Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Margaret A. Tolbert

Second Advisor

Barbara Ervens

Third Advisor

Jose L. Jimenez

Fourth Advisor

Rainer M. Volkamer

Fifth Advisor

Owen B. Toon

Abstract

Cirrus clouds, composed of water ice, cover 30% of the global surface and affect climate by scattering incoming solar radiation and scattering and absorbing outgoing terrestrial radiation. Despite their ubiquity, ice clouds are not accurately represented in global climate models and provide one of the largest uncertainties in predictions of climate change. This is, in part, because our understanding of ice particle formation processes in the atmosphere is low. Recent evidence has shown that pure ice clouds are dominated by heterogeneous ice nucleation, and, therefore, are dependent on presence of rare (5) ice freezing nuclei. While it is known that mineral dust aerosol is an efficient ice nucleus, much less is known about organic-inorganic species, which composed a significant fraction of cirrus ice residuals.

In this study we have probed the ice nucleation efficiencies of organic, organic-sulfate, and organic-sea-salt species using a Knudsen cell flow reactor and a Raman microscope coupled to an environmental cell. Specifically, we have probed the ice nucleation efficiency of thin films of crystalline monocarboxylic acids from 180 to 200 K, liquid-liquid phase separated particles containing ammonium sulfate and organic polyols from 210 to 235 K, simulated secondary organic aerosol derived from aqueous processes with and without ammonium sulfate from 215 to 230 K, and synthetic sea-salt particles with and without a proxy for dissolved organic carbon from 215 to 225 K. The hygroscopic phase transitions of the above particles were also explored up to 260 K. From these experiments we find that certain subsets of organic-inorganic particles may be very efficient heterogeneous ice nuclei in the upper troposphere. These subsets include crystalline organics with a high O:C ratio, sugar-like glassy organics particles or coatings/shells, and effloresced ammonium sulfate or sea-salt with liquid organic/inorganic coatings.

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