An Investigation on Secondary Organic Aerosol Induced Restructuring in Soot Aggregates

• Author / Creator

  Leung, Kaiser K

• Currently the understanding of morphological evolution of soot aggregates under the influence of secondary organic aerosol (SOA) and the process of SOA restructuring are not well known. As a consequence, the uncertainty in the estimation of the radiative forcing from soot aggregates is high. The objective of this work is to investigate the relationship between soot restructuring due to SOA coatings and the aggregate properties, and to investigate the effects of water on SOA coatings. Two sets of photo-oxidation experiments were conducted to investigate the restructuring of soot aggregates induced by secondary organic aerosol (SOA) coating and its humidity dependence. The first set of photo-oxidation experiments involved soot aggregates generated from three sources, an ethylene premixed burner, an inverted diffusion burner, and a diesel generator. Soot aggregates were treated by denuding then size-selected by a differential mobility analyzer and injected into a smog chamber, and subsequently exposed to SOA using p¬-xylene as a precursor. For a given initial mobility diameter, the diesel aggregates were less dense with smaller primary particles than the aggregates from the two burners. The change in mobility of aggregates between the initial and final structures displayed a linear dependence on the number of primary particles in the aggregate. The linear relationship could allow modelers to predict the evolution of aggregate morphology induced by SOA using a single parameter. The second set of photo-oxidation experiments investigated the relative humidity (RH) dependence on the restructuring of aggregates induced by SOA coatings. Soot aggregates in these experiments were generated from the ethylene premixed burner, classified by mobility diameter and injected into a smog chamber. The aggregates were then exposed to SOA coatings using p--xylene as a precursor and subsequently subjected to one of the RHs: < 12, 20, 40, 60, 85%. At RH < 12%, a uniform mobility growth with increasing coating mass was observed, indicating the coating was too viscous to induce aggregate restructuring. At RH above 20%, restructuring of aggregates were observed in the form of a decrease in mobility diameter with increasing coating mass. Interestingly the degree of restructuring increased with increasing RH, indicating that elevated humidity decreased coating viscosity and increased surface tension. Appreciable water uptake by the SOA coating was observed for RH above 60%, and the hygroscopicity parameter for SOA coating generated from p-xylene was determined. The result of the second series of photo-oxidation experiments has significant implications on atmospheric restructuring of soot aggregates induced by SOA coatings. Overall the significance of the work reported here may contribute to the better understanding of the evolution of soot aggregate morphology due under the influence of SOA coatings in the atmosphere, allowing aggregate modelers to predict the compaction of aggregates knowing the aggregate properties.

• Subjects / Keywords

  • Secondary Organic Aerosol
  • Soot Aggregate
  • Restructuring

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3D50GB4H

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.