Uncovering the Photochemical Conversion of Atmospheric Chlorinated Organics on Mineral Dust: In‐Field Evidence of a New Source of Dioxin

Abstract Hazardous chemicals are typically assessed based on their inherent toxicity, often neglecting the fact that their atmospheric secondary transformation products may exhibit increased toxicity and persistence, potentially exceeding the risks associated with the parent chemicals. Chlorinated volatile organic compounds (CVOCs) are a significant class of commercial chemicals, but their secondary conversion in the atmosphere remains largely unknown. Herein, by combining laboratory and in‐field experiments, we have identified a new conversion pathway that the CVOCs can be photochemically transformed into polychlorinated dibenzo‐ p ‐dioxins and dibenzofurans (PCDD/Fs) on mineral dust particulates under atmospheric conditions. We showed that mineral components, particularly Fe‐ and Al‐related oxides, can efficiently convert monochlorobenzene, dichloromethane, and perchloroethylene into PCDD/Fs under light irradiation. By combing reaction product measurements and density functional theory (DFT) calculations, we found that the α‐Fe 2 O 3 exhibited much higher propensity for dioxin formation than γ‐Al 2 O 3 , as evidenced by its lower reaction energy barriers for both the initial phenol formation and subsequent chlorination processes. In particular, histopathological assays showed the photochemically‐reacted α‐Fe 2 O 3 can cause severe damage to the lung and brain tissues of mice, underscoring the need to reassess the toxicity of commercial CVOCs and their secondary transformation products.