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Open Access Publications from the University of California

Chemical and Physical Investigation of Secondary Organic Aerosol Formation

  • Author(s): Nakao, Shunsuke
  • Advisor(s): Cocker, David R.
  • et al.

The overall objective of this dissertation is to advance the understanding of anthropogenic influences on SOA formation with a major focus on SOA formation from the photooxidation of aromatic hydrocarbons. This study advances the poor understanding of aromatic SOA formation through multi-generational reactions by investigating the significance of major intermediate species - specifically phenolic compounds and glyoxal. Phenolic compounds are identified to play a significant role in aromatic SOA formation (approximately 20% of aromatic SOA formed via the phenolic route, under low NOx conditions). The formation of bicyclic hydroperoxides, currently assumed in aromatic reaction mechanisms, is supported by chemical analysis utilizing a soft-ionization technique. SOA formation from glyoxal uptake onto aerosol, however, is shown to have a negligible effect on SOA formation from the oxidation of aromatic hydrocarbons (RH less than 80%) in an environmental chamber, contrary to the current belief based on simpler systems. Therefore, glyoxal is excluded as an intermediate species of aromatic SOA at least in the experimental conditions of this study.

Another facet of the body of work details development and application of a new real-time aerosol density measurement system. A recently proposed empirical relationship between SOA density and elemental composition (O/C and H/C) is evaluated against the extensive database of this study, extending the applicability of the empirical relationship towards aromatic compounds. In addition, SOA formation from diesel exhaust photooxidation is investigated by the combination of the real-time density measurement and other physical/chemical analysis, demonstrating that mass-based measurement techniques are critical in interpreting the physical processes during diesel SOA formation, i.e., evaporation of semi-volatile organics from fractal-like primary organic aerosol, as well as condensation of secondary organic compounds onto fractal-like particles. Finally, real-time density measurement is applied to the photooxidation of CH3I, which produces fractal-like iodine oxide particles (IOP); density measurement is critical for determination of mass-based aerosol formation yields. Nearly all reacted iodine is found in particle phase when there is sufficiently high O3, which was reasonably modeled in the absence of NOx. A discrepancy between observed and modeled IOP formation in the presence of NOx suggests incomplete understanding in iodine chemistry involving NOx.

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