Structural and Light-Absorption Characteristics of Complex Water-Insoluble Organic Mixtures in Urban Submicrometer Aerosols

Submicrometer aerosols in the urban atmosphere of Nagoya, Japan, were collected in late winter and early spring, and the water-insoluble organic matter (WISOM) in the samples were fractionated into six subfractions based on their polarities by using solvent and normal-phase solid-phase extractions: nonpolar (Fl), low polar (F2 and F3), and medium-polar (F4, F5, and F6) fractions. The overall structural characteristics of these subfractions were then analyzed using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and high-resolution aerosol mass spectrometry. Quantitative information related to the overall chemical characteristics of the WISOM in the different polarity fractions, including their elemental compositions, the relative abundances of different functional groups and their fragments from electron impact ionization, was obtained. These water-insoluble fractions accounted for half of the total light absorption by the extracted aerosol matter at 400 nm. The contributions of the medium-polar fractions to both the total organic carbon and light absorption by the extracts were dominant among the contributions from the six subfractions. Large molecules with aromatic and heteroatomic (0 and N) groups, including charge transfer complexes, might have greatly contributed to the light absorption by the fraction F4, which is the largest fraction of the extracted water-insoluble organic matter.