Fine particulate matter (PM_2.5) causes millions of premature deaths each year worldwide. Oxidative potential (OP) has been proposed as a better metric for aerosol health effects than PM_2.5 mass concentration alone. In this study, we report for the first time online measurements of PM_2.5 OP in wintertime Beijing and surroundings based on a dithiothreitol (DTT) assay. These measurements were combined with co-located PM chemical composition measurements to identify the main source categories of aerosol OP. In addition, we highlight the influence of two distinct pollution events on aerosol OP (spring festival celebrations including fireworks and a severe regional dust storm). Source apportionment coupled with multilinear regression revealed that primary PM and oxygenated organic aerosol (OOA) were both important sources of OP, accounting for 41 ± 12 % and 39 ± 10 % of the OP_v^DTT (OP normalized by the sampled air volume), respectively. The small remainder was attributed to fireworks and dust, mainly resulting from the two distinct pollution events. During the 3.5-day spring festival period, OP_v^DTT spiked to 4.9 nmol min^-1 m^-3 with slightly more contribution from OOA (42 ± 11 %) and less from primary PM (31 ± 15 %). During the dust storm, hourly-averaged PM_2.5 peaked at a very high value of 548 μg m^-3 due to the dominant presence of dust-laden particles (88 % of total PM_2.5). In contrast, only mildly elevated OP_v^DTT values (up to 1.5 nmol min^-1 m^-3) were observed during this dust event. This observation indicates that variations in OP_v^DTT cannot be fully explained using PM_2.5 alone; one must also consider the chemical composition of PM_2.5 when studying aerosol health effects. Our study highlights the need for continued pollution control strategies to reduce primary PM emissions, and more in-depth investigations into the source origins of OOA, to minimize the health risks associated with PM exposure in Beijing.

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