Source-Specific Molecular Signatures for Light-Absorbing Organic Aerosols

SOURCE-SPECIFIC MOLECULAR SIGNATURES FOR LIGHT-ABSORBING ORGANIC AEROSOLS Amanda Susan Willoughby Old Dominion University, 2015 Director: Dr. Patrick G. Hatcher Organic aerosols (OA) are universally regarded as an important component of the atmosphere based on quantitative significance as well as the far-reaching impact they have on global climate forcing and human health. Despite the acknowledged importance, OA amounts and impacts remain the largest uncertainties regarding radiative forcing estimates. Incomplete chemical characterization of aerosol organic matter (OM) and a lack of concrete source apportionment is a major source of this uncertainty. The primary focus of this study is to provide much needed molecular details regarding ambient OA from key emission sources, and establish links between molecular and optical properties. Complete chemical characterization of OA has been a longstanding obstacle for the atmospheric community. In this thesis Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was used to evaluate the molecular properties of OM extracted from ambient aerosols collected from marine, biomass burning, urban, and mixed sources. A method was developed to evaluate the water-insoluble OM in addition to the frequently analyzed water-soluble OM to provide a more complete evaluation of the molecular components that define each emission source. Molecular formulas facilitated by FTICRMS reveal that water-insoluble OM contains numerous aliphatic and sulfur-containing compounds, and represent important anthropogenic components not previously recognized. Principal component analysis reveals source-specific molecular characteristics for each of the emission sources. Aromatic nitrogen species are a distinguishing component for biomass burning aerosols, structurally diverse and highly processed aliphatic and oxygenated species are key components of urban aerosols, and marine aerosols contain a large number of biologically-derived organic compounds and organosulfates. Atmospheric aging reactions including oligomerization and reactions with atmospherically-relevant inorganic species were explored using Kendrick mass defect analysis and evaluation of specific mass differences. Oligomerization of OM with isoprene-related compounds and reactions with inorganic species are apparent in each of the emission sources indicating widespread importance, and variations in extent of reaction indicate these urban and mixed source aerosols have undergone more atmospheric aging. Finally, the link between molecular character and related fluorescence properties, specifically fluorescence, was explored by coupling FTICR-MS with excitation emission matrix spectroscopy (EEMs). Fluorescent signatures observed in each of the aerosols supports their widespread importance in the process of radiative forcing. Biomass burning aerosols display a unique feature associated with freshly-emitted OM, likely linked to the abundance of highly-aromatic nitrogen-containing compounds. The urban aerosols display unique diesel-like features likely associated with the PAH-like compounds identified in the mass spectra, highlighting their anthropogenic influence. This study provides unique qualitative and analytical approaches for enhancing the understanding of the molecular properties, the atmospheric transformations, and associated optical properties of atmospheric OM necessary for ultimately eliminating the uncertainties associated with OA and their net global impacts.