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

含碳燃料的燃烧通常会产生含碳纳米颗粒——“烟灰”或“黑碳”，通常是直径dp~15-40纳米的“初级颗粒”的分形聚集体。柴油发动机、森林火灾或工业过程中产生的烟尘是每年造成500万至900万人死于空气污染的主要原因，也是导致气候变暖的主要因素。另一方面，有意生产的“黑碳”是第一批纳米结构材料之一，是用于传感器、催化剂支撑和能源系统的新材料家族的一部分，可能是燃料脱碳的途径。碳纳米颗粒的行为取决于颗粒的结构，其特征在于初级颗粒的尺寸分布、聚集结构和碳的亚纳米准化学排列（如石墨畴的尺寸）。有了这种结构，就有可能预测这些粒子将如何与光相互作用、沉积在肺部或被收集到气体净化装置中。烟尘形成的主流模型是，首先，初级颗粒形成，然后布朗混凝产生含有初级颗粒的聚集体，这些初级颗粒来自燃烧系统的许多部分。我的团队有了一个发现，对这个模型进行了重大更新，这是本提案的重点。我们发现，对于大多数燃烧系统来说，在燃烧室形成到排放到大气中的这段时间里，每个烟灰集合体都不与其他烟灰颗粒相互作用。因此，所排放的煤烟聚集体携带有关燃烧条件在设备中分布的信息，在材料性质和形态上有所不同。例如，平均而言，大小团聚体似乎来自火焰的不同部分，并且具有不同的初级粒径和不同程度的石墨化。由于煤烟颗粒是煤烟的初级颗粒，其粒径和性质在聚集体内部是均匀的，但在聚集体之间却有很大的差异，因此这种新的煤烟结构模型被称为外部混合假说（EMH）。在接下来的5年里，我的团队将通过开发更好的图像处理方法（需要获得高分辨率的煤烟结构信息）、气溶胶动力学模型（需要将排气后的条件与火焰中的条件联系起来）和更好的煤烟测量方法（利用EMH为我们提供了比以前更准确的煤烟结构表示）来探索EMH。这些都是迈向定量理解烟尘形成和开发气溶胶测量方法的重要一步，这些方法可以支持环境监测、环境科学和工程纳米颗粒合成——这些领域不仅将使用这项研究，而且还将为在这里训练的HQP提供就业机会（3名本科生、1名MASc学生和2名博士生）。

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).