Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Margaret A. Tolbert

Second Advisor

Joshua A. Gordon

Third Advisor

Rainer Volkamer

Fourth Advisor

Veronica Vaida

Fifth Advisor

Eleanor C. Browne

Abstract

The phase state and water content of atmospheric particles influences climate, air quality, and the hydrologic cycle through impacts on particle optical properties, heterogeneous chemistry, and the partitioning of water between the particle and gas phase. A significant fraction of liquid atmospheric particles contain soluble inorganic compounds that can undergo efflorescence, i.e., the process of particle crystallization with concurrent loss of particle-phase water. Despite the importance of particle phase state, there is no comprehensive understanding of particle crystallization and empirical observations are a necessity. In the laboratory studies presented here, phase transformations of aqueous and crystalline inorganic microparticles were studied using a novel long-working distance optical trap. The optical trap was developed with the motivation of probing crystallization induced by particle collisions (coagulation). A key result from these studies was the first observations of a previously unexplored particle crystallization pathway that we termed “contact efflorescence”. Contact efflorescence describes the crystallization of a metastable aqueous microdroplet initiated by surface contact with an externally located particle. These studies demonstrate that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient relative humidity (≤80%). The occurrence of contact efflorescence is demonstrated using soluble crystalline contact nuclei as well as amorphous (non-crystalline) organic contact nuclei. These results point toward a transient crystal nucleation pathway relevant at the moment of aerosol contact and suggest far more particle types can induce crystallization of atmospheric aerosols via coagulation than is currently considered in atmospheric chemistry and climate models.

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