Size Dependent Morphology of Organic Aerosol

Restricted (Penn State Only)
Author:
Altaf, Muhammad Bilal
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 23, 2017
Committee Members:
  • Miriam Freedman, Dissertation Advisor
  • Miriam Freedman, Committee Chair
  • David Boehr, Committee Member
  • Raymond Schaak, Committee Member
  • Eugene Clothiaux, Outside Member
Keywords:
  • Aerosol
  • Organic Aerosol
  • Transmission Electron Microscopy
  • Morphology
  • Phase Separation
  • Cloud Condensation Nuclei
  • Cavity Ring-Down Spectroscopy
  • TEM
  • Atmospheric Chemistry
  • Climate
  • Biogenic Aerosol
  • Secondary Organic Aerosol
Abstract:
The Earth’s atmosphere is composed of a wide variety of gas phase species and particulate matter that have a large impact on our climate. Though our understanding of the climate system has improved significantly over the past few decades, the impact of aerosol particles remains uncertain. Aerosol particles can affect climate through the absorption and scattering of radiation (aerosol direct effect) and by serving as cloud condensation nuclei (aerosol indirect effect). It is known that aerosol particles cause a net cooling effect on the planet, but the magnitude of cooling is unclear and remains under investigation. A large part of this uncertainty is due to an incomplete understanding of the complex physical and chemical properties of aerosol particles such as composition, morphology, and phase state. In this dissertation, we focus on investigating the role of particle size and composition in determining morphology. We have discovered that for some organic aerosol systems, particle morphology depends on size, where small particles are homogeneous and large particles are phase separated. To explore the origins of this size dependent behavior, we have worked with a model organic aerosol system, poly(ethylene glycol)-400 mixed with ammonium sulfate. We have used cryogenic-transmission electron microscopy to probe the effect of phase separation mechanism on particle morphology by varying the organic aerosol composition. Our results suggest that a size dependent morphology occurs due to an activated process, where the presence or absence of an activation barrier to phase separation controls the resulting morphology. We have also explored the kinetics and thermodynamics of the phase separation process that results in a size dependent morphology by varying the experimental drying rates. Drying rates that span over four orders of magnitude were investigated. We have observed that at the fastest drying rates, the size of the transition region where both phase separated and homogeneous morphologies exist is on the order of ~100 nm. At the slowest drying rates, the transition region shifts to smaller diameters and the width narrows to ~3 nm. Our results suggest that a size dependent morphology persists to the slowest drying rates. Thus, we conclude that an underlying thermodynamic effect results in this size dependent behavior, rather than solely a kinetic phenomenon. To determine the atmospheric implications of a size dependent morphology, we have used a cloud condensation nuclei counter to probe the effect of a homogeneous vs. a phase separated morphology on cloud condensation nuclei (CCN) activity. We have found that the activation diameters differ for particles which have the same composition, but varying morphology. Since aerosol optical properties are a sensitive measure of particle structure, we have studied the morphology-resolved optical properties of organic aerosol using cavity ring-down spectroscopy. Our spectroscopy data indicate that the optical properties of core-shell and partially engulfed particles are approximately equal, but different than the prediction for homogeneous particles. By accurately parametrizing aerosol particle morphology, optical properties, and CCN activity we will be able to better predict heterogeneous chemistry in the atmosphere, in addition to the aerosol direct and aerosol indirect effects which play a critical role in constraining climate forcing.