Date of Award
Master of Science (MS)
Characterization of particulate matter (PM), more specifically the carbonaceous fraction, is essential for understanding atmospheric processes, source determination, and health impacts. In this thesis, a novel approach to both the quantification and characterization of carbonaceous atmospheric PM was developed and validated on model compounds and collected ambient PM from a local source within Grand Forks, ND.
Thermal optical analysis (TOA) is a commonly used method for the determination of organic (OC) and elemental (EC) carbon within atmospheric PM that yields quantitative results, i.e., total concentrations of OC and EC. However, for speciation of OC, there is no universal method. Typical approaches include solvent extraction followed by gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS), thermal desorption aerosol GC-MS (TAG), aerosol mass spectrometry (AMS), and pyrolysis GC-MS. In this thesis, thermal desorption (TD) coupled with pyrolysis (Pyr) GC-MS (TD-Pyr-GC-MS) was employed for characterization of carbonaceous PM and determination of specific tracers that were used for source apportionment. This method was developed to be used in combination with quantitative TOA data and qualitative results for both concentrations of OC, and its characterization.
TOA of PM revealed a wide range of OC that makes up the total PM concentration (25 –75%), showing a wide variability in composition of atmospheric PM. Quantification by TOA supported the significance of the pyrolytic fraction, in which 73 – 87 % of the OC evolved at temperatures above 400 °C. The comprehensive speciation of OC assessed sequentially with thermal TD (evolving at 300 °C) and Pyr (˃ 400 °C) coupled to GC-MS enabled the investigation of both low and high molecular weight species’ tracers. The TD fraction showed a high abundance of long chain alkanes (waxes) with an odd number of carbon atoms, indicating biogenic origin, along with fatty acids (FAs) and fatty acid methyl esters (FAMEs). Furthermore, the generally ignored Pyr fraction showed a series of homologous compounds, which included n-alkenes, n-alkylbenzenes, light polycyclic aromatic hydrocarbons (PAHs), n-alkanes, and substituted phenols, many of which are thought to be derived from the breakdown of larger molecular weight biogenic sources, e.g., plant waxes and triacylglycerides (TGs). The sequential pyrolytic temperatures steps used in this thesis were essential in understanding the overall composition of PM collected in the Grand Forks area.
Furthermore, the model compounds analyzed in this study with TD-Pyr-GC-MS, i.e., TGs and fatty acids, provided unique insights into the mechanisms of pyrolysis. Moreover, the process of decomposition through hydrodeoxygenation vs. decarboxylation were assessed through analysis of these compounds. In addition, the Pyr of TGs and fatty acids, were shown to form specific homology profiles, mainly n-alkylbenzenes and 2-ring PAHs, which further supported their presence in atmospheric PM.
Nespor, Brett Thomas, "Organic Speciation Of Air Particulate Matter With Thermal Desorption – Pyrolysis – Gas Chromatography – Mass Spectrometry" (2018). Theses and Dissertations. 2421.