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| Titel |
Effect of air composition (N2, O2, Ar, and H2O) on CO2 and CH4 measurement by wavelength-scanned cavity ring-down spectroscopy: calibration and measurement strategy |
| VerfasserIn |
H. Nara, H. Tanimoto, Y. Tohjima, H. Mukai, Y. Nojiri, K. Katsumata, C. W. Rella |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 5, no. 11 ; Nr. 5, no. 11 (2012-11-12), S.2689-2701 |
| Datensatznummer |
250003168
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| Publikation (Nr.) |
 copernicus.org/amt-5-2689-2012.pdf |
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| Zusammenfassung |
| We examined potential interferences from water vapor and atmospheric
background gases (N2, O2, and Ar), and biases by isotopologues of
target species, on accurate measurement of atmospheric CO2 and CH4
by means of wavelength-scanned cavity ring-down spectroscopy (WS-CRDS).
Changes of the background gas mole fractions in the sample air substantially
impacted the CO2 and CH4 measurements: variation of CO2 and
CH4 due to relative increase of each background gas increased as Ar
< O2 < N2, suggesting similar relation for the
pressure-broadening effects (PBEs) among the background gas. The
pressure-broadening coefficients due to variations in O2 and Ar for
CO2 and CH4 are empirically determined from these experimental
results. Calculated PBEs using the pressure-broadening coefficients are
linearly correlated with the differences between the mole fractions of
O2 and Ar and their ambient abundances. Although the PBEs calculation
showed that impact of natural variation of O2 is negligible on the
CO2 and CH4 measurements, significant bias was inferred for the
measurement of synthetic standard gases. For gas standards balanced with
purified air, the PBEs were estimated to be marginal (up to 0.05 ppm for
CO2 and 0.01 ppb for CH4) although the PBEs were substantial (up
to 0.87 ppm for CO2 and 1.4 ppb for CH4) for standards balanced
with synthetic air. For isotopic biases on CO2 measurements, we
compared experimental results and theoretical calculations, which showed
excellent agreement within their uncertainty. We derived instrument-specific
water correction functions empirically for three WS-CRDS instruments
(Picarro EnviroSense 3000i, G-1301, and G-2301), and evaluated the
transferability of the water correction function from G-1301 among these
instruments. Although the transferability was not proven, no significant
difference was found in the water vapor correction function for the
investigated WS-CRDS instruments as well as the instruments reported in the
past studies within the typical analytical precision at sufficiently low
water concentrations (<0.7% for CO2 and <0.6% for CH4).
For accurate measurements of CO2 and CH4 in ambient
air, we concluded that WS-CRDS measurements should be performed under
complete dehumidification of air samples, or moderate dehumidification
followed by application of a water vapor correction function, along with
calibration by natural air-based standard gases or purified air-balanced
synthetic standard gases with the isotopic correction. |
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