Semester

Spring

Date of Graduation

2019

Document Type

Thesis

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Fabien Goulay

Committee Member

Brian Popp

Committee Member

Justin Legleiter

Committee Member

Kenneth Showalter

Committee Member

Alan Bristow

Abstract

My research focuses on investigating the impact of moisture-induced and oligomer-induced viscosity changes on OH-initiated oxidation of semi-solid aerosols,and the role of gas-liquid interfaces in regulating aqueous aerosol chemistry. Saccharides, which are a major constituent of aqueous atmospheric aerosols, are chosen as model molecules to form highly oxygenated organic aerosols. The experiments are performed using an atmospheric pressure flow-tube reactor with both online VUV-AMS (Vacuum-Ultraviolet Aerosol Mass Spectrometer) and offline GC-MS analysis techniques. The decay rates of saccharide are determined by measuring the loss signal of saccharide in the particle phase as a function of OH exposure (time-integrated total concentration of OH radical). A reaction-diffusion model is developed to interpret the observed kinetics behavior. These results highlight that the chemical transformation of semi-solid aerosols is kinetically limited by bulk diffusion and that of aqueous aerosol is dependent on surface-bulk partitioning.

The kinetics of the OH-initiated oxidation of semi-solid monosaccharide particles are obtained over a range of relative humidity (RH) in order to investigate the impact of moisture-induced viscosity changes on the mechanisms of oxidative aging of semi-solid aerosols. The reactive uptake coefficient of monosaccharide ( increases by a factor of 2.4 as the surrounding RH is increased from 10% to 30%. A reaction-diffusion kinetic model with a constant diffusion coefficient is developed to investigate the impact of bulk molecule diffusion on kinetics behavior of semi-solid aerosols. This study suggests that the diffusion of the bulk reactant from the particle inner core to its surface is the rate-limiting step in oxidation of the semi-solid aerosols.

In order to investigate the oligomer-induced viscosity changes on reactive properties of semi-solid aerosols, reactive uptake coefficients are measured over a range of monosaccharide:disaccharide molar ratio ranging between 1:1 and 4:1 at 30% RH. The reactive uptake coefficient of monosaccharide is found to decrease by a factor of 5 as the molar ratio changing from 4:1 to 1:1. The observed decay behaviors can be reproduced by using a simple compositional Vignes relationship to predict the composition-dependent diffusion coefficients of the saccharides. Simulation results suggest that a gradient diffusivity arises due to concentration gradients across the particle through heterogeneous oxidation of semi-solid particles. These findings illustrate the impact of bulk composition on reactant bulk diffusivity, which determines the rate-limiting step during the chemical reaction of semi-solid multi-component particles.

For equimolar monosaccharide-disaccharide aqueous aerosols, the reactive uptake coefficient of monosaccharide is 5.02±1.12 and the reactive uptake coefficient of disaccharide is 0.39±0.10. Molecular dynamics simulations of the mixed aqueous solutions reveal the formation of a ~10 Å disaccharide exclusion layer below the water surface. The monosaccharide concentration is predicted to be low at the surface and to increase rapidly within the first 10 Å of the air-water interface. The observed decays are consistent with a poor spatial overlap of the OH radical at the interface with the disaccharide in the particle bulk. These findings highlight the critical importance of partitioning of bulk reactant at the gas-liquid interface in determining the reaction rate of reactive species in aqueous aerosols.

Comments

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