10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Impact of the Assumptions of Soot Nanostructure and Aggregation on Particle Sizing Using Time-Resolved Laser-Induced Incandescence
MADHU SINGH, Randy Vander Wal, The Pennsylvania State University
Abstract Number: 386 Working Group: Aerosol Modeling
Abstract Soot emissions contribute to deteriorating air-quality and have an adverse impact on human health and the environment. It has, thus, become imperative to characterize soot for its particle size, morphology, composition and optical properties, for instance, to better understand the material and the consequence of its formation. Towards particle sizing, time-resolved laser-induced incandescence (TiRe-LII) is one such non-intrusive, in-situ optical diagnostic technique used to estimate soot primary particle size. TiRe-LII uses the conductive cooling profile of laser-heated soot to infer primary particle diameter. Conductive cooling is believed to occur after approximately 100 ns of the laser pulse, with sublimation and radiation dominating cooling during the first 100 ns. Here, it is tested experimentally using three model carbon blacks with ranging particle diameters as measured by transmission electron microscopy (TEM). The carbon blacks are dispersed as an aerosol and irradiated by the primary harmonic, 1064 nm, of a Nd:YAG laser at a fluence of 200 mJ/cm2. The incandescence signal, collected by a spectrograph-camera system, is temporally resolved by controlling and stepping out the gate delay in time, allowing the collection of detailed temperature-time profiles. Incandescence data is acquired as the particle cools, from which the particle’s time-temperature history is extracted. Soot particle temperature is calculated by multi-wavelength pyrometry across a 600 nm spectral band by fitting the acquired incandescence spectra to Planck’s black-body radiation law. Good fits to black-body profiles justify the use of a wavelength-independent emissivity with a value equal to unity. This assertion is supported by changed nanostructure and consequently, altered optical properties upon laser annealing. The effect of annealing on nanostructure is shown by TEM images. Aggregate morphology and primary particle size remain equivalent to the original material, while the particle’s nanostructure changes. Based on Kirchoff’s law, an alteration in optical emissivity post annealing is demonstrated by UV-VIS absorption, used here as a surrogate measure of the particle’s emission. This data also shows a reduction in the material’s optical band-gap after annealing, supporting increased π-conjugation and aromatic structure, all pointing to a changed and wavelength-independent emissivity. Primary particle diameters found from fitting experimentally measured TiRe-LII signals with existing analytical and numerical models do not match the particle diameters as directly visualized by TEM. TiRe-LII models typically assume point-contacting spheres and a constant accommodation coefficient in order to generate a temperature profile and infer particle size. The thermal accommodation coefficient is shown to be temperature dependent based on experimental conductive cooling profiles and differs substantially between these materials. Soot particles exist as aggregates and modeling them as point-contacting spheres is an over-simplification leading to inaccurate particle sizing. Aggregate structure in the form of intra-aggregate connectivity and shielding is an underlying cause for erroneous particle sizing. Effect of aggregation and the thermal accommodation coefficient as currently incorporated in current LII models is isolated and demonstrated individually, all other parameters being constant. Time-temperature profiles vary significantly as the degree of aggregation and value of the thermal accommodation coefficient change, pointing to inaccuracies in particle sizing by models.