Understanding of the complex morphology and optical properties of combustion-generated carbonaceous aerosols has been a challenging research topic. Calculation of aerosol radiative forcing and satellite aerosol retrievals depends critically on the knowledge of aerosol optical properties, which are a function of particle morphology, size, and refractive index. In addition, aerosol morphology is an important control parameter in industrial aerosol generation and use. Ensembles of aerosols often include a variety of complex morphologies, but these morphologies currently cannot be separated and very little is known about their influence on other aerosol parameters. In this dissertation, a novel charge-based technique for classifying fractal-like aerosol agglomerates based on their morphology is demonstrated. Using this technique, the formation mechanism and optical properties of fractal-like carbonaceous aerosols from a high-temperature combustion system (premixed flame) are investigated. Contrary to previous observations of a universal mass fractal dimension of ≈1.8 for fractal-like aerosol aggregates formed in the dilute-limit of a premixed flame via 3-dimensional diffusion-limited cluster aggregation (DLCA) processes, minority populations (≈3%) of aggregates yielding low mass fractal dimensions between 1.2 and 1.51 were observed. Two hypotheses are presented to explain this observation.
To improve our understanding of the validity of optical theories for fractal-like chain aggregates, real-time optical measurements of fractal-like aggregates were compared with the prediction by three optical theories, namely Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). The RDG approximation agreed within 10% with the experimental results and the exact electromagnetic calculations of the IEFS theory, while volume-equivalent Mie theory overpredicted the experimental scattering coefficient by a factor of ≈3.2. The theoretical and experimental values of scattering and absorption coefficients agreed well within their uncertainties (≈10%) for the traditional soot refractive index value of 1.57- i 0.56.
In contrast to the fractal-like aggregates produced by sooting flames, low-temperature smoldering combustion of commonly occurring wildland fuels produced different carbonaceous particles, that is amorphous, spherical “tar balls”, which had only recently been named. Optical measurements performed using real-time, in situ instrumentations indicated that brown carbon, a form of organic carbon (OC), which exhibits strong spectrally-dependent light absorption, is an important component of tar balls.
This dissertation concludes with the development of a novel software package to simulate the growth of 3-dimensional carbonaceous fractal agglomerates and create their 2-dimensional pixelated projection images by restricting them to stable orientations as commonly encountered for real world fractal agglomerates collected on filter media for electron microscopy.
|Commitee:||Arnott, William P., Cline, Joseph I., Herald, Christopher M., Leitner, David M., Zielinska, Barbara|
|School:||University of Nevada, Reno|
|School Location:||United States -- Nevada|
|Source:||DAI-B 69/12, Dissertation Abstracts International|
|Subjects:||Atmospheric sciences, Optics|
|Keywords:||Aerosols, Brown carbon, Carbonaceous aerosols, Fractal, Morphology, Optics, Soot|
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