Description (deu)
ABSTRACT
Condensation Nuclei Counters (CNC's) are frequently used to measure the particle concentration in the atmosphere. However, it is not clear if all atmospheric particles are activated as condensation nuclei and grow to visible size in order to be detected. Investigations of heterogeneous nucleation and droplet growth in supersaturated vapour are required to determine the detection limit and sensitivity of CNCs. Therefore a process controlled expansion cloud chamber with an online data acquisition system has been developed. The droplets, growing in a fast, pressure defined expansion cloud chamber, are illuminated by a He-Ne laser beam. The intensity of the light, scattered under selectable fixed scattering angle, the transmitted light intensity and the gas pressure are monitored simultaneously during expansion and droplet growth. From these data the initial supersaturation in the chamber and size and concentration of the growing droplets can be derived without any empirical calibration. Droplet growth have been measured at various initial supersaturations. The present droplet growth theory has been extended and good agreement with the experimental data has been found under the assumption of heat and mass accomodation coefficients being unity. The particle concentrations of the urban atmospheric aerosol in Vienna have been measured at various supersaturations. Accordingly Kelvin equivalent size ranges between 0,002 and 0,1 μm diameter. In satisfactory agreement with measurements of Whitby et al. a mode of the number size distribution has been found, which is identified as nucleation mode.