Heterogeneous Nucleation of Water Vapor on Monodispersed Ag and NaCl Particles with Diameters Between 6 and 18 nm

Abstract

During the 1980 workshop of the working group on ultrafine aerosols (WUFA), the heterogeneous nucleation of supersaturated water vapor on well-defined aerosols was investigated. Ultrafine Ag and NaCl aerosols were generated using a constant temperature tube furnace. By means of electrostatic classification of the polydispersed primary aerosol, monodispersed fractions with electrical mobility particle diameters of 6, 8, 12, and 18 nm were obtained. After passing through a humidifier, the vapor-saturated monodispersed aerosol entered a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC)). Supersaturation was then achieved by a fast adiabatic expansion, and heterogeneous nucleation was initiated by aerosol particles acting as condensation nuclei. The number concentrations of the droplets growing in the expansion chamber were measured for stepwise increasing supersaturations. At well-defined “critical” saturation ratios, the onset of heterogeneous nucleation was observed. For the insoluble, hydrophobic Ag-aero-sols, the “critical” supersaturations were found to be considerably higher than the values calculated according to Kelvin's equation. For the soluble, hygroscopic NaO aerosols, however, the measured “critical” supersaturations were much lower than predicted by Kelvin's equation. Thus, a significant dependence of the heterogeneous nucleation process on particle size and chemical composition was observed experimentally. Quantitative comparison of the experimental “critical” supersaturations with calculations based on the macroscopic theory of heterogeneous nucleation shows satisfactory agreement for Ag particles, whereas for NaCl particles some deviations are encountered. Increasing the supersaturation beyond the “critical” value generally leads to a constant droplet number concentration, indicating that all aerosol particles are activated as condensation nuclei. Thus, even the considered ultrafine hydrophobic aerosol particles can in principle be detected by means of condensation nuclei counters if sufficiently high vapor supersaturations are achieved in the measuring system.