Carbon nanoparticle structure: implications for environmental impacts, measurement and formation

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Combustion of carbon-containing fuels often results in carbonaceous nanoparticles - "soot" or "black carbon", typically fractal aggregates of "primary particles" with diameter dp~15-40 nanometers. Soot produced in diesel engines, forest fires, or industrial processes is a major contributor to 5-9 million air pollution deaths annually, and a major climate-warming agent. On the other hand, intentionally produced "black carbon" was one of the first nanostructured materials, part of a family of new materials used for sensors, catalyst supports, and energy systems, and possibly a route to fuel decarbonization.  The behavior of carbon nanoparticles depends on structure of the particles, characterized by the size distribution of the primary particles, the aggregate structure, and the sub-nanometer quasi-chemical arrangement of carbon (eg, size of graphitic domains). Given this structure, it is possible to predict how these particles will interact with light, deposit in lungs, or be collected in gas cleaning devices. The prevailing model for soot formation is that first, primary particles form, then Brownian coagulation produces aggregates containing primary particles drawn from many parts of the combustion system. My team has made a discovery resulting in a significant update to this model, the focus of this proposal. What we have found is that for most combustion systems, each soot aggregate does not interact with other soot particles between the time it is formed in a combustion chamber and the time that it is emitted to the atmosphere.  As a result, the emitted soot aggregates carry information about the distribution of combustion conditions in the device, differing in material properties and morphology.  For example, on average, large and small aggregates appear to come from different parts of the flame, and have different primary particle size and different degrees of graphitization.  Because the soot particles are soot primary particle sizes and properties are uniform within aggregates but very different between aggregates, this new structural model of soot is called the External Mixing Hypothesis (EMH). Over the next 5 years my team will explore the EMH through development of better image processing methods (needed to obtain high-resolution soot structural information), aerosol dynamics models (needed to connect post-exhaust conditions to in-flame conditions) and better soot measurement methods (capitalizing on the EMH which provides us with a much more accurate representation of soot structure than previously available).  These are important steps towards a long term vision of understanding soot formation quantitatively and developing aerosol measurement methods that can support environmental monitoring, environmental science, and engineered nanoparticle synthesis- the fields that will not only use this research but provide career opportunities for the HQP trained here (3 undergraduates, 1 MASc student, and 2 PhD students).