Morphology, composition, and atmospheric processing of soot particles

Combustion-generated soot aerosols play an important role in climate forcing due to their strong light-absorbing properties. Quantitative measurement of BC is challenging because BC often occurs in highly non-spherical soot particles of complex morphology. The task is further complicated because of the lack of an unambiguous chemical definition of the material. Here we present the development and application of a new technique for determining particle morphology and composition. Simultaneous measurements of mobility diameter, vacuum aerodynamic diameter, and non-refractory composition were used to determine the particle mass, volume, density, composition, dynamic shape factor, and fractal dimension. Under certain conditions, particle surface area and the number and size of the primary spherules composing the soot fractal aggregates were also determined.; The particle characterization technique described above was applied to the following four studies: (1) Characterization of flame-generated soot particles.{09} Depending on flame conditions, either fractal or near-spherical particles were generated and their properties interpreted in terms of the mechanism for soot formation. (2) Coating and denuding experiments were performed on soot particles. The results yielded information about morphology changes to the entire soot particle and to the internal black carbon structure due to atmospheric processing. The denuding experiments also provided particle surface area, which was used to determine the atmospheric lifetime of fractal soot particles relative to spheres. (3) An inter-comparison study of instruments measuring the black carbon content of soot particles was conducted. The detailed characterization of soot particles enabled a number of assumptions about the operation of the selected instruments to be tested. (4) Ambient particles were sampled in Mexico City. In the early morning, ambient particles were detected with a fractal morphology similar to that of diesel particles. As the morning progressed, the fractal particles increased in mass and became more spherical. These changes were attributed to atmospheric processing by photochemistry.