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| Titel |
The stability and calibration of water vapor isotope ratio measurements during long-term deployments |
| VerfasserIn |
A. Bailey, D. Noone, M. Berkelhammer, H. C. Steen-Larsen, P. Sato |
| 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 ; 8, no. 10 ; Nr. 8, no. 10 (2015-10-27), S.4521-4538 |
| Datensatznummer |
250116652
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| Publikation (Nr.) |
 copernicus.org/amt-8-4521-2015.pdf |
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| Zusammenfassung |
| With the recent advent of commercial laser absorption spectrometers, field
studies measuring stable isotope ratios of hydrogen and oxygen in water
vapor have proliferated. These pioneering analyses have provided invaluable
feedback about best strategies for optimizing instrumental accuracy, yet
questions still remain about instrument performance and calibration
approaches for multi-year field deployments. With clear scientific potential
for using these instruments to carry out monitoring of the hydrological
cycle, this study examines the long-term stability of the isotopic biases
associated with three cavity-enhanced laser absorption
spectrometers – calibrated with different systems and approaches – at two
remote field sites: Mauna Loa Observatory, Hawaii, USA, and Greenland
Environmental Observatory, Summit, Greenland. The analysis pays particular
attention to the stability of measurement dependencies on water vapor
concentration and also evaluates whether these so-called concentration
dependences are sensitive to statistical curve-fitting choices or
measurement hysteresis. The results suggest evidence of monthly-to-seasonal
concentration-dependence variability – which likely stems from low
signal-to-noise at the humidity-range extremes – but no long-term
directional drift. At Mauna Loa, where the isotopic analyzer is calibrated
by injection of liquid water standards into a vaporizer, the largest source
of inaccuracy in characterizing the concentration dependence stems from an
insufficient density of calibration points at low water vapor volume mixing
ratios. In comparison, at Summit, the largest source of inaccuracy is
measurement hysteresis associated with interactions between the reference
vapor, generated by a custom dew point generator, and the sample tubing.
Nevertheless, prediction errors associated with correcting the concentration
dependence are small compared to total measurement uncertainty. At both
sites, changes in measurement repeatability that are not predicted by
long-term linear drift estimates are a larger source of error, highlighting
the importance of measuring isotopic standards with minimal or well
characterized drift at regular intervals. Challenges in monitoring isotopic
drift are discussed in light of the different calibration systems evaluated. |
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