Fundamental Studies of Nanoparticle Formation in Air Pollution
Projects
Nucleation in aqueous systems
Water is one of the most important species involved in atmospheric and industrial processes. New particle formation in the atmosphere almost always involves water, while in industry aerodynamic and turbomechanical flows are strongly impacted by nucleation and condensation, and water is usually one of the key components. The experimental portion of the program is investigating phase transitions in supersonic nozzles in aqueous systems using conventional pressure trace measurements, small angle neutron scattering (SANS) experiments, and Tunable Diode Laser Absorption Spectroscopy.
Despite impressive advances in the field of nucleation research, it is still not possible to reliably calculate the absolute nucleation rates from the gas phase as a function of gas phase composition and temperature. An approach based on molecular simulations, for example transition state theory that accurately describes the growth and evaporation kinetics of small clusters, would be invaluable. Our approach is to use statistical simulations that have the accuracy of ab initio calculations at a computational cost that is affordable for large-scale simulations. The method is unique in that it is self-correcting and places the bulk of computational effort in the configurations that dominate the process being simulated.
Nuggets: D2O Spectroscopy; Boundary Layers
Soot Inception
Soot is emitted from both mobile and stationary sources and is one of the major sources of atmospheric particulate matter. Understanding the fundamental aspects of soot formation has been hampered by a lack of suitable instrumentation to quantify and molecularly characterize the incipient soot particles in the 1 – 10 nm size range. In this project we are generating soot in a flat flame burner and characterizing the size distributions of the incipient particles directly by sampling using a scanning mobility particle sizer (SMPS) that has a lower cutoff diameter of 3nm, as well as in situ using small angle neutron scattering (SANS). The molecular components in the incipient particles are characterized with several aerosol mass spectrometry techniques.
The starting point for the theoretical work is a soot model that includes gas phase reaction mechanism, particle inception, growth, coagulation, and, finally, particle transport in flames. Many practical combustion processes contain metals that can enhance or reduce soot production. Experiments are also examining the interaction between metals and soot by generating particles from ethylene-oxygen flames seeded with ferrocene. Mathematical models are being developed that incorporate molecular mechanisms of metal oxide particle inception and carbon deposition.
Nuggets: SANS from flames; Direct size distribution measurements in flames; chemical composition of early soot particles
Nucleation in the Atmosphere
A key component of this research is linking atmospheric nucleation events to those observed in the laboratory and explored experimentally. To facilitate this, we renovated a laboratory at the Bodega Marine Laboratory (BML), on the Pacific Coast about 100 miles north of San Francisco, to provide a clean environment with power and network capabilities for remote sensing of nucleation events. BML is ideally suited for these measurements because a) it is a research laboratory that is part of the UC Davis campus, the home of one of the PIs on this project (Wexler), b) the site is very clean in that there are few local emissions and the wind is mostly out of the west, and c) nucleation in the marine boundary layer is very important for global energy balance especially the indirect effect of clouds.
Nuggets: Aerosol evolution near roadways; Nucleation in a clean marine environment
