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| Titel |
Continuous measurements of water vapor isotopic compositions using an integrated cavity output spectrometer: calibrations and applications |
| VerfasserIn |
L. Wang, K. Caylor, D. Dragoni |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021860
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| Zusammenfassung |
| The 18O and 2H of water vapor can be used to investigate couplings between biological
processes (e.g., photosynthesis or transpiration) and hydrologic processes (e.g., evaporation)
and therefore serve as powerful tracers in hydrological cycles. A typical method for
determining δ18O and δ2H fluxes in landscapes is a “Keeling Plot” approach, which uses
field-collected vapor samples coupled with a traditional isotope ratio mass spectrometer
to infer the isotopic composition of evapotranspiration. However, fractionation
accompanying inefficient vapor trapping can lead to large measurement uncertainty and the
intensive laboring involved in cold-trap make it almost impossible for continuous
measurements. Over the last 3-4 years a few groups have developed continuous
approaches for measuring δ18O and δ2H that use laser absorption spectroscopy
(LAS) to achieve accuracy levels similar to lab-based mass spectrometry methods.
Unfortunately, most LAS systems need cryogenic cooling, constant calibration to a
reference gas, and substantial power requirements, which make them unsuitable for
long-term field deployment at remote field sites. In this research, we tested out a
new LAS–based water vapor isotope analyzer (WVIA, Los Gatos Research, Inc,
Mountain View, CA) based on Integrated Cavity Output Spectroscopy (ICOS) and
coupled this instrument with a flux gradient system. The WVIA was calibrated
bi-weekly using a dew point generator and water with known δ18O and δ2H signatures.
The field work was performed at Morgan-Monroe State Forest Ameriflux tower
site (central Indiana) between August 8 and August 27, 2008. The combination
method was able to produce hourly δ18O and δ2H fluxes data with reproducibility
similar to lab-based mass spectrometry methods. Such high temporal resolution
data were also able to capture signatures of canopy and bare soil evaporation to
individual rainfall events. The use of the ICOS water vapor analyzer within a gradient
system has the potential to greatly expand the use of continuous δ18O and δ2H
fluxes measurements to address a wide range of ecohydrological research topics. |
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