10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Characterization Of Laser Derivatized Soot Nanostructure Towards Identifying Its Source
MADHU SINGH, Randy Vander Wal, The Pennsylvania State University
Abstract Number: 383 Working Group: Carbonaceous Aerosol
Abstract Soot source identification by characterizing nanostructure of the laser annealed material is based on the premise that soots originating from different combustion sources differ in their nanostructure because of the variation in formation conditions. Conditions of temperature, pressure, fuel, fuel-air-ratio, fuel-air mixing are specific to each source during combustion and differ between sources. Thus, soot produced from different sources will vary to reflect its formation conditions. TEM imaging with the application of image analysis algorithms is used to quantify nanostructure parameters. When sufficiently distinct, the contrasting nanostructure can identify the soot origin. Where similar nanostructure or poorly contrasted nanostructure is observed, subtle differences may be magnified by laser derivatization to enhance lamellae recognition and quantification. Derivatization is a process by which a compound is chemically changed, producing a new compound that has properties amenable to a particular analytical method. Laser derivatization is used here in a similar manner to enhance subtle differences in soot nanostructure upon laser heating. Due to the extremely rapid heating (~ 5 nanoseconds to reach 3700°C) and cooling (microsecond) timescales observed when soot is irradiated with a Q-switched laser pulse, soots anneal incompletely due to apparent kinetic limitations on these timescales. Convolved with these changes are small differences in initial nanostructure and composition that become magnified by the fast heating and rapid quenching.
Source identification is important to pollution receptor models that require knowledge of source contributions at receptor sites. In this study soot samples from known sources are imaged via transmission electron microscopy (TEM) before and after laser heat treatment. Image processing algorithms quantify nanostructure characteristics including fringe length, fringe tortuosity, fringe spacing, stacking and curvature for nascent and laser heated soot. These metrics collectively comprise the identification marker or “fingerprint” for soot. Unknown samples can then be compared against information available from existing samples to identify its source or have a close estimate of what the source may be, both qualitatively and quantitatively. Quantitative comparison for identification uses statistical analyses. By this approach, laser derivatization uses the changed soot nanostructure post laser heat treatment to identify the soot source.