HOMOGENEOUS NUCLEATION IN A LAMINAR FLOW DIFFUSION - INTRODUCTION

New particle formation by homogeneous nucleation has been experimental ly studied in this thesis. The nucleation rate was measured using a la minar flow diffusion chamber technique, where flow containing vapour i s rapidly cooled and critical supersaturations for homogeneous nucleat ion have been obtained. The characteristics of this method have been e xamined and the general principles of the method have been applied to two different measurement systems. Nucleation rate of dibutyl phthalat e (DBP) vapour has been measured using a chamber, which was constructe d and developed in the University of Helsinki. The nucleation rate was measured in the temperature range from 246 K to 317 K using nitrogen as a carrier gas. The effect of carrier gas on the nucleation process was examined repeating some of the measurements in helium and argon. T he measured data agree well with other data in the literature and are considered as reliable. However, significant discrepancy between vario us theoretical predictions and experimental results was found. It is c oncluded that the nucleation theories based on macroscopic measurables can not completely describe microscopical phenomena of nucleation. Th e laminar flow diffusion chamber method for nucleation rate measuremen ts was applied to a commercial instrument used for aerosol particle de tection (Ultrafine Condensation Particle Counter). The measurements we re performed for n-butanol vapour. Nitrogen, helium and air were used as carrier gases. It was found that this system is not suitable in its present form for quantitative measurements of nucleation rate. Howeve r, the results are explained qualitatively in view of the investigatio ns of this work.