Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Margaret A. Tolbert

Second Advisor

O. Brian Toon

Third Advisor

Linnea Avallone

Abstract

Cirrus clouds are ubiquitous in the upper troposphere. The formation of ice in cirrus remains one of the most uncertain aspects of climate change. Through laboratory experiments we hope to better understand the pathways available for ice formation in these clouds. In the atmosphere, less than 1 in 1000 particles serve as surfaces for ice formation. Currently it is unclear why one particle would nucleate ice preferentially compared to another. A Raman-spectroscopy-based system for studying ice nucleation and aerosol properties has been constructed and used to investigate various types of aerosol particles. Studies of both laboratory-generated and ambient aerosol particles have been performed in order to explore heterogeneous ice nucleation at the level of the individual particle. A major focus of this thesis is the investigation of heterogeneous ice nucleation on particles with multiple chemical components and various morphologies. Heterogeneous ice nucleation on solid ammonium sulfate particles containing a coating of insoluble organic material was measured under typical upper tropospheric conditions. Raman spectroscopy was used to gain insight into the distribution of chemical species within individual particles. This technique was also used for the analysis of complex ambient aerosol. Selective depositional ice nucleation events were used to identify ice nucleus particles and Raman spectroscopy was used for chemical identification of these particles. The vast majority of ambient particles, in both the background and ice-active particle fractions, were observed to contain a detectable amount of organic material. Some oxygenated organic particles were observed to serve as efficient ice nuclei. To determine atmospheric conditions that could favor a heterogeneous pathway for cirrus ice formation on oxygenated organic aerosol, the state transitions and ice nucleation ability of highly viscous glassy or amorphous organic aerosols were also examined.

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