Quantum chemical studies of atmospherically relevant molecular clusters and their role in particle formation

DePalma, Joseph William, Jr.
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University of Delaware
The goal of this dissertation is to gain a more complete understanding of the chemical mechanisms governing new particle formation (NPF), a key process that influences the concentration of atmospheric particulate matter. NPF is a gas-to-particle process, first beginning with the nucleation of ambient vapors into small molecular clusters which then grow rapidly by condensation of additional trace vapors to larger, climatically relevant sizes. The climatically relevant size is greater than approximately 50 nm in diameter, where the newly formed particle may act as a cloud condensation nucleus (CCN), i.e. the seed for cloud droplet formation. CCN have important implications for Earth's radiation balance and it has been suggested that nearly 50% of all CCN arise from NPF. Despite the prevalence of this process, the early stages of NPF and how many of the newly formed particles go on to have climatic effects is not well understood, hindering climate modeling efforts. Further developing a fundamental, chemical understanding of exactly how NPF occurs will ultimately reduce the uncertainties in climate modeling. In this dissertation, the tools of computational chemistry are used to explore various chemical aspects of NPF. The key species in NPF are sulfuric acid, water, ammonia, amines and various types of carbon-containing molecules. These components form the molecular clusters which are the seeds of CCN and ultimately cloud droplets. The structures and thermochemistry of both charged and uncharged clusters were explored to elucidate the chemical mechanisms of their formation. Hydrogen bonding and electrostatics drive cluster formation. Water has a minor role in cluster formation, offering a small enhancement of cluster stability. Even with these strong interactions, the cluster growth potential energy surface is not smooth, and activation barriers must be traversed in order to form stable clusters. Highly oxidized organics can form stable oligomers in both the gas and condensed phase and can influence fundamental chemical processes. From a broader perspective, the work in this dissertation shows that trace atmospheric gases such as sulfuric acid, ammonia, amines and oxidized organics are key players in the formation of new particles and ultimately are drivers of global climate. [Abstract shortened by ProQuest/UMI.]