Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Veronica Vaida

Second Advisor

Barbara Ervens

Third Advisor

Margaret Tolber

Abstract

Atmospheric organics originate from biogenic and anthropogenic sources including volatile organic compounds (VOCs) and oxidation products of these VOCs. In the atmosphere, the organics can be processed and become important components of atmospheric aerosols. Methylglyoxal (CH3COCHO) is a known oxidation product of VOCs and has been observed in field studies and incorporated into atmospheric models. While the gas-phase chemistry of methylglyoxal is fairly well understood, its modeled concentration and role in the formation of secondary organic aerosols (SOA) remains controversial. In this thesis, I investigate aqueous chemistry of methylglyoxal to better understand the link between VOCs and the formation of SOA. The gas-phase hydration is a process not previously considered to occur in water-restricted environments, such as the atmosphere, but could have important consequences for the atmospheric processing of organics. Methylglyoxal is an interesting molecule for studying gas-phase hydration of because it contains both a ketone and an aldehyde group. This allows for the simultaneous analysis of the effect of gas-phase hydration on both carbonyl groups. I examine the gas-phase hydration of methylglyoxal leading to diol and hydrates (water clusters), and examine their water and photon mediated chemistry. The gas-phase hydration of methylglyoxal has important consequences to its atmospheric processing. Methylglyoxal diol and its hydrates have a lower vapor pressure than the parent aldehyde and a tendency to form intermolecular hydrogen bonds. The ability to more readily form hydrogen bonds can increase the diol and diol hydrate contributions to aerosol growth by partitioning more readily into the aqueous phase. This would increase methylglyoxal content in aerosols and affect the organic content of aqueous-phase aerosols and cloud droplets. Additionally, hydration of methylglyoxal to form the diol can alter the electronic state of the parent aldehyde. Formation of methylglyoxal diol will eliminate the n→π* transition of the aldehyde carbonyl, which undergoes near-ultraviolet (UV) photochemistry. This allows for methylglyoxal diol to form new products via UV photochemistry through its remaining ketone carbonyl and opens the way for other photochemistry through excitation of the OH vibrational overtone in the near-infrared (IR). To examine the hydration of methylglyoxal and the formation of methylglyoxal diol I employed a variety of spectroscopic techniques including, Fourier transform IR spectroscopy (FTIR), cavity ringdown spectroscopy (CRDS), incoherent broad band cavity enhanced absorption spectroscopy (IBBCEAS), UV-visible spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. The FTIR and CRD are used to observe and characterize the formation of methylglyoxal diol and its hydrates. The IBBCEAS was used to investigate the changes in the UV absorption cross section of methylglyoxal with increasing relative humidity. UV-visible and NMR spectroscopy was used to analyze the aqueous-phase hydration of methylglyoxal and its photolysis products. These results may impact the understanding of the atmospheric fate and processing of methylglyoxal and the role of organics in atmospheric chemistry. Hydration of aldehydes such as methylglyoxal has implications for modification of atmospheric radical production and provides important degradation pathways of potential SOA precursors.

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