Nontargeted Analysis Reveals Organic Compounds That Drive Oxidative Potential in Ambient Particulate Matter

Air pollution adversely affects human health, with studies consistently linking it to exposure to particulate matter. However, differential toxicity remains to be established to improve monitoring and mitigation efforts. This study examines the oxidative potential (OP) of fine particulate matter using the dithiothreitol (DTT) assay as a proxy for toxicity. We analyzed 42 ambient particle filter samples from Frankfurt, Germany, and Beijing, China, focusing on water-soluble organic compounds. The aqueous extracts were analyzed for metal content via inductively coupled plasma-mass spectrometry and for organic compounds through ultrahigh performance liquid chromatography-high-resolution mass spectrometry. Utilizing a chelating agent to remove metals allowed separation of the effects of bulk aerosol and organics only. A hierarchical cluster analysis identified key compounds linked to OP, including yet unknown quinones, C₉H₆O₆ (an oxidation product of o-xylene), and C₉H₈O₄ (an oxidation product of trimethylbenzene), suggesting secondary products from aromatic precursors. We also found phthalic acid, a tracer for biomass burning and vehicular exhaust, and nitrosalicylic acid, an OP-enhancing compound. A volume-normalized OP of up to 4 nmol DTT min^-1 m^-3 was observed after metal removal, underscoring the importance of stable organic compounds and isolating metals to understand their relationship with OP in ambient particulate matter.