Abstract
Organic constituents of atmospheric particular matter (PM) contribute significantly to prompt generation of reactive oxygen species (ROS) in particle phase, which is often quantified in terms of its oxidative potential (OP). In this study, we used complementary methods of the advanced molecular characterization and the OP quantitation to explore potential of different organic species to enhance ROS generation in ambient aerosol samples. We used seven PM2.5 samples collected in the Midwest US, and extracted them sequentially into a set of solvents with different polarity: first non-polar hexane (HEX), then moderately polar dichloromethane (DCM) and finally polar deionized (DI) water. The DI extracts were further separated using solid phase extraction to isolate water-soluble organic carbon (WSOC) from inorganic ions. The OP of obtained extracts was analyzed using dithiothreitol (DTT) assay and reported here in terms of the DTT consumption rate (called DTT activity). The molecular composition of the same extracts was analyzed using a tandem platform of high-performance liquid chromatography (HPLC) coupled with a photodiode array detector (PDA) and a high-resolution mass spectrometer (HRMS). Highest DTT activities were observed in WSOC fractions (>1.1 μM min−1) followed by DCM (0.75–1.1 μM min−1) and HEX (<0.75 μM min−1) fractions, respectively. These results were complemented by the HPLC-PDA-HRMS molecular characterization data indicating numerous redox-active polar organics detected in the WSOC fractions. In particular, chemical component with elemental formula of C11H11N - attributed to dimethyl quinoline (DMQ) - was found systematically across all DTT-active fractions. The unprotonated nitrogen atom in the pyridine ring of quinolines facilitates electron transfer in the redox reactions, catalyzing generation of ROS in the quinoline containing samples. This is the first study that unambiguously identifies quinoline as a promoter of ROS generation determined within complex mixtures of ambient organic aerosols.
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