Investigations into the dynamics of aerosols in enclosures as used for air pollution studies

This thesis treats aerosol behaviour under various conditions in enclosed spaces. Knowledge of this behaviour is of importance for the use of aerosol-filled enclosures as a supply of aerosol, as a means for aerosol characterization and for so-called smog chambers for air pollution research (mainly for the investigation of photochemical processes by which usually particles are formed). Chapter 1 gives a brief literature survey, indicating the limited attention given hitherto to aerosol behaviour in enclosed spaces. After decades of almost exclusive interest from meteorologists, since the beginning of the sixties, enclosed aerosols have become an important research subject in nuclear safety. Nuclear aerosol research, however, pertains mainly to high aerosol mass concentrations. In chapter 2 a model is given, describing aerosol removal from enclosed spaces. Usually, aerosol deposition on the walls occurs from a cloud with homogeneous space distribution due to thermal convection. The rate-limiting step of deposition is the transport through the boundary layer at the walls. Relations are derived for a number of removal mechanisms: sedimentation, diffusion, thermophoresis, diffusiophoresis, electrophoresis and photophoresis. From literature data and Fuchs' theory a relation is derived for the dependence of the boundary layer thickness δfor diffusive deposition on the diffusion coefficient D of aerosol particles in usual containments. Boundary conditions for non-stirred deposition of aerosols by sedimentation and diffusion are given. Chapter 3 summarizes the most important means used for experiments. In chapter 4 the experimental results and conclusions are presented. It consists of four sections dealing with unheated dry enclosures, heated dry enclosures, enclosures with a heated pool of liquid, and smog chambers, respectively. Observations on the behaviour of monodisperse and polydisperse aerosols in unheated dry enclosures support the model derived for sedimentation and diffusion. Additionally, the relation between δand D derived in chapter 2 is validated. Particles larger than a few tenths of a micron are removed by sedimentation; particles smaller than about 0.1 μm are subject to diffusive deposition, though often electrophoresis plays an important role (particularly, in case of particles of about 0.1 μm). It is shown that aerosol measurements in containments can be used for determination of dynamic shape factors and aerodynamic diameters. Aerosols in heated dry containments are removed by thermophoresis. Observations on a large variety of aerosols and enclosures are in fair agreement with the model on thermophoretic removal derived in chapter 2. Interpretation of the experimental results yields the thermophoretic properties of aerosols. The results support the theories of Brock and Stetter, and are in disagreement with those of Derjaguin and Epstein. In accordance with theoretical predictions, thermophoretic decay constants are obtained independent of particle size (≥0.5 μm), of particle material and of temperature. Gentle heating of the floor of a containment is shown to have a stabilizing effect on the enclosed aerosol. Introduction of aerosol in a containment with a heated pool of liquid on the floor leads to particle growth by vapour condensation followed by an accelerating removal by sedimentation. Experimental observations support a model describing aerosol behaviour in such systems. The knowledge of this aerosol behaviour allows study of condensation on aerosol particles and particle growth under conditions of variable degrees of supersaturation. The degree of supersaturation can be varied by means of the heating power dissipated in the pool of liquid as well as by means of solving substances in the liquid. Investigations of aerosol behaviour in photochemical smog chambers show insignificant deposition due to thermophoresis or photophoresis under these conditions. Observed aerosol stabilities could be explained by electrophoretic deposition, yielding a coherent picture with electrophoresis in other investigations. Coagulation constants of aerosol could be calculated from measurements on number concentrations of aerosols in enclosed spaces. The results obtained agree fairly well with Davies' theory on coagulation. Decay curves of number concentration of aerosols in metal vessels can be used for assessment of diffusion coefficients.