Redox potential and cytotoxicity of N-heterocyclic aromatic SOA from indole oxidation in the atmosphere

Nitrogen-containing heterocyclic aromatic compounds (NHACs) significantly contribute to urban air pollution but remain understudied. This study investigates the formation and transformation of secondary organic aerosol (SOA) from prototypical NHACs, indole, under various day- and night-time atmospheric processes. We examined the relationship between the evolving redox potential and cytotoxicity of indole-derived SOA (Indole-SOA) and its chemical alternations. Results show that Indole-SOA exhibits high oxidative potential (OP^DTT of 88–268 pmol min⁻¹ μg⁻¹) and antioxidant capacity (AOC of 0.41–0.83 nmol trolox μg⁻¹), exceeding most biogenic and anthropogenic organic aerosols. Indole-SOA induces significant cytotoxicity in lung epithelial cells, characterized by oxidative stress, mitochondrial dysfunction, and CYP1A1-driven detoxification pathways. The strongly correlated OP^DTT and cytotoxicity of Indole-SOA are influenced by atmospheric agings and are closely related to particulate-phase products of aromatic carbonyl and reduced-nitrogen compounds. Nighttime chemistry involving O₃ and NO₃• produces Indole-SOA with lower yields but higher redox potential and cytotoxicity. Furthermore, Indole-SOA mixing with ambient PM_2.5 shows a positive redox interaction, with the synergistic effect on OP^DTT determined by Indole-SOA type and proportion. Molecular markers of Indole-SOA can be identified in Shanghai urban PM_2.5, indicating potential health risks from indole and its derivatives in Chinese megacities.