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| Titel |
Design and performance of a Nafion dryer for continuous operation at CO2 and CH4 air monitoring sites |
| VerfasserIn |
L. R. Welp, R. F. Keeling, R. F. Weiss, W. Paplawsky, S. Heckman |
| 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 ; 6, no. 5 ; Nr. 6, no. 5 (2013-05-14), S.1217-1226 |
| Datensatznummer |
250017889
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| Publikation (Nr.) |
 copernicus.org/amt-6-1217-2013.pdf |
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| Zusammenfassung |
| In preparation for routine deployment in a network of greenhouse gas
monitoring stations, we have designed and tested a simple method for drying
ambient air to near or below 0.2% (2000 ppm) mole fraction
H2O using a Nafion dryer. The inlet system was designed for use with
cavity ring-down spectrometer (CRDS) analyzers such as the Picarro model
G2301 that measure H2O in addition to their principal analytes, in
this case CO2 and CH4. These analyzers report dry-gas mixing ratios without drying
the sample by measuring H2O mixing ratio at
the same frequency as the main analytes, and then correcting for the dilution
and peak broadening effects of H2O on the mixing ratios of the other
analytes measured in moist air. However, it is difficult to accurately
validate the water vapor correction
in the field. By substantially lowering the amount of H2O in the
sample, uncertainties in the applied water vapor corrections can be reduced
by an order of magnitude or more, thus eliminating the need to determine
instrument-specific water vapor correction coefficients and to verify the
stability over time. Our Nafion drying inlet system takes advantage of the
extra capacity of the analyzer pump to redirect 30% of the dry gas
exiting the Nafion to the outer shell side of the dryer and has no
consumables. We tested the Nafion dryer against a cryotrap
(−97 °C) method for removing H2O and found that in wet-air
tests, the Nafion reduces the CO2 dry-gas mixing ratios of the sample gas
by as much as 0.1 ± 0.01 ppm due to leakage across the
membrane. The effect on CH4 was smaller and varied within
± 0.2 ppb, with an approximate uncertainty of 0.1 ppb.
The Nafion-induced CO2 bias is partially offset by sending the dry
reference gases through the Nafion dryer as well. The residual bias due to
the impact of moisture differences between sample and reference gas on the
permeation through the Nafion was approximately −0.05 ppm for
CO2 and varied within ± 0.2 ppb for CH4. The
uncertainty of this partial drying method is within the WMO compatibility
guidelines for the Northern Hemisphere, 0.1 ppm for CO2 and
2 ppb for CH4, and is comparable to experimentally
determining water vapor corrections for each instrument but less subject to
concerns of possible drift in these corrections. |
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