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

Probing the subtropical lowermost stratosphere and the tropical upper troposphere and tropopause layer for inorganic bromine

“…The instruments of the NASA-ATTREX package most important for the present study consist of a fast UV photometer for measurement of ozone (Gao et al, 2012), a gas chromatograph (Unmanned Aircraft System Chromatograph for Atmospheric Trace Species -UCATS; Wofsy et al, 2011, andMoore et al, 2003) as well as a Picarro instrument (Harvard University Picarro Cavity Ringdown Spectrometer -HUPCRS; Crosson, 2008;Rella et al, 2013;Chen et al, 2013) to measure CH 4 , CO 2 , and CO, a whole-air sampler (GWAS; Schauffler et al, 1998Schauffler et al, , 1999 to analyze a large suite of stable trace gases, and a three-channel scanning limb mini-DOAS (differential optical absorption spectroscopy) instrument for spectroscopic detection of O 3 , NO 2 , BrO, OClO, IO, O 4 , O 2 , H 2 O vapor , H 2 O liquid , and H 2 O solid in the UV-visnear-IR spectral ranges (e.g., Weidner et al, 2005;Platt and Stutz, 2008;Kritten et al, 2010Kritten et al, , 2014Kreycy et al, 2013;Stutz et al, 2016).…”

“…The Picarro analyzer uses wavelength-scanned cavity ringdown spectroscopy (WS-CRDS) technology to make high-precision measurements of greenhouse gases (Crosson, 2008;Rella et al, 2013;Chen et al, 2013). HUPCRS reports concentrations of CO 2 , CH 4 , and CO every ∼ 2.2 s and the data are averaged to 10 s. In-flight precision for CH 4 is 0.2 ppb in 10 s. Briefly, the analyzer uses three distributed feedback (DFB) diode lasers in the spectral region of 1.55 to 1.65 µm.…”

Comparison of nitrous oxide (N<sub>2</sub>O) analyzers for high-precision measurements of atmospheric mole fractions

“…ments tested provide water vapor measurements and a correction function to transfer wet ambient air measurements to the dry mole fraction. This correction accounts for dilution and spectroscopic effects such as pressure broadening (Chen et al, 2013). In this study, we test the water vapor correction applied by the manufacturer of the different instruments.…”

Evaluation of a new JMA aircraft flask sampling system and laboratory trace gas analysis system

“…The mean biases are nearly the same or smaller than the WMO-recommended limits of 2 ppb for CH 4 and 0.1 ppb for N 2 O, but the standard deviations are still relatively large compared to the mean values when several outliers with larger errors by the GC analyses were omitted. and Chen et al (2013) reported that the difference between the continuous (CRDS) and flask (GC/FID) aircraft measurements for CH 4 showed a small bias of 0.4 ± 1.3 ppb during their field intercomparisons. Our results are consistent with these previous studies showing that the bias in measuring CH 4 mole fraction by our WS-CRDS analyzer is smaller than the WMO's interlaboratory compatibility goal.…”