Browsing by Author "Haverd, V"

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    Absolute calibration of the intramolecular site preference of N-15 fractionation in tropospheric N2O by FT-IR spectroscopy
    (American Chemical Society, 2009-03-15) Griffith, DWT; Parkes, SD; Haverd, V; Paton-Walsh, C; Wilson, SR
    Nitrous oxide (N2O) plays important roles in atmospheric chemistry both as a greenhouse gas and in stratospheric ozone depletion. Isotopic measurements of N2O have provided an invaluable insight into understanding its atmospheric sources and sinks. The preference for N-15 fractionation between the central and terminal positions (the "site preference") is particularly valuable because it depends principally on the processes involved in N2O production or consumption, rather than the N-15 content of the substrate from which it is formed. Despite the value of measurements of the site preference, there is no internationally recognized standard reference material of accurately known and accepted site preference, and there has been some lack of agreement in published studies aimed at providing such a standard. Previous work has been based on isotope ratio mass spectrometry (IRMS); in this work we provide an absolute calibration for the intramolecular site preference of N-15 fractionation of working standard gases used in our laboratory by a completely independent technique-high-resolution Fourier transform infrared (FT-IR) spectroscopy. By reference to this absolute calibration, we determine the site preference for 25 samples of tropospheric N2O collected under clean air conditions to be 19.8 parts per thousand +/- 2.1 parts per thousand. This result is in agreement with that based on the earlier absolute calibration of Toyoda and Yoshida (Toyoda, S.; Yoshida, N. Anal. Chem. 1999, 71, 4711-4718) who found an average tropospheric site preference of 18.7 parts per thousand +/- 2.2 parts per thousand. We now recommend an interlaboratory exchange of working standard N2O gases as the next step to providing an international reference standard. © 2009, American Chemical Society
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    Measured deuterium in water vapour concentration does not improve the constraint on the partitioning of evapotranspiration in a tall forest canopy, as estimated using a soil vegetation atmosphere transfer model
    (Elsevier, 2011-06-15) Haverd, V; Cuntz, M; Griffith, DWT; Keitel, C; Tadros, CV; Twining, JR
    Partitioning the evapotanspiration (ET) flux in a forest into its component fluxes is important for understanding the water and carbon budgets of the ecosystem. We use non-linear parameter estimation to determine the vertical profile of the Lagrangian timescale (T(L)) and partitioning of ET that simultaneously optimise agreement between modelled and measured vertical profiles of temperature, water vapour, carbon dioxide concentrations, and deuterated water vapour for a two-week period in November 2006. High precision real-time trace gas measurements were obtained by FTIR spectroscopy. Modelled temperature and concentration profiles are generated using a Lagrangian dispersion theory combined with source/sink distributions of HDO, H(2)O, sensible heat, and CO(2). These distributions are derived from an isotopically enabled multilayer Soil Vegetation Atmospheric Transfer (SVAT) model subject to multiple constraints. The soil component of the model was tested in isolation using measured deuterium content of soil chamber evaporate, while the leaf component was tested using isotopic analyses of leaf and xylem water, combined with leaf-level gas exchange measurements. Optimisation of T(L) and the partition of ET was performed twice: once using only temperature, H(2)O and CO(2) profiles and a second time including HDO as well. The modelled vertical concentration profiles resulting from inclusion of HDO in the cost function demonstrate our ability to make consistent estimates of both the scalar source distributions and the deuterium content of the water vapour sources. However, introducing measurements of deuterium in water vapour does not significantly alter resulting estimates of normalised T(L) (0.4 +/- 0.1 at canopy top) and the partition of ET(85 +/- 2% transpiration), suggesting that the additional data and modelling required to use deuterium are not warranted for the purpose of partitioning ET using the framework presented here. Crown Copyright (C) 2011 Published by Elsevier B.V.