Polar organic oxygenates in PM2.5 at a southeastern site in the United States

A field study was undertaken in Research Triangle Park, NC, USA during the summer of 2000 to identify classes of polar oxygenates in PM2.5 containing carbonyl and/or hydroxyl functional groups and, to the extent possible, determine the individual particle-bound oxygenates that make up each class. The selected site was in a semi-rural environment with expected impacts from both biogenic and anthropogenic sources. PM2.5 samples were collected and analyzed for gravimetric mass, inorganic composition by ion chromatography, polar oxygenated compounds by gas chromatography-mass spectroscopy after derivatization of the solvent extracts, organic and elemental carbon (EC) by a thermal–optical method, and chemical functional groups by infrared (IR) spectroscopy. The chemical analyses also included determining the polarity of the PM2.5 by measuring the organic carbon (OC) concentrations in a series of extraction solvents of increasing polarity. The ambient PM2.5 mass concentrations ranged between 9 and 30 μg m−3, with an average mass composition of 28% sulfate, 6% nitrate, 12% ammonium, 39% OC compound, 2% EC, and 12% liquid water at a relative humidity of 43%. OC analyses of the organic extracts indicated approximately 89% of the extractable OC was more polar in nature than that extracted by n-hexane. IR analysis of the PM2.5 was consistent with the presence of significant levels of polar carbonyl and hydroxyl functional groups. The polar nature of the organic fraction was further supported by the observation that the inorganic fraction accounted for only about 80% of measured liquid water concentrations in the PM2.5. The mass spectra data were consistent with the presence of the following classes of oxygenates: oxo monocarboxylic acids, trihydroxy monocarboxylic acids, dihydroxy dicarboxylic acids, hydroxy dicarboxylic acids, normal dicarboxylic acids, oxo dicarboxylic acids, methoxy dicarboxylic acids, tricarboxylic acids, triols, and photooxidation products of α-pinene and toluene. In particular, five secondary organic aerosol compounds observed in a smog chamber irradiation of an α-pinene/NOx/air mixture were detected in ambient PM2.5.