Mechanistic investigation of nanoparticle formation in vehicle exhaust and laboratory experiments

Airborne particles which are ubiquitous in the atmosphere have profound influences on air quality, public health and global climate. On a small urban scale, vehicle-emitted nanoparticles have been shown to have adverse impacts on public health. On a much larger global scale, uncertainties on nanoparticle formation in atmosphere pose an important challenge to our understanding of the indirect radiative forcing of aerosols on global climate change. To mitigate the adverse health impacts as well as better understand the climatic impacts resulting from nanoparticles, a better understanding of their formation mechanisms in various environments is required.;In this thesis, I aim to improve our understanding of nanoparticle formation processes by applying an advanced microphysical model to kinetically investigate nucleation phenomena. In addition, advanced quantum-chemical methods are employed to obtain thermochemistries associated with the initial few steps of the nucleation process. The derived thermochemistries are then incorporated into an aerosol microphysical model to improve the accuracy of nucleation phenomena simulations.;Based on these studies, I came to four main conclusions: (1) For diesel vehicles running on high sulfur fuel or low sulfur fuel but equipped with diesel particulate filters, binary H2SO4-H2O homogeneous nucleation appears to be the dominant mechanism for nanoparticle formation; (2) For vehicles running on low sulfur fuel but not equipped with diesel particulate filters, non-volatile cores produced during engine combustion may be the source of nanoparticles observed in vehicle exhaust; (3) Application of quantum-chemical methods in deriving the thermochemistries of small water clusters substantially reduce systematic over-predictions of nucleation rates by the classical water nucleation theory over wide temperature and saturation ratio ranges; (4) In a case study of a recent laboratory experiment, identification of SO3-(SO3)m(H2O) n clusters of very high stability using quantum-chemical methods suggests that the formation of SO3-(SO3) m(H2O)n may be a dominant pathway for the first few steps of the observed nucleation.;The work presented in this thesis provides insights into nucleation mechanisms in several important environments. They are particularly helpful in developing an optimum strategy to reduce or even remove nanoparticles from vehicle exhaust or understanding the mechanisms of secondary aerosol formation in the lower troposphere.