Overview of methods to characterize the mass, size, and morphology of soot

Combustion and other high-temperature processes can produce solid aerosol nanoparticles with complex morphologies, including fractal-like aggregates of primary particles. Characterizing these morphologies, as well as particle mass, is key to understanding their behavior in natural and engineered systems, and it can provide clues to the origin of the particles. We focus here on the characterization of soot, although most of the techniques apply to other aerosol aggregates. A complete description of these aerosols would include the mass and morphology of every particle. In practice, it is possible to obtain detailed information on individual particles from microscopy of extracted samples. A particular focus of this review, tandem classifier/detector systems can determine 2-dimensional mass and mobility distributions that may be interpreted through the lens of fractal models. Very fast in situ light scattering measurements can be used to determine the structure factor, related to fractal dimension, and the aggregate and primary particle size distributions. These approaches are complementary when there are appropriate models to connect morphological details to optical and transport characteristics of the particles. Over the last few decades these models have become more sophisticated, requiring more information on the particle structure and properties, but also facilitating more sophisticated inferences from in-situ and online measurement techniques.