Atmospheric processing reshapes the VOC health-risk profile downwind of a petrochemical complex: a source-to-receptor...

Volatile organic compounds (VOCs) emitted from a large petrochemical complex in Daesan, South Korea pose significant health risks. Conventional risk assessments often rely on static source emissions and may not fully capture the chemical evolution of toxicity during atmospheric transport, leading to potential exposure misclassification where relatively low-toxicity precursors form more harmful secondary compounds downwind. To address this limitation, this study integrates real-time field measurements with mechanistic atmospheric simulations to re-evaluate health risks and identify population-specific Chemicals of Concern (COCs), defined based on their contribution to cumulative health risk (hazard index, HI, and carcinogenic risk, CR). Fifty-six VOCs were quantified at the source using a proton-transfer-reaction mass spectrometer (PTR-MS), along with inorganic precursors (NO_x). A coupled AERMOD-MCM (Master Chemical Mechanism) framework was applied to estimate residential exposure 4.4 km downwind under wintertime conditions, providing a conservative baseline with reduced photochemical activity. The results reveal a shift in dominant risk drivers during atmospheric transport. While physical dilution reduced primary reactive alkenes by ∼26.5%, atmospheric chemistry altered this trend, with secondary formation increasing formaldehyde concentrations by 35%. Importantly, the acrolein-excluded hazard index (robust HI) remained approximately 4.0 at the residential receptor, exceeding the safety threshold (HI > 1), indicating sustained non-carcinogenic risk independent of acrolein. In contrast, the total HI reached 294; however, this value is largely driven by acrolein and should be interpreted as an upper-bound estimate given known uncertainties in PTR-MS quantification. These findings demonstrate that reactive VOC precursors (e.g., ethylene, propylene) can substantially contribute to downwind health risk through secondary pollutant formation. Risk assessments based solely on primary emissions may therefore underestimate toxicity associated with atmospheric processing. Overall, this study highlights the importance of incorporating chemical transformation into health risk assessments and supports the development of reactivity-based emission control strategies that account for both primary emissions and secondary formation.