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

The toxicity of PM 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 were investigated in 2017/2018 and 2022 in Xiamen, China. The decrease rate of volume-normalized DTT (DTT) (38%) was lower than that of PM (55%) between the two sampling periods. However, the mass-normalized DTT (DTT) increased by 44%. Clear seasonal patterns with higher levels in winter were found for PM, most chemical constituents and DTT but not for DTT. 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 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 and DTT due to their different main contributors. The PMF-MLR model showed that the contributions (nmol/(min·m)) of vehicle emission, coal + biomass burning, ship emission and secondary aerosol to DTT 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.