Decoupling between PM2.5 oxidative potential and light extinction: Roles of chemical composition and aerosol thermodynamic state.

Fine particulate matter (PM2.5) can induce adverse health effects through oxidative stress, yet its oxidative burden is not always reflected by conventional indicators such as mass concentration or light extinction. This study investigated the coupling and decoupling between oxidative potential (OP) and light extinction using PM2.5 observations at two urban sites in southern Taiwan. A quadrant framework integrating volume-normalized OP and light extinction identified an "invisible killer" regime with high redox activity but low optical impact. Under coupling conditions, mass-normalized OP and mass extinction efficiency were mainly associated with shared chemical components, particularly water-soluble organic carbon. Under decoupling conditions, high aerosol liquid water content (ALWC) was associated with optics-dominant states through enhanced hygroscopic growth and light extinction, whereas low ALWC corresponded to higher OP-to-extinction ratios. Aerosol pH showed a weaker secondary association, consistent with literature-supported effects of acidity on aerosol chemistry and potential metal dissolution. Source apportionment indicated that secondary nitrate primarily enhanced light extinction, while region-specific source characteristics contributed to OP variability. These findings show that optical improvements may not correspond to proportional reductions in particle redox activity, supporting complementary use of OP-informed metrics in air quality assessment.