Intercomparison of fog water samplers

Abstract

During the Po Valley Fog Experiment 1989, 2 fogwater collectors were operated simultaneously at the ground and the results were compared to each other. The chemical analyses of the samples as well as the collection efficiencies showed remarkable differences between both collectors. Some differences in the solute concentrations in the samples of both collectors could be expected due to small differences in the 50% cut-off diameters (5 < d₅₀ < 7 μm). The large differences in the collection efficiencies however cannot be explained by these small variations of d₅₀, because normally only a small fraction of the water mass is concentrated in the size range of 5-7 μm droplets. It will be shown, that it is not sufficient to characterize a fogwater collector only by its cut-off diameter. The results of several wind tunnel calibration tests show, that the collection efficiencies of the fogwater collectors are a function of windspeed and shape of the droplet spectra. Calculations to simulate the performance of each collector give an idea of the shape of the collection efficiency curves, especially for the larger droplets. Differences in fogwater concentrations sampled simultaneously in 2 levels of the FISBAT-tower (Fuzzi et al. 1992) by identical collectors can occur due to changing collection efficiencies caused by different windspeeds. Since it is also possible to simulate the time evolution of the solute concentration in the fogwater samples, it will be shown, that changes in the solute concentration can be explained to a large extent by changing microphysical conditions.