Diploma Thesis
Fluxes of reactive and non-reactive trace gases near the forest floor
Michael Riederer (02/2008-05/2009)
Support: Thomas Foken, Johannes Lüers
Teilbetreuung durch Prof. Meixner (MPI für Chemie Mainz)
This work investigates fluxes and concentrations of reactive as well as non-reactive trace gases in the ground level layers of the atmosphere in an ecosystem with high vegetation. In the course of the EGER IOP2 experiment at the coniferous Weidenbrunnen investigation site in the Fichtelgebirge Mountains (Northern Bavaria, Germany), data was collected from June 29th to July 2nd. Besides meteorological parameters, collected by an automatic weather station, concentration measurements of the trace gases CO2, H2O, O3, NO and NO2 at five, as well as 220Rn and 222Rn at four sampling heights inside the lowest meter above the forest floor were conducted. Afterwards, miscellaneous modelling approaches, based on, e.g., common profile equations or hydrodynamical research, are used for trace gas flux determination. A hydrodynamical multilayer model, which accounts for the existence of three different layers within the lowest meter of the atmospheric boundary layer, where the transition of molecular to turbulent exchange takes place, provides the most feasible results. These are compared with fluxes measured by eddy covariance technique and static soil chambers. Oftentimes, the spatial heterogeneity of the forest ecosystem originates considerable differences of the fluxes, despite relatively small displacement of the measurement locations. The varying understorey vegetation impeded satisfactory comparisons in case of the sensible and latent heat flux. If these are intended for future experiments, the spatial arrangement of the measurement setup is to be reconsidered. However, this was not a problem for the comparison between eddy covariance determined and modelled O3 fluxes, which fit quite well most of the time and validate the hydrodynamical multilayer modelling approach. The fact that the static soil chamber delivers much lower CO2 and 222Rn surface fluxes, compared to the model, requires a discerning consideration of the applied soil chamber system. This is essential, because the surface flux is a basic parameter of the turbulent eddy diffusion coefficient (K) determination. With an unconventional approach, based on Fick’s first law, K-profiles are provided, as groundwork for further concentration profile modelling, especially of reactive trace gases, which are strongly affected by transportation velocity and residence time. Choosing, e.g., the modelled 222Rn surface flux for the calculations, results, on average, in 1.2∙10-4 to 6.5∙10-3 m2 s-1 higher turbulent eddy diffusion coefficients than using the originally designated soil chamber flux. A modern soil chamber system, with an improved exponential fit of the concentration rise in the chamber, is suggested. In the course of this work, observed differences between modelled and measured fluxes initiated causal research, especially in terms of soil close decoupling events. For the detection of those, a two step surface concentration modelling approach was developed. Compared to empirical surface concentrations, conclusions about separation of coupled and decoupled conditions become possible. The usage of two different non-reactive trace gases 222Rn and water vapour, results in more than 80% consistence.