Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

Biomass burning aerosols play an important role in the chemistry and physics of the atmosphere and therefore, affect global climate. Biomass burning aerosols are generally aqueous and have a strong saccharidic component due to the combustion and pyrolysis of cellulose, a major component of foliar fuel. This class of aerosol is known to affect both the absorption and scatter of solar radiation. Also, biomass burning aerosols contribute to cloud formation through their action as cloud-condensation nuclei. Many questions exist about the chemical speciation and chemical aging of biomass burning aerosols and how this affects their atmospheric properties and ultimately, global climate. Also, knowledge of the chemical components of these aerosols is important in the search for chemical tracers that can give information about the point or regional source, fuel type, and age of a biomass burning aerosol parcel. Levoglucosan was chosen for these studies as a model compound for biomass burning aerosols because of its high measured concentrations in aerosol samples. Levoglucosan often dominates the aerosol composition by mass. In this dissertation, laboratory proxy systems were developed to study the solution-phase chemistry of levoglucosan with common atmospheric reactants found in biomass burning aerosols (i.e. H+, •OH). To mimic these natural conditions, acid chemistry was studied using sulfuric acid in water (pH=4.5). The hydroxyl radical (•OH) was produced by the Fenton reaction which consists of iron, hydrogen peroxide and acid (H2SO4) in aqueous solvent. For studies in aqueous sulfuric acid, oligomers of levoglucosan were measured by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS). A rational mechanism is proposed based on both the acid-catalyzed cationic ring-opening of levoglucosan and nucleophilic attack of ROH from levoglucosan on the hemi-acetal carbon to produce pyranose oligomers through the formation of glycosidic bonds. Oligomer formation is further supported by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Reactions of levoglucosan with •OH produced from Fenton chemistry were studied in solution. Two modes of oligomerization (2000 u) were observed for reaction times between 1 and 7 days using MALDI-TOF-MS and laser desorption ionization (LDI) TOF-MS. Single-mass unit continuum mass distributions with dominant -2 u patterns were measured and superimposed by a +176/+162 u oligomer series. This latter oligomer pattern was attributed to a Criegee rearrangement (+14 u) of levoglucosan, initiated by •OH, forming a lactone (176 u). The acid-catalyzed reaction of any ROH from levoglucosan (+162 u) forms an ester through transesterification of the lactone functionality, whereupon propagation forms polyesters. Proposed products and chemical mechanisms are suggested as sources and precursors of humic-like substances (HULIS), which are known to possess a large saccharic component and are possibly formed from biomass burning aerosols. These products could also serve as secondary tracers, giving further information on the source and age of the aerosol.

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