This thesis discusses the formation of aerosol particles by homogeneous nucleation of supersaturated vapor, and the subsequent or simultaneous growth of particles by condensation. Experiments, theory, and numerical simulations are used to approach the underlying goal of understanding the aerosol evolution process in photochemically reactive systems, such as Los Angeles smog.
A comprehensive size-sectionalized model was developed for simulating the evolution of a multicomponent aerosol size distribution through homogeneous nucleation, condensational growth, coagulation, and various deposition mechanisms.When applied to atmospheric photochemistry, the model predicted that the number of new particles nucleated is controlled by the ratio between the rates of homogeneous nucleation and condensational growth. A simple model was devised for predicting the number and size evolution of particles which would be formed by a burst of homogeneous nucleation. An interesting aspect of the model was its prediction of suppression of homogeneous nucleation by seed aerosol through bulk vapor depletion. Later these predictions were verified qualitatively in two systems. One was a physiochemically well characterized system where nucleation was driven by a high initial supersaturation ratio, in which nucleation was faster than predicted by classical nucleation theory, and suppression of nucleation was only slight. The second system was our outdoor smog chamber.
In a large outdoor smog chamber, toluene and NOx were allowed to photochemically react. Gas phase concentrations and the resulting aerosol distribution were followed with time, for various initial concentrations of reactants and seed aerosol. A few thousand seed particles per cm3 (sub-ambient concentrations) were sufficient to suppress homogeneous nucleation that would have resulted in several times as many particles. Operation of the chamber in dual mode allowed the influence of a single parameter, varied between the two sides of the bag, to be clearly observed, thus avoiding many of the difficulties that arise from comparing experiments conducted at different times and different temperature and sunlight histories.
