Date of Award

Spring 1-1-2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemistry & Biochemistry

First Advisor

Rainer Volkamer

Second Advisor

Joost de Gouw

Third Advisor

Paul Ziemann

Abstract

Photochemical reactions are important in the atmosphere, in both the gas and condensed phases. Glyoxal is an interesting molecule since its properties make it a unique indicator of the rate of VOC oxidation. Glyoxal also forms secondary organic aerosols as a result of multiphase reactions, including light-absorbing imidazoles. Imidazole-2-carboxaldehyde (IC) has recently been shown to facilitate autophotocatalytic aerosol growth through oxidation within and on surfaces of aerosols. These molecules have been found in the condensed-phase of aerosols and aqueous environments. Photosensitizers are molecules that can transfer energy to initiate chemicals reaction that would otherwise be energetically unfavorable. This thesis presents gas-phase products from photosensitized heterogeneous reactions on aqueous interfaces by light-emitting diode cavity enhanced differential absorption spectroscopy (LED-CE-DOAS). In Chapter 2, IC is used as a photosensitizer to study the formation of HO2 radicals from aqueous films containing IC and citric acid using a coated-wall flow tube reactor. The formation rate of HO2 radicals (PHO2) is indirectly measured by converting gas-phase NO into NO2. PHO2 was observed to be a function of IC mass loading and actinic flux. A mechanism between the excited IC and citric acid as a potential H-donor is proposed in Chapter 2. In Chapter 3, the photosensitizer humic acid was added to a quartz cell reactor to investigate the formation of glyoxal from fatty acids at the air-water interface. Glyoxal was observed in the presence and absence of humic acid, and increased for higher carboxylic acid concentrations. In the absence of humic acid, fatty acid photochemistry triggered the formation of glyoxal precursors using radiation below 420 nm. These gas-phase results of HO2 and glyoxal can be expanded to reactions at the surface and inside the condensed-phase of particles and the bulk.

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