Changes in source contributions to the oxidative potential of PM_2.5 in urban Xiamen, China

The toxicity of PM_2.5 does not necessarily change synchronously with its mass concentration. In this study, the chemical composition (carbonaceous species, water-soluble ions, and metals) and oxidative potential (dithiothreitol assay, DTT) of PM_2.5 were investigated in 2017/2018 and 2022 in Xiamen, China. The decrease rate of volume-normalized DTT (DTT_v) (38%) was lower than that of PM_2.5 (55%) between the two sampling periods. However, the mass-normalized DTT (DTT_m) increased by 44%. Clear seasonal patterns with higher levels in winter were found for PM_2.5, most chemical constituents and DTT_v but not for DTT_m. The large decrease in DTT activity (84%-92%) after the addition of EDTA suggested that water-soluble metals were the main contributors to DTT in Xiamen. The increased gap between the reconstructed and measured DTT_v and the stronger correlations between the reconstructed/measured DTT ratio and carbonaceous species in 2022 were observed. The decrease rates of the hazard index (32.5%) and lifetime cancer risk (9.1%) differed from those of PM_2.5 and DTT_v due to their different main contributors. The PMF-MLR model showed that the contributions (nmol/(min·m³)) of vehicle emission, coal + biomass burning, ship emission and secondary aerosol to DTT_v in 2022 decreased by 63.0%, 65.2%, 66.5%, and 22.2%, respectively, compared to those in 2017/2018, which was consistent with the emission reduction of vehicle exhaust and coal consumption, the adoption of low-sulfur fuel oil used on board ships and the reduced production of WSOC. However, the contributions of dust + sea salt and industrial emission increased.