Experimentelle Bestimmung der Depositionsgeschwindigkeit luftgetragener Partikel mit Hilfe der Eddy-Kovarianzmethode über einem Fichtenaltbestand im Solling

The aim of this dissertation was to determine the deposition velocity of atmospheric particles for a spruce stand as a function of the particle size and the atmospheric conditions using the eddy correlation method. For this purpose, measurements with a high time resolution (1 Hz or more) are needed for particle concentration (as a function of particle size) as well as for vertical wind velocity at one point above the forest. An optical particle counter PCS-2000 (Palas) was used for the measurement of particle concentration, and a sonic anemometer USAT3 (Metek) was used for the measurement of vertical wind velocity. The measurements were part of the research program "Atmospharische Aerosolforschung" of the Federal Ministry of Education and Research, and were carried out at the Solling site in Germany. The PCS-2000 is able to detect atmospheric particles between 0.2 and 10 µm diameter. For this range, a mean particle concentration of about 800 particles per square centimeter was found with the maximum of concentration in the lowest size class. Generally, the diary of particle concentration seems to depend strongly on the relative humidity of the air. In detail, the particle concentration depends on a variety of processes, such as particle transport, particle formation, particle modification and especially particle deposition. The relative contribution of each of these processes to the resulting amount of concentration has to be calculated using sophisticated models. At times with high relative humidity, a significant shift of the concentration maximum from less then 0.2 µm diameter to between 1 and 2 µm diameter was found for the particle spectrum. During this times, the spectrum is mainly determined by fog particles. This implication could be confirmed by control measurements with a digital camera, too. Unfortunately, time series of particle concentration with resolutions of 1 Hz and more registered by the PCS-2000 measurements are extremely noisy. This phenomenon can be completely traced back to the measurement technique of the PCS-2000 which detects each particle separately. As a direct result of the high statistical uncertainties of very fast concentration measurements, the deposition velocities calculated from data measured by the PCS-2000 are characterized by a high statistical error. This error can be keenly predicted using the (measured or expected) standard deviation of vertical wind speed, the mean particle concentration, the volume of air which is analyzed by the device for particles per time, and the measurement frequency. The lower the particle concentration and the lower the volume of air which is analyzed for particles per time, the higher is the statistical error of the calculated particle flux and deposition velocity. Furthermore, particle fluxes determined by the eddy correlation method are unrealistically high and not explainable by material fluxes between atmosphere and vegetation at times with high relative humidity. This phenomenon is caused by very fast and continuous size changes of the particles in the humid air. Altogether, a reasonable analysis of the recorded data set was only possible for a small part of the whole measurement period (in respect to particle concentration, wind speed and humidity) and generally only for the lower part of the particle spectrum detected by the PCS-2000. For the whole measurement period, mean deposition velocities of 1.05 cm/s (0.22 µm diameter particle size), 1.23 cm/s (0.30 µm) and 1.47 (0.42 µm) were found. This means that deposition velocity grows with the particle size in the investigated range from 0,2 to 0,5 µm diameter. The amount of deposition velocity depends mainly on the intensity of mechanically induced turbulence as well as on the thermal stability of the atmosphere. The most important task for future micro-meteorological investigations relating to particle deposition should be the design and the development of reasonable particle analyzer for fast, size-specific concentration measurements.