Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapor isotopes

Global Biogeochemical Cycles (Impact Factor: 4.53). 03/2009; 23. DOI: 10.1029/2008GB003331

ABSTRACT 1] The carbon and oxygen isotopes of CO 2 and the oxygen isotopes of H 2 O are powerful tracers for constraining the dynamics of carbon uptake and water flux on land. The role of land biota in the atmospheric budgets of these isotopes has been extensively explored through the lens of leaf-scale observations. At the ecosystem scale, kinetic fractionation is associated with molecular and turbulent diffusion. Intuitively, air turbulence, being nondiscriminative in diffusing materials, should act to erase the kinetic effect. Using the first canopy-scale isotopic flux measurements, we show just the opposite: that in the terrestrial environment, air turbulence enhances the effect, rather than suppressing it. The sensitivity of kinetic fractionation to turbulence is striking in situations where the canopy resistance is comparable to or lower than the aerodynamic resistance. Accounting for turbulent diffusion greatly improves land surface model predictions of the isoforcing of 18 O-CO 2 and transpiration enrichment of leaf water in 18 O-H 2 O in field conditions. Our results suggest that variations in surface roughness across the landscape can contribute to spatial variations in the composition of atmospheric 18 O-CO 2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric 18 O-CO 2 . In comparison, air turbulence has a limited effect on the isoforcing of 13 C-CO 2 .

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    ABSTRACT: Studying the evaporation process and its link to the atmospheric circulation is central for a better understanding of the feedbacks between the surface water components and the atmosphere. Stable water isotopes are ideal tools to investigate surface evaporation as they are naturally available tracers of water phase changes in the atmosphere. The strength of isotope fractionation processes depends on environmental conditions such as relative humidity and temperature. In this study, we use five months of deuterium excess (d) measurements at the hourly to daily timescale from a cavity ring-down laser spectrometer to characterise the evaporation source of low-level continental water vapour at the long-term hydrometeorological monitoring site Rietholzbach in northeastern Switzerland. To reconstruct the phase change history of the air masses in which we measure the d signature and to diagnose its area of surface evaporation we apply an established Lagrangian moisture source diagnostic. With the help of a correlation analysis we investigate the strength of the relation between d measurements and the moisture source conditions. Temporal episodes with a duration of a few days of strong anticorrelation between d and relative humidity as well as temperature are identified. The role of plant transpiration, the large-scale advection of remotely evaporated moisture, the local boundary layer dynamics at the measurement site and recent precipitation at the site of evaporation are discussed as reasons for the existence of these modes of strong anticorrelation between d and moisture source conditions. The relation between d in atmospheric water vapour at the measurement site and the relative humidity conditions at the location of evaporation exhibits distinct characteristics for land surface evaporation and ocean evaporation. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux can be deduced from the d-relative humidity relation at the seasonal timescale as well as for individual events. The slope of the relation between d and the diagnosed moisture source relative humidity provides a novel framework to estimate the transpiration fraction of land evapotranspiration at the local to continental scale. Over the whole analysis period (August to December 2011) a transpiration fraction of the evapotranspiration flux over the continental part of the moisture source region of 63% is found albeit with a large event-to-event variability (0% to 99%) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 82% (varying between 65% and 94%) is found. Such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important land-atmosphere coupling parameter.
    Atmospheric Chemistry and Physics 11/2013; 13(11):29721-29784. · 4.88 Impact Factor
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    ABSTRACT: [1] This study investigates the response of the global mean and spatial variations of the δ18O value of atmospheric CO2 (δCa) to changes in soil CO2 hydration rates, relative humidity, the δ18O value of precipitation and water vapor, visible radiation, temperature, and ecosystem flux partitioning. A three-dimensional global transport model was coupled to a mechanistic land-surface model and was used to calculate isotopic fluxes of CO2 and H2O and the resulting δCa. The model reproduced the observed global mean and north-south gradient in δCa. The simulated seasonal amplitude and phases of CO2 and δCa agreed well at some but not all locations. Sensitivity tests with relative humidity increased by 3.2% from its original value decreased δCa by 0.21‰. Similarly, a global 3.3‰ decrease in the isotopic composition of both precipitation and water vapor (δWP and δWAV, respectively) caused a 2.6‰ decrease in δCa. A 1 K increase in atmospheric temperatures also affected δCa, but there was a very small δCa response to realistic changes in light levels. Experiments where leaf and soil CO2 fluxes were repartitioned revealed a non-trivial change to δCa. The predicted north-south δCa gradient increased in response to an increase in soil CO2 hydration rates. However, the δCa gradient also had a large response to global changes in δWP and δWAV. This result is particularly important since most models fail to deplete δWP enough at middle and high-latitudes, where the influence of δWP and δWAV on the δCa gradient is strongest.
    Journal of Geophysical Research: Biogeosciences 11/2013; · 3.02 Impact Factor
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    ABSTRACT: We have developed a mid-infrared continuous-wave quantum cascade laser direct-absorption spectrometer (QCLS) capable of high frequency (≥1 Hz) measurements of 12CH4 and 13CH4 isotopologues of methane (CH4) with in situ 1-s RMS ? precision of 1.5 ‰ and Allan-minimum precision of 0.2 ‰. We deployed this QCLS in a well-studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg CH4 m-2 hr-1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 ± 0.8 mgCH4 m-2 hr-1) over the same period. Mean ? composition of emitted CH4 calculated using EC isoflux methods was -71 ± 8 ‰ (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of -64.5 ± 0.8 ‰. Ebullitive fluxes, representing ˜10% of total CH4 fluxes at this site, were on average 1.2 ‰ enriched in 13C compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process-based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.
    Journal of Geophysical Research Atmospheres 05/2012; 117(D10):10301-. · 3.44 Impact Factor

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