Organic compounds make up a significant fraction of atmospheric aerosol mass. Their identification and quantification are needed for understanding aerosol’s role in deteriorating air quality and affecting global climate. The traditional method for determining organic compounds in aerosol involves solvent extraction, concentration of extracts followed by separation and detection using gas-chromatography-mass spectrometer (GC/MS). This method is labor intensive and time consuming. Our lab has recently developed an in-injection port thermal desorption (TD)-GC/MS method to quantify nonpolar organic compounds such as alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs). This TD-GC/MS method does not require any sample treatment and can detect individual nonpolar organic compounds as...[ Read more ]

Organic compounds make up a significant fraction of atmospheric aerosol mass. Their identification and quantification are needed for understanding aerosol’s role in deteriorating air quality and affecting global climate. The traditional method for determining organic compounds in aerosol involves solvent extraction, concentration of extracts followed by separation and detection using gas-chromatography-mass spectrometer (GC/MS). This method is labor intensive and time consuming. Our lab has recently developed an in-injection port thermal desorption (TD)-GC/MS method to quantify nonpolar organic compounds such as alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs). This TD-GC/MS method does not require any sample treatment and can detect individual nonpolar organic compounds as low as a few nanograms per sample. These advantages over the traditional solvent extraction-GC/MS approach allow us to quantify aerosol organics in filter samples which previously could not be utilized because of insufficient aerosol materials for the solvent extraction-GC/MS method. In this thesis work, I have focused on the applications of the injection port TD-GC/MS method in studying aerosol nonpolar organic compounds.

In the first application, emission samples collected from laboratory-simulated coal combustion in power plants were quantified for contents of PAHs, alkanes and hopanes in 13 aerosol size bins in the size range of 0.3-10 μm. Quantification of individual aerosol organics in such a fine size resolution was only made possible by our newly developed analytical method. PAH emissions were found to have a fine and coarse mode and the fine mode was enhanced during coal combustion in the absence of O₂. The coarse mode PAHs were the dominant mode in combustions with 20% O₂. This was postulated to be a result of the newly formed PAHs through pyrosynthesis condensing onto fly ash particles. The n-Alkanes and hopane did not show discernable patterns in their size distribution, consistent with the knowledge that these compounds could not be formed during combustion and could only derive from devolatilization from the parent coal particles.

In the second application, forty-three ambient aerosol samples collected in Klang Valley, Malaysia were analyzed by the TD-GC/MS method. The detailed chemical characterization indicated that both anthropogenic and biomass burning sources had important contributions to aerosols at this site. This set of samples were collected using two quartz fiber filters connected in series, i.e, a front and a back filters. Materials collected on the back filters were thought mainly from the adsorption of gaseous organic in the sampling air stream, the amounts of which approximated the positive sampling artifact caused by adsorption of gaseous species for the front filter. The low detection limit of the TD-GC/MS allowed quantification of individual organic compounds in the back filters. This in turns allowed us to examine the positive sampling artifact issue for individual compounds. Previous only the sampling artifact for the bulk OC could be studied because of analytical limitation. Both n-alkanes and hopanes were detected in the back filters. The back/front filter ratios for these individual organic compounds were found to be generally higher than the back/front ratio for the bulk OC. Out measurements of individual SVOC compounds indicate that we are still far from a quantitative understanding of the sampling artifact issues.

In summary, this thesis work has demonstrated the great potential of the newly developed GC/MS method in studying aerosol organic chemistry. Further applications in laboratory and field studies of atmospheric aerosols are recommended.