Chemistry and Physics of Incipient Soot

Dr. Hai Wang
Mechanical and Aerospace Engineering
University of Southern California

Ultra fine soot particles do not contribute to the mass of air particulates significantly, but they can have notable impact in human health and the climate. Small soot particles can penetrate deeper into human airways and be absorbed and transported more effectively into the blood stream than large particles. Soot may act as Cloud Condensation Nuclei (CCN), increase the cloud density, and perturb the regional to global climate. In addition, the light absorbing and scattering properties of soot are critical to the global, atmospheric energy balances. All of these processes are more directly related to particle size distribution and chemical compositions than particle mass. It will be inevitable for future regulatory developments to consider these additional particle properties. For the same reason, the existing soot models will have to be drastically improved beyond a prediction of particle mass. They should be able to predict the particle size distribution function (PSDF) with some accuracy. Secondly, these models should be able to follow the evolution of chemical composition of soot particles in flames. Soot formation is also one of the unsolved fundamental combustion problems. Although we are now able to predict the rate of soot formation to some degree of accuracy, our understanding of particle nucleation and mass growth in the nanometer size range remains vague. For example, in most soot models particle nucleation is assumed to begin with the coalescence of two polycyclic aromatic hydrocarbon (PAH) molecules. Yet, by the thermodynamic argument it is obvious that the condensed-phase PAHs should preferentially evaporate rather than condense in flames. Recent theoretical studies (e.g., kinetic Monte Carlo and molecular dynamics) have shed some new light on the nucleation processes. However, the validity of theoretical findings is yet to be compared to experiments.

Over the past few years, we have developed and used an array of techniques to follow the nucleation and growth of incipient soot particles. These techniques include Small Angle Neutron Scattering, probe sampling/Scanning Mobility Particle Sizer, Atomic Force Microscopy and Particle Ionization Aerosol Mass Spectrometry. These techniques allowed us to look at soot nucleation and mass growth in laminar steady premixed flames at a resolution unseen before. What emerged from a joint considerations of various experimental observations are (1) particle size distribution functions are generally bimodal, indicating that the soot nucleation follows a second-order reaction kinetics with respect to the monomer; (2) incipient soot particles can have C/H ratios as small as 1 and can contain a large amount of aliphatics or aliphatic side chains; (3) these particles are generally liquid-like in that they spread on a substrate upon impact; (4) nanometer sized particles can undergo mass growth without the presence of H atoms in the gas phase, and as such the HACA mechanism appears to be incomplete to explain soot mass growth in flames.

Hai Wang received his Ph.D. in Fuel Science from Penn State in 1992. He was a Professional Research Staff at Princeton University from 1994 to 1996. He joined the faculty of Mechanical Engineering at the University of Delaware in 1997. Currently, he is a Professor of Aerospace and Mechanical Engineering at the University of Southern California. He was the recipient of the NSF CAREER Award in 1999. His research interests include combustion, high-temperature chemical kinetics, heterogeneous atmospheric chemistry, catalysis, hypersonics, nanomaterials synthesis, nano-structured chemical sensors, and photovoltaic.