5.1 Vapour stable water isotopes in different urban vegetation

This study showed that extended (i.e. >2 months) periods, continuous sequential in-situ monitoring of δvproduces reliable ad novel data in urban green space environments with a temperate climate. Our distributed network of inlet ports sampling the atmospheric boundary at different heights above contrasting urban green space vegetation produced reliable high-resolution data with a 2-hourly resolution for each inlet. However, there is no doubt that the method is very labour intensive and requires almost daily maintenance including checking the correct operation of the CRDS, ventilation systems and pumps. Detailed, biweekly data checks of the different inlets are also necessary to detect condensation in the tubes or other unwanted memory effects in the CRDS. In particular, the in-situ setup requires a secured environment for the CRDS and vapour tubing (e.g. a locked and fenced box and securing pipes adapted to the study site). Overall, however, in-situ monitoring of δv needs less regular maintenance than in-situ soil water monitoring due to greater condensation issues from deep soil compared to atmospheric vapour (cf. ).
Monitored δv data fluctuated around the LMWL in equal distribution over the entire study period indicating no dominance of non-equilibrium fractionation , but disequilibrium occurred at shorter time scales. We found limited difference between the two vegetation covers reflecting a generally well mixed boundary. δv of grassland showed a slightly higher temporal variability and also higher variance along the height profile compared to the tree site. The only significant difference was that the surface air (at 0.15 m height) above the grassland was more enriched, though this was rapidly attenuated with height. An in-situ study by Griffis et al. found similar effects of surface evaporation enriching surface boundary layer water vapour and atmospheric loss of light vapour fraction above grassland through the underlying process of kinetic fractionation during evaporation , while tree canopy protects from such loss.
At the event scale, δv showed clear isotopic responses after rain. The response timing was dependent on the time of day being more marked around noon when radiation input is elevated. This is due to the fact that δv at hourly timescale is controlled by airmass advection which increases with higher solar radiation . At seasonal scale, lc-excess was low in summer and higher in autumn reflecting higher ET in warmer months. Griffis et al. explained the seasonal amplitude of δv to be driven by Rayleigh processes that are strongly modulated by evaporation and entrainment, i.e. inflow of an air parcel to another.