Doctoral Dissertation Oral Defense of Farnaz Khosravi
Aug 7, 2025 11:00 AM | The formation of soot nanoparticles in combustion environments remains one of the most complex and least understood phenomena in aerosol science, particularly at the early stages where molecular precursors transition into solid-phase nanoparticles. This dissertation focuses on advancing the detection and characterization of flame-formed carbonaceous and metal aerosols smaller than 5 nm by advancing the application of state-of-the-art aerosol diagnostic techniques. The primary focus of this dissertation is the integration of High-Resolution Differential Mobility Analysis (HR-DMA) coupled with a novel Condensation Particle Counting (CPC) diagnostic technique to achieve high-resolution measurements of the size distribution functions (SDFs) of sub-5 nm particles. This dissertation evaluates the performance of a prototype Water CPC (WCPC) in detecting hydrophobic sub-5 nm charged and neutral carbonaceous flame-formed aerosols. To enhance condensation growth activation of the smallest possible aerosols within a WCPC, a two-stage saturator is installed at the WCPC inlet and operated with different working fluid, which is either DiEthylene Glycol (DEG), normal Butanol (nBA), Isopropanol (IPA), or Ethanol (EtOH). Experimental results demonstrate that the concurrent supersaturation of water and fluid enhances the detection of charged materials as small as 1.5 nm with 50% efficiency. Notably, the evaporative flow generated in the saturator inlet serves as a natural sheath, directing the aerosol sample toward the centerline of the growth tube, where supersaturation and temperature fields are more uniform, enabling the fluid-WCPC to be used also as an aerosol sizing instrument. This dissertation further advances the detection of electrically neutral nanosized aerosols, an area that has remained largely unexplored due to the inherent difficulties in measuring uncharged particles. By adapting HR-DMA and fluid-WCPC systems, the dissertation presents the first robust approach for detecting and characterizing neutral sub-5 nm aerosols. This capability addresses a critical gap in the current understanding of soot inception, enabling direct investigation of the early-stage formation of both neutral and naturally charged flame-formed aerosols. I also provide a tool to monitor and sample both hydrophilic and hydrophobic aerosols smaller than 5nm in the atmosphere. Additionally, HR-DMA is also employed in a new Planar Mixing Layer Flame (PMLF) configuration to capture the evolution of incipient soot SDFs down to ~0.9 nm, revealing distinct high- and low-temperature soot nucleation pathways, with results validated by in-situ laser diagnostics. Beyond combustion-generated aerosols, this dissertation explores broader applications of HR-DMA, including the characterization of sub-10 nm platinum (Pt) nanoparticles synthesized via Reactive Spray Deposition Technology (RSDT) for building Membrane Electrode Assemblies (MEAs), functional for electrochemical applications. By employing rapid dilution sampling coupled with HR-DMA, this dissertation reveals how Pt nanoparticles evolve in size and acquire charge through interactions with flame-generated ions, providing key insights into controlling nanoparticle synthesis mechanisms during their high-temperature synthesis in flame. Together, this dissertation establishes a versatile experimental framework that leverages state-of-the-art techniques to explore the earliest stages of aerosol formation in flames. These methodologies provide critical insights into gas-to-particle conversion processes, significantly improving our ability to characterize combustion-generated aerosols with greater accuracy and resolution. At the same time, the techniques provide tools to monitor the fate of such flame-formed nanosized aerosols, once emitted in the atmosphere.
