Improved measurements of soot using new particle morphology models

点击翻译按钮获取中文摘要

The effect of aerosols (including "soot") on climate is large but also has a large uncertainty.  Failure to reduce climate-active emissions may force humanity to disperse light-scattering particles in the atmosphere to prevent runaway global warming. A precise knowledge of the role of aerosols in climate will be needed to do this "geoengineering".****Particles are the largest contributors to air-pollution-related deaths, and black carbon from engines may be particularly toxic, but it is not known which chemical or physical characteristics of particles cause this toxicity. Particle emissions from many devices are regulated and the regulations may become more sophisticated, placing new requirements on emission measurement equipment. Particle structure complicates these measurements.****Aerosol instruments respond to non-spherical aerosols in different ways depending on the measurement principle (mass, mobility, stopping distance, light scattering, absorption, count), and the particle morphology as well as material properties affect the measurement. For decades, aggregate aerosols (such as soot) have been modeled as fractals composed of "primary particles" of diameter dp, with a fractal dimension of Df~1.8. The contact between particles and short range configuration is captured in the "fractal prefactor" kF. This 3-parameter geometric model has been a very useful approximation but may have been taken as far as it can go. Researchers have long observed (based on TEM) that primary particles do not have a uniform size but the size variations were assumed to be randomly dispersed through the aerosol population. Recently we have shown, for the first time,  systematic trends of primary particle size with aggregate size. Effectively, the soot from real combustion systems is an "external mixture" of nearly ideal fractal aggregates with very different primary particle diameters. These variations can be important because the important aerosol physical characteristics are typically non-linear functions of primary particle diameter: the use of a single average primary particle diameter in models will result in an incorrect prediction of physical properties (or incorrect inversion of aerosol measurements). The challenge is to develop a simple model that can capture key variations - the core objective of the proposed research. We plan to do this using our extensive TEM image database with corresponding measurements by SMPS, CPMA, light scattering and absorption measurements.****A better model of structure opens a new window onto the physical processes of particle formation. The relation between combustion conditions and the particle morphology will be studied in two controlled experimental campaigns (a model gas flare and and an optical engine) and interpreted through a new simulation of particle formation.  The proposed funding will leverage other funding to support 3 high-quality PhD projects.**