Proposal Number: CTS-0553764; Principal Investigator: Violi, Angela Institution: University of Michigan Ann Arbor
Particulate emissions in the nanoparticle size range are related to two pressing environmental problems - the health impacts of fine particles and global warming. Epidemiological studies have shown a correlation between increased morbidity and increases in measured ambient particulate concentrations. The high number concentration and small size of nanoparticles lead to high rates of deposition deep in the lung. The ultrafine particles (< 0.1 micron) have been found to promote acute pulmonary response, and they impair the ability of the macrophages (the scavenger cells in lungs) to engulf and remove particles from the extracellular milieu. Therefore, nanoparticles emitted by combustion sources are a very serious health concern because of both their size and the carcinogens with which they are associated. It is therefore important to characterize the chemical and physical properties of atmospheric particles. Combustion is the main process through which man continuously injects particles into the atmosphere. More importantly, these particles are produced at the smallest sizes physically possible in the form of clusters with nanometric dimensions The key feature of this proposal is its innovative multiscale characterization of nano-particle formation in combustion environments, through the use of novel simulation methodologies at disparate (spatial/temporal) regimes. The use of atomistic models, such as Molecular Dynamics, can allow us to follow the transformations that occur during nanoparticle formation in a chemically specific way, providing information on both the chemical structure and the configuration of the nanoparticles and their agglomeration. Carbonaceous nanoparticle agglomeration is influenced by large length and time scale motions that extend to mesoscopic scales, i.e., one micrometer or more in length and one microsecond or more in time. In order to increase the time and length scales accessible in simulations and be able to simulate nanoparticle assembly, it is necessary to describe the particles on a more coarse-grained (CG) level. The primary research objective of this proposal is to study nanoparticle coagulation and assembly using an unique multi-scale coarse-graining approach. With this methodology the effective forces between whole groups of atoms in the molecular system are mapped into much simpler effective forces for coarse-grained sites on the molecules (or nanoparticles). These resulting forces are, in effect, the potential of mean forces between the coarse-grained sites (i.e., groupings of atoms). As a result, the effective phase space of the system is significantly reduced in size, as are the number of costly force calculations. The CG model will allow the simulations of the nanoparticle systems in this project to bridge upward in both length and time scale, so as to better access the properties influenced by those scales. This approach provides a connection between the various time and length scales in the nanoparticle self-assembly problem, together with an unprecedented opportunity for the understanding of the atomistic interactions underlying carbonaceous nanoparticle structures and growth. The integrated multi-scale simulation approach provides a detailed, molecular level description of the structure of fine particles, which are generated during the combustion of hydrocarbons. With the results obtained using this novel approach, it will be possible to significantly broaden this research into important directions, such as the influence of these particles on human health and global warming. The integration of experimental studies and theoretical simulations, in multi-level time and particle size scales, will increase the overall understanding of nanoparticle formation and transformation. This is an important aspect of the intellectual contribution, beyond the increased scientific understanding at any one scale. The research activities proposed will advance discovery through the development of computational models that will elucidate for the first time ever the mechanisms of nanoparticle formation in a chemically specific way. The broader educational impacts of the proposed project will be significant as well, through the direct involvement of graduate students in the research project, as well as outreach activities in scientific education and to underrepresented groups. The research and educational activities combined will advance understanding through the teaching of computational chemistry and molecular modeling, which will provide students the opportunity to gain a deeper understanding of the physical and chemical processes involved using these multimedia-based approaches for effective teaching.
