Diurnal Evolution of the Oxygen Isotope Anomaly of CO2 above a Forest Ecosystem
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The oxygen isotope anomaly (∆17O) of atmospheric CO2 has been proposed as a better tracer of the gross CO2 fluxes between the atmosphere and the biosphere than the δ18O signal, due to physiochemical processes in the biosphere following mass-dependent isotope fractionation. In the first part of this thesis, we prepared and characterised an automated system to measure the oxygen isotope anomaly (∆17O) of atmospheric CO2. The fractionation factors of the CO2-O2 isotope exchange reaction are determined at different reaction temperatures experimentally, and the results confirm that the fractionation factor has a temperature dependence. There is an offset between the fractionation factors found experimentally in this study and fractionation factors which are calculated theoretically. The reason for this offset is still unknown, however, it could be due to real gas effects compared to ideal gas effects, or due to inconsistencies between the isotopic scales. In the second part of this thesis, a mixed layer model simulating a convective atmospheric boundary layer above a forest ecosystem is used to simulate the diurnal evolution of the ∆17O signature of atmospheric CO2. The model is used to investigate the relative contributions from entrainment, soil, and plant to the temporal budget of ∆17O of atmospheric CO2 under different meteorological conditions, and we find that the contribution from the plant is the most sensitive to changes in the meteorology (specifically, humidity), indicating the temporal budget is dependent on photosynthesis activity.