PhD Thesis
Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem
Thomas Wrzesinsky (01/1999-02/2004)
Support: Otto Klemm
Summary
For mountainous forests in Central Europe fog deposition is likely to play an important role in the cycling of water, nutrients, and pollutants. Quantifying the amount of occult deposition was very difficult in the past due to technical limitations. In this project a deposition measurement setup was developed, tested and operated at the ecosystem research site “Waldstein”. The system consisted of a fast droplet spectrometer (measuring range from 1.5 to 50 µm diameter in 40 size classes) and an ultrasonic anemometer to measure the vertical wind component. From April 2001 through March 2002 a continuous data set of fogwater deposition had been obtained. In addition, visibility and the chemical composition of fog water (pH, electrical conductivity, Na+, K+, NH4+, Mg2+, Ca2+, Cl–, NO3–, SO42– und PO43–) were determined. Fog water was collected using an active heatable cloudwater collector which was developed during this project. Samples were taken every 8 h during fog events. The median sample volume was 249 ml. A total of 223 days with fog were observed during the measuring period. The percentage of fog occurrence was 25.7 %.
Quality control of data included stationarity and turbulence tests. Corrected data sets were then used for flux calculations. During one year of measurements (2001/2002) a total water input of 108 kg ha–1 by turbulent deposition and 17 kg ha–1 by sedimentation was observed. Hence, turbulent deposition is dominating the total fog precipitation of 125 mm with a percentage of 86 %. Fogwater deposition shows a significant seasonal cycle. The highest monthly amounts were observed during late fall and winter (24 mm in January), lowest during the summertime (1 mm in August). Measured droplet count spectra show peak values at diameters of 2, 6, and 9 µm, respectively. The median droplet mass spectrum has a maximum at 12 µm. Typical mass spectra have peak values at 9, 12, or 15 µm diameter. For the liquid water content the median is 156 mg m–3 with a maximum of 2639 mg m–3 (5-min averages). The largest contribution to total fogwater flux is in the droplet diameter class from 14.5 to 15.5 µm. Droplets smaller than 7 µm are emitted (net balance), whereas larger droplets are deposited. For the measuring period the amount of precipitation by rain and snow was 1414 mm. Therefore, the percentage of fogwater input amounts to about 8 % of the total input.
During the measurements 253 samples of fog water were taken. For comparison, wet only precipitation samples were taken on a weekly basis, too. The chemical composition of fog and rain water is highly variable. For fog water, median pH was 4.14, median concentrations were 621 µeq l–1 for NH4+, 487 µeq l–1 for NO3–, and 321 µeq l–1 for SO42–, respectively. These three major ions contribute 87 % to the total concentration. Fogwater concentrations are significantly higher than those in rain. Enrichment factors are 18.1 (NH4+), 13.1 (NO3–), and 11.5 (SO42–), respectively.
The fog deposition for the main ions was calculated from concentrations and fogwater fluxes. The inputs are 9.8 kg ha–1 for NH4+ (7.9 kg ha–1 wet only), 27.9 kg ha–1 for NO3– (25.1) and 14.0 kg ha–1 for SO42– (15.0), respectively. Therefore, occult deposition is a major contributor to total ion inputs at that site, playing approximately the same role as rain and snow precipitation. Nitrogen input is 13.9 kg N ha–1 a–1 (11.8 wet only). Throughfall measurements for the measuring period show a nitrogen input of 23.3 kg N ha–1 a–1. Hence, throughfall is nearly the same as the sum of fog, rain, and snow. The role of dry deposition (e.g., the input of nitrate by particles or gaseous HNO3) in the ecosystem balance has not been quantified and has to be considered when trying to close the nitrogen cycles as well as other nutrient fluxes. Another aspect is the turnover within the tree canopy which may play a significant role in closing the balance.