Multivariate analysis of water-soluble organic carbon molecular formulas in daily samples

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Conference Proceeding

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Water-soluble organic carbon (WSOC) in aerosols is a highly complex mixture with thousands of compounds that can be identified using ultrahigh resolution mass spectrometry. In this study, molecular formula characterization of aerosol WSOC was done on individual daily aerosol samples (N = 34) collected consecutively from June 25 to July 28 in 2010 at the Storm Peak Laboratory near Steamboat Spring, Colorado (3220m a.s.l.). The aerosol samples were analyzed using ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry after a reverse phase solid phase extraction procedure. 2000-5000 monoisotopic molecular formulas were identified in each of the daily samples with a majority of the molecular formulas present in multiple samples. Relative abundance weighted oxygen-to-carbon ratios (O/Cw) and hydrogen-to-carbon ratios (H/Cw) were calculated for each of the daily aerosol samples based on the identified molecular formulas. The overall O/Cw values for each of the daily samples are very similar ranging between 0.47 and 0.55. Similarly, the overall H/Cw values were mostly around 1.55. Relative abundance double bond equivalents (DBEw) were also calculated for each of the samples. DBEw values were approximately 5 for most of the samples with a few exceptions of DBEw > 5.4. Daily aerosol WSOC samples were examined using hierarchical cluster analysis. The hierarchical cluster analysis was conducted on the identified monoisotopic molecular formulas and their relative abundance for all of the aerosol samples. Eight of the samples were analyzed twice and all of the eight replicates were closely paired in the cluster analysis dendrogram which partially validates the results. The cluster analysis indicates that the aerosols have similar composition when the air masses have similar origins. For example, aerosol samples collected from July 14th to July 18th are located in the same branch of the dendrogram and their back trajectory analysis indicates the air came from the west. The aerosol WSOC samples with exceptionally high DBEw are located in the same dendrogram branch as well. These results indicate aerosol WSOC samples from these days have similar composition with similar unsaturated compounds (e.g., C16H16O8(CH2)1-9). Cluster analyses were also done on the individual molecular formulas to examine their relationships. The cluster analysis results indicate a majority of the CHO compounds are clearly distinct from compounds with heteroatoms perhaps reflecting their different formation pathways. Within the CHO compound clusters, we commonly observed paired CHO compounds differing by CH2, CH2O or O in the molecular formulas. These results help to unravel the complex nature of WSOC and provide insights to the high molecular weight compound formation mechanisms.

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Fall Meeting 2013