Organic PM2.5: Fractionation by Polarity, FTIR Spectroscopy, and OM/OC Ratio for the Pittsburgh Aerosol

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Abstract
A polarity-based extraction/fractionation method validated with standard compounds was used to characterize organic aerosol samples collected during the Pittsburgh Air Quality Study (PAQS). Organic extracts were separated into 5 polarity classes by sequential elution with hexane, dichloromethane, ethyl acetate, acetone, and methanol. Organic mass (OM) and carbon mass (OC) were measured in samples, their extracts, and their corresponding fractions yielding OM/OC ratios and the contribution of each polarity fraction to total OM. The study average OM/OC ratio for each fraction [(OM/OC)fraction] varied from 1.37 for the hexane fraction to 2.25 for the methanol fraction. OM/OC ratios for “non-extractable” organics ((OM/OC)N-E) were also predicted; the study average (OM/OC)N - E was 2.54, consistent with ratios of 2.1–3.2 for water-soluble organic aerosol species. Annual average ratios with and without the contributions of the “non-extractable” material [(OM/OC)total and (OM/OC)extract, respectively] were 2.05 ± 0.18(1σ) and 1.91 ± 0.24(1σ), similar to OM/OC of atmospherically relevant oligomers and aged aerosols measured elsewhere. Ratios were somewhat higher during summer/winter than spring/fall, probably because of a greater contribution of oxidized species such as dicarboxylic acids (summer), levoglucosan (winter and/or summer), and humic-like-substances (HULIS; winter and/or summer). We hypothesize that the OM/OC of atmospheric aerosols approaches a value of 1.9–2.1 as it ages and oligomerizes in the atmosphere. The annual-average contributions of each fraction to the total collected OM mass were, 16.8, 14.0, 11.7, 11.5, 19.3, and 26.7% for the hexane-, dichloromethane-, ethyl acetate-, acetone-, methanol-, and “non-extractable” fractions, respectively. Thus non-polar and very polar species dominated the OM mass throughout the year. Fourier transformed infrared (FTIR) spectroscopy was used to further characterize the composition of extracts and fractions. This method can be used to fractionate organic PM for toxicological studies as well as organic aerosol characterization.