Biosphere-Atmosphere Gas Exchange Measurements using Fourier Transform Infrared Spectrometry

Field measurements of biosphere-atmosphere gas exchange are of great importance because they provide the possibility to study greenhouse gas dynamics and its feedback mechanisms in detail. This thesis contributes to the further development of concentration and flux measurement techniques to study biosphere-atmosphere exchange processes, by exploring the possibilities of using an in-situ Fourier Transform Infrared (FTIR)-analyzer for ecosystem research. This instrument is capable of measuring CO2, CH4, N2O, CO, and δ CO2 simultaneously. It was combined with different flux measurement techniques, such as the flux gradient technique, the ratio-nocturnal boundary layer technique, and the flux chamber technique. The system was used in four different field campaigns and several laboratory studies, and details about the system were published in van Asperen et al. (2015a). This thesis focuses on the use of the system to a) apply and assess different (new) flux measurement techniques, and b) study different flux and ecosystem processes. Several aspects of different flux measurement techniques were assessed. The parameterization of the diffusion coefficient, required for the flux gradient technique, was studied. In this thesis, it is shown that common parameterizations from the literature underestimate the ecosystem CO2 fluxes. A new type of parameterization, which combines eddy covariance diffusion measurements and meteorological parameterizations, is described and evaluated. This approach enables reliable flux gradient measurements for multiple gases. Furthermore, a new flux measurement method, the ratio-nocturnal boundary layer (R-NBL) technique was tested. The R-NBL technique infers the fluxes from the simultaneous increase of at least two gases in the boundary layer, and the accompanying (eddy covariance) flux measurements of one of the gases. This technique was compared to eddy covariance flux measurements for the first time, and a good agreement was demonstrated. The measurements show that the R-NBL technique is able to detect very small N2O fluxes, and a detection limit of 0.004 nmol m −2 s−1 for N2O fluxes was estimated. Such a low detection limit is not reached by other micrometeorological techniques, which makes the R-NBL technique very suitable for measuring trace gas fluxes in homogeneous ecosystems. In addition to the direct flux measurements, the FTIR-analyzer was also employed in a forest ecosystem to investigate the spatial variation of gas concentrations in a forest, which is important in order to better understand the storage of gases below the forest canopy. The storage component is important for the flux calculations from forest ecosystems. Horizontal concentration measurements inside the canopy showed large spatial and temporal variation of gas concentrations within 10 meters distance. The vertical concentration profile was found to be very different for different gases. For correct determination of the storage component, it was concluded that multiple vertical concentration profile measurements are needed within the canopy. Different process level studies were performed by use of the measurement set-up. N2O production mechanisms could be studied in a 15N-labeling experiment, in which different agricultural fertilizers were used. The results showed that the FTIR-analyzer is capable of measuring different isotopologues and isotopomers of N2O at low concentrations. The experiment revealed the fast and relatively large loss of fertilizer-nitrogen (1%) via N2O emission right after fertilizer application. Furthermore, the role of photoand thermal degradation in arid ecosystem carbon dynamics could be studied in the field and the laboratory. No photodegradation induced CO2 and CO fluxes were found in the field. Thermal degradation fluxes were observed in the field (for CO) and in the laboratory (for CO and CO2). The thermal CO production in the field was